<- RFC Index (1501..1600)
RFC 1583
Obsoletes RFC 1247
Obsoleted by RFC 2178
Network Working Group J. Moy
Request for Comments: 1583 Proteon, Inc.
Obsoletes: 1247 March 1994
Category: Standards Track
OSPF Version 2
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is
unlimited.
Abstract
This memo documents version 2 of the OSPF protocol. OSPF is a
link-state routing protocol. It is designed to be run internal to a
single Autonomous System. Each OSPF router maintains an identical
database describing the Autonomous System's topology. From this
database, a routing table is calculated by constructing a shortest-
path tree.
OSPF recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic. OSPF provides
support for equal-cost multipath. Separate routes can be calculated
for each IP Type of Service. An area routing capability is
provided, enabling an additional level of routing protection and a
reduction in routing protocol traffic. In addition, all OSPF
routing protocol exchanges are authenticated.
OSPF Version 2 was originally documented in RFC 1247. The
differences between RFC 1247 and this memo are explained in Appendix
E. The differences consist of bug fixes and clarifications, and are
backward-compatible in nature. Implementations of RFC 1247 and of
this memo will interoperate.
Please send comments to ospf@gated.cornell.edu.
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RFC 1583 OSPF Version 2 March 1994
Table of Contents
1 Introduction ........................................... 5
1.1 Protocol Overview ...................................... 5
1.2 Definitions of commonly used terms ..................... 6
1.3 Brief history of link-state routing technology ......... 9
1.4 Organization of this document .......................... 9
2 The Topological Database .............................. 10
2.1 The shortest-path tree ................................ 13
2.2 Use of external routing information ................... 16
2.3 Equal-cost multipath .................................. 20
2.4 TOS-based routing ..................................... 20
3 Splitting the AS into Areas ........................... 21
3.1 The backbone of the Autonomous System ................. 22
3.2 Inter-area routing .................................... 22
3.3 Classification of routers ............................. 23
3.4 A sample area configuration ........................... 24
3.5 IP subnetting support ................................. 30
3.6 Supporting stub areas ................................. 31
3.7 Partitions of areas ................................... 32
4 Functional Summary .................................... 34
4.1 Inter-area routing .................................... 35
4.2 AS external routes .................................... 35
4.3 Routing protocol packets .............................. 35
4.4 Basic implementation requirements ..................... 38
4.5 Optional OSPF capabilities ............................ 39
5 Protocol data structures .............................. 41
6 The Area Data Structure ............................... 42
7 Bringing Up Adjacencies ............................... 45
7.1 The Hello Protocol .................................... 45
7.2 The Synchronization of Databases ...................... 46
7.3 The Designated Router ................................. 47
7.4 The Backup Designated Router .......................... 48
7.5 The graph of adjacencies .............................. 49
8 Protocol Packet Processing ............................ 50
8.1 Sending protocol packets .............................. 51
8.2 Receiving protocol packets ............................ 53
9 The Interface Data Structure .......................... 55
9.1 Interface states ...................................... 58
9.2 Events causing interface state changes ................ 61
9.3 The Interface state machine ........................... 62
9.4 Electing the Designated Router ........................ 65
9.5 Sending Hello packets ................................. 67
9.5.1 Sending Hello packets on non-broadcast networks ....... 68
10 The Neighbor Data Structure ........................... 69
10.1 Neighbor states ....................................... 72
10.2 Events causing neighbor state changes ................. 75
10.3 The Neighbor state machine ............................ 77
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10.4 Whether to become adjacent ............................ 83
10.5 Receiving Hello Packets ............................... 83
10.6 Receiving Database Description Packets ................ 86
10.7 Receiving Link State Request Packets .................. 89
10.8 Sending Database Description Packets .................. 89
10.9 Sending Link State Request Packets .................... 90
10.10 An Example ............................................ 91
11 The Routing Table Structure ........................... 93
11.1 Routing table lookup .................................. 96
11.2 Sample routing table, without areas ................... 97
11.3 Sample routing table, with areas ...................... 98
12 Link State Advertisements ............................ 100
12.1 The Link State Advertisement Header .................. 101
12.1.1 LS age ............................................... 102
12.1.2 Options .............................................. 102
12.1.3 LS type .............................................. 103
12.1.4 Link State ID ........................................ 103
12.1.5 Advertising Router ................................... 105
12.1.6 LS sequence number ................................... 105
12.1.7 LS checksum .......................................... 106
12.2 The link state database .............................. 107
12.3 Representation of TOS ................................ 108
12.4 Originating link state advertisements ................ 109
12.4.1 Router links ......................................... 112
12.4.2 Network links ........................................ 118
12.4.3 Summary links ........................................ 120
12.4.4 Originating summary links into stub areas ............ 123
12.4.5 AS external links .................................... 124
13 The Flooding Procedure ............................... 126
13.1 Determining which link state is newer ................ 130
13.2 Installing link state advertisements in the database . 130
13.3 Next step in the flooding procedure .................. 131
13.4 Receiving self-originated link state ................. 134
13.5 Sending Link State Acknowledgment packets ............ 135
13.6 Retransmitting link state advertisements ............. 136
13.7 Receiving link state acknowledgments ................. 138
14 Aging The Link State Database ........................ 139
14.1 Premature aging of advertisements .................... 139
15 Virtual Links ........................................ 140
16 Calculation Of The Routing Table ..................... 142
16.1 Calculating the shortest-path tree for an area ....... 143
16.1.1 The next hop calculation ............................. 149
16.2 Calculating the inter-area routes .................... 150
16.3 Examining transit areas' summary links ............... 152
16.4 Calculating AS external routes ....................... 154
16.5 Incremental updates -- summary link advertisements ... 156
16.6 Incremental updates -- AS external link advertisements 157
16.7 Events generated as a result of routing table changes 157
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16.8 Equal-cost multipath ................................. 158
16.9 Building the non-zero-TOS portion of the routing table 158
Footnotes ............................................ 161
References ........................................... 164
A OSPF data formats .................................... 166
A.1 Encapsulation of OSPF packets ........................ 166
A.2 The Options field .................................... 168
A.3 OSPF Packet Formats .................................. 170
A.3.1 The OSPF packet header ............................... 171
A.3.2 The Hello packet ..................................... 173
A.3.3 The Database Description packet ...................... 175
A.3.4 The Link State Request packet ........................ 177
A.3.5 The Link State Update packet ......................... 179
A.3.6 The Link State Acknowledgment packet ................. 181
A.4 Link state advertisement formats ..................... 183
A.4.1 The Link State Advertisement header .................. 184
A.4.2 Router links advertisements .......................... 186
A.4.3 Network links advertisements ......................... 190
A.4.4 Summary link advertisements .......................... 192
A.4.5 AS external link advertisements ...................... 194
B Architectural Constants .............................. 196
C Configurable Constants ............................... 198
C.1 Global parameters .................................... 198
C.2 Area parameters ...................................... 198
C.3 Router interface parameters .......................... 200
C.4 Virtual link parameters .............................. 202
C.5 Non-broadcast, multi-access network parameters ....... 203
C.6 Host route parameters ................................ 203
D Authentication ....................................... 205
D.1 AuType 0 -- No authentication ........................ 205
D.2 AuType 1 -- Simple password .......................... 205
E Differences from RFC 1247 ............................ 207
E.1 A fix for a problem with OSPF Virtual links .......... 207
E.2 Supporting supernetting and subnet 0 ................. 208
E.3 Obsoleting LSInfinity in router links advertisements . 209
E.4 TOS encoding updated ................................. 209
E.5 Summarizing routes into transit areas ................ 210
E.6 Summarizing routes into stub areas ................... 210
E.7 Flushing anomalous network links advertisements ...... 210
E.8 Required Statistics appendix deleted ................. 211
E.9 Other changes ........................................ 211
F. An algorithm for assigning Link State IDs ............ 213
Security Considerations .............................. 216
Author's Address ..................................... 216
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RFC 1583 OSPF Version 2 March 1994
1. Introduction
This document is a specification of the Open Shortest Path First
(OSPF) TCP/IP internet routing protocol. OSPF is classified as an
Interior Gateway Protocol (IGP). This means that it distributes
routing information between routers belonging to a single Autonomous
System. The OSPF protocol is based on link-state or SPF technology.
This is a departure from the Bellman-Ford base used by traditional
TCP/IP internet routing protocols.
The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force. It has been designed expressly for
the TCP/IP internet environment, including explicit support for IP
subnetting, TOS-based routing and the tagging of externally-derived
routing information. OSPF also provides for the authentication of
routing updates, and utilizes IP multicast when sending/receiving
the updates. In addition, much work has been done to produce a
protocol that responds quickly to topology changes, yet involves
small amounts of routing protocol traffic.
The author would like to thank Fred Baker, Jeffrey Burgan, Rob
Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo
Medin, Kannan Varadhan and the rest of the OSPF working group for
the ideas and support they have given to this project.
1.1. Protocol overview
OSPF routes IP packets based solely on the destination IP
address and IP Type of Service found in the IP packet header.
IP packets are routed "as is" -- they are not encapsulated in
any further protocol headers as they transit the Autonomous
System. OSPF is a dynamic routing protocol. It quickly detects
topological changes in the AS (such as router interface
failures) and calculates new loop-free routes after a period of
convergence. This period of convergence is short and involves a
minimum of routing traffic.
In a link-state routing protocol, each router maintains a
database describing the Autonomous System's topology. Each
participating router has an identical database. Each individual
piece of this database is a particular router's local state
(e.g., the router's usable interfaces and reachable neighbors).
The router distributes its local state throughout the Autonomous
System by flooding.
All routers run the exact same algorithm, in parallel. From the
topological database, each router constructs a tree of shortest
paths with itself as root. This shortest-path tree gives the
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route to each destination in the Autonomous System. Externally
derived routing information appears on the tree as leaves.
OSPF calculates separate routes for each Type of Service (TOS).
When several equal-cost routes to a destination exist, traffic
is distributed equally among them. The cost of a route is
described by a single dimensionless metric.
OSPF allows sets of networks to be grouped together. Such a
grouping is called an area. The topology of an area is hidden
from the rest of the Autonomous System. This information hiding
enables a significant reduction in routing traffic. Also,
routing within the area is determined only by the area's own
topology, lending the area protection from bad routing data. An
area is a generalization of an IP subnetted network.
OSPF enables the flexible configuration of IP subnets. Each
route distributed by OSPF has a destination and mask. Two
different subnets of the same IP network number may have
different sizes (i.e., different masks). This is commonly
referred to as variable length subnetting. A packet is routed
to the best (i.e., longest or most specific) match. Host routes
are considered to be subnets whose masks are "all ones"
(0xffffffff).
All OSPF protocol exchanges are authenticated. This means that
only trusted routers can participate in the Autonomous System's
routing. A variety of authentication schemes can be used; a
single authentication scheme is configured for each area. This
enables some areas to use much stricter authentication than
others.
Externally derived routing data (e.g., routes learned from the
Exterior Gateway Protocol (EGP)) is passed transparently
throughout the Autonomous System. This externally derived data
is kept separate from the OSPF protocol's link state data. Each
external route can also be tagged by the advertising router,
enabling the passing of additional information between routers
on the boundaries of the Autonomous System.
1.2. Definitions of commonly used terms
This section provides definitions for terms that have a specific
meaning to the OSPF protocol and that are used throughout the
text. The reader unfamiliar with the Internet Protocol Suite is
referred to [RS-85-153] for an introduction to IP.
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Router
A level three Internet Protocol packet switch. Formerly
called a gateway in much of the IP literature.
Autonomous System
A group of routers exchanging routing information via a
common routing protocol. Abbreviated as AS.
Interior Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous
System has a single IGP. Separate Autonomous Systems may be
running different IGPs.
Router ID
A 32-bit number assigned to each router running the OSPF
protocol. This number uniquely identifies the router within
an Autonomous System.
Network
In this memo, an IP network/subnet/supernet. It is possible
for one physical network to be assigned multiple IP
network/subnet numbers. We consider these to be separate
networks. Point-to-point physical networks are an exception
- they are considered a single network no matter how many
(if any at all) IP network/subnet numbers are assigned to
them.
Network mask
A 32-bit number indicating the range of IP addresses
residing on a single IP network/subnet/supernet. This
specification displays network masks as hexadecimal numbers.
For example, the network mask for a class C IP network is
displayed as 0xffffff00. Such a mask is often displayed
elsewhere in the literature as 255.255.255.0.
Multi-access networks
Those physical networks that support the attachment of
multiple (more than two) routers. Each pair of routers on
such a network is assumed to be able to communicate directly
(e.g., multi-drop networks are excluded).
Interface
The connection between a router and one of its attached
networks. An interface has state information associated
with it, which is obtained from the underlying lower level
protocols and the routing protocol itself. An interface to
a network has associated with it a single IP address and
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mask (unless the network is an unnumbered point-to-point
network). An interface is sometimes also referred to as a
link.
Neighboring routers
Two routers that have interfaces to a common network. On
multi-access networks, neighbors are dynamically discovered
by OSPF's Hello Protocol.
Adjacency
A relationship formed between selected neighboring routers
for the purpose of exchanging routing information. Not
every pair of neighboring routers become adjacent.
Link state advertisement
Describes the local state of a router or network. This
includes the state of the router's interfaces and
adjacencies. Each link state advertisement is flooded
throughout the routing domain. The collected link state
advertisements of all routers and networks forms the
protocol's topological database.
Hello Protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On multi-access networks the Hello
Protocol can also dynamically discover neighboring routers.
Designated Router
Each multi-access network that has at least two attached
routers has a Designated Router. The Designated Router
generates a link state advertisement for the multi-access
network and has other special responsibilities in the
running of the protocol. The Designated Router is elected
by the Hello Protocol.
The Designated Router concept enables a reduction in the
number of adjacencies required on a multi-access network.
This in turn reduces the amount of routing protocol traffic
and the size of the topological database.
Lower-level protocols
The underlying network access protocols that provide
services to the Internet Protocol and in turn the OSPF
protocol. Examples of these are the X.25 packet and frame
levels for X.25 PDNs, and the ethernet data link layer for
ethernets.
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1.3. Brief history of link-state routing technology
OSPF is a link state routing protocol. Such protocols are also
referred to in the literature as SPF-based or distributed-
database protocols. This section gives a brief description of
the developments in link-state technology that have influenced
the OSPF protocol.
The first link-state routing protocol was developed for use in
the ARPANET packet switching network. This protocol is
described in [McQuillan]. It has formed the starting point for
all other link-state protocols. The homogeneous Arpanet
environment, i.e., single-vendor packet switches connected by
synchronous serial lines, simplified the design and
implementation of the original protocol.
Modifications to this protocol were proposed in [Perlman].
These modifications dealt with increasing the fault tolerance of
the routing protocol through, among other things, adding a
checksum to the link state advertisements (thereby detecting
database corruption). The paper also included means for
reducing the routing traffic overhead in a link-state protocol.
This was accomplished by introducing mechanisms which enabled
the interval between link state advertisement originations to be
increased by an order of magnitude.
A link-state algorithm has also been proposed for use as an ISO
IS-IS routing protocol. This protocol is described in [DEC].
The protocol includes methods for data and routing traffic
reduction when operating over broadcast networks. This is
accomplished by election of a Designated Router for each
broadcast network, which then originates a link state
advertisement for the network.
The OSPF subcommittee of the IETF has extended this work in
developing the OSPF protocol. The Designated Router concept has
been greatly enhanced to further reduce the amount of routing
traffic required. Multicast capabilities are utilized for
additional routing bandwidth reduction. An area routing scheme
has been developed enabling information
hiding/protection/reduction. Finally, the algorithm has been
modified for efficient operation in TCP/IP internets.
1.4. Organization of this document
The first three sections of this specification give a general
overview of the protocol's capabilities and functions. Sections
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4-16 explain the protocol's mechanisms in detail. Packet
formats, protocol constants and configuration items are
specified in the appendices.
Labels such as HelloInterval encountered in the text refer to
protocol constants. They may or may not be configurable. The
architectural constants are explained in Appendix B. The
configurable constants are explained in Appendix C.
The detailed specification of the protocol is presented in terms
of data structures. This is done in order to make the
explanation more precise. Implementations of the protocol are
required to support the functionality described, but need not
use the precise data structures that appear in this memo.
2. The Topological Database
The Autonomous System's topological database describes a directed
graph. The vertices of the graph consist of routers and networks.
A graph edge connects two routers when they are attached via a
physical point-to-point network. An edge connecting a router to a
network indicates that the router has an interface on the network.
The vertices of the graph can be further typed according to
function. Only some of these types carry transit data traffic; that
is, traffic that is neither locally originated nor locally destined.
Vertices that can carry transit traffic are indicated on the graph
by having both incoming and outgoing edges.
Vertex type Vertex name Transit?
_____________________________________
1 Router yes
2 Network yes
3 Stub network no
Table 1: OSPF vertex types.
OSPF supports the following types of physical networks:
Point-to-point networks
A network that joins a single pair of routers. A 56Kb serial
line is an example of a point-to-point network.
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Broadcast networks
Networks supporting many (more than two) attached routers,
together with the capability to address a single physical
message to all of the attached routers (broadcast). Neighboring
routers are discovered dynamically on these nets using OSPF's
Hello Protocol. The Hello Protocol itself takes advantage of
the broadcast capability. The protocol makes further use of
multicast capabilities, if they exist. An ethernet is an
example of a broadcast network.
Non-broadcast networks
Networks supporting many (more than two) routers, but having no
broadcast capability. Neighboring routers are also discovered
on these nets using OSPF's Hello Protocol. However, due to the
lack of broadcast capability, some configuration information is
necessary for the correct operation of the Hello Protocol. On
these networks, OSPF protocol packets that are normally
multicast need to be sent to each neighboring router, in turn.
An X.25 Public Data Network (PDN) is an example of a non-
broadcast network.
The neighborhood of each network node in the graph depends on
whether the network has multi-access capabilities (either broadcast
or non-broadcast) and, if so, the number of routers having an
interface to the network. The three cases are depicted in Figure 1.
Rectangles indicate routers. Circles and oblongs indicate multi-
access networks. Router names are prefixed with the letters RT and
network names with the letter N. Router interface names are
prefixed by the letter I. Lines between routers indicate point-to-
point networks. The left side of the figure shows a network with
its connected routers, with the resulting graph shown on the right.
Two routers joined by a point-to-point network are represented in
the directed graph as being directly connected by a pair of edges,
one in each direction. Interfaces to physical point-to-point
networks need not be assigned IP addresses. Such a point-to-point
network is called unnumbered. The graphical representation of
point-to-point networks is designed so that unnumbered networks can
be supported naturally. When interface addresses exist, they are
modelled as stub routes. Note that each router would then have a
stub connection to the other router's interface address (see Figure
1).
When multiple routers are attached to a multi-access network, the
directed graph shows all routers bidirectionally connected to the
network vertex (again, see Figure 1). If only a single router is
attached to a multi-access network, the network will appear in the
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RFC 1583 OSPF Version 2 March 1994
**FROM**
* |RT1|RT2|
+---+Ia +---+ * ------------
|RT1|------|RT2| T RT1| | X |
+---+ Ib+---+ O RT2| X | |
* Ia| | X |
* Ib| X | |
Physical point-to-point networks
**FROM**
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|N2 |
+---+ +---+ * ------------------------
| N2 | * RT3| | | | | X |
+----------------------+ T RT4| | | | | X |
| | O RT5| | | | | X |
+---+ +---+ * RT6| | | | | X |
|RT5| |RT6| * N2| X | X | X | X | |
+---+ +---+
Multi-access networks
**FROM**
+---+ *
|RT7| * |RT7| N3|
+---+ T ------------
| O RT7| | |
+----------------------+ * N3| X | |
N3 *
Stub multi-access networks
Figure 1: Network map components
Networks and routers are represented by vertices.
An edge connects Vertex A to Vertex B iff the
intersection of Column A and Row B is marked with
an X.
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directed graph as a stub connection.
Each network (stub or transit) in the graph has an IP address and
associated network mask. The mask indicates the number of nodes on
the network. Hosts attached directly to routers (referred to as
host routes) appear on the graph as stub networks. The network mask
for a host route is always 0xffffffff, which indicates the presence
of a single node.
Figure 2 shows a sample map of an Autonomous System. The rectangle
labelled H1 indicates a host, which has a SLIP connection to Router
RT12. Router RT12 is therefore advertising a host route. Lines
between routers indicate physical point-to-point networks. The only
point-to-point network that has been assigned interface addresses is
the one joining Routers RT6 and RT10. Routers RT5 and RT7 have EGP
connections to other Autonomous Systems. A set of EGP-learned
routes have been displayed for both of these routers.
A cost is associated with the output side of each router interface.
This cost is configurable by the system administrator. The lower
the cost, the more likely the interface is to be used to forward
data traffic. Costs are also associated with the externally derived
routing data (e.g., the EGP-learned routes).
The directed graph resulting from the map in Figure 2 is depicted in
Figure 3. Arcs are labelled with the cost of the corresponding
router output interface. Arcs having no labelled cost have a cost
of 0. Note that arcs leading from networks to routers always have
cost 0; they are significant nonetheless. Note also that the
externally derived routing data appears on the graph as stubs.
The topological database (or what has been referred to above as the
directed graph) is pieced together from link state advertisements
generated by the routers. The neighborhood of each transit vertex
is represented in a single, separate link state advertisement.
Figure 4 shows graphically the link state representation of the two
kinds of transit vertices: routers and multi-access networks.
Router RT12 has an interface to two broadcast networks and a SLIP
line to a host. Network N6 is a broadcast network with three
attached routers. The cost of all links from Network N6 to its
attached routers is 0. Note that the link state advertisement for
Network N6 is actually generated by one of the attached routers: the
router that has been elected Designated Router for the network.
2.1. The shortest-path tree
When no OSPF areas are configured, each router in the Autonomous
System has an identical topological database, leading to an
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+
| 3+---+ N12 N14
N1|--|RT1|\ 1 \ N13 /
| +---+ \ 8\ |8/8
+ \ ____ \|/
/ \ 1+---+8 8+---+6
* N3 *---|RT4|------|RT5|--------+
\____/ +---+ +---+ |
+ / | |7 |
| 3+---+ / | | |
N2|--|RT2|/1 |1 |6 |
| +---+ +---+8 6+---+ |
+ |RT3|--------------|RT6| |
+---+ +---+ |
|2 Ia|7 |
| | |
+---------+ | |
N4 | |
| |
| |
N11 | |
+---------+ | |
| | | N12
|3 | |6 2/
+---+ | +---+/
|RT9| | |RT7|---N15
+---+ | +---+ 9
|1 + | |1
_|__ | Ib|5 __|_
/ \ 1+----+2 | 3+----+1 / \
* N9 *------|RT11|----|---|RT10|---* N6 *
\____/ +----+ | +----+ \____/
| | |
|1 + |1
+--+ 10+----+ N8 +---+
|H1|-----|RT12| |RT8|
+--+SLIP +----+ +---+
|2 |4
| |
+---------+ +--------+
N10 N7
Figure 2: A sample Autonomous System
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RFC 1583 OSPF Version 2 March 1994
**FROM**
|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
----- ---------------------------------------------
RT1| | | | | | | | | | | | |0 | | | |
RT2| | | | | | | | | | | | |0 | | | |
RT3| | | | | |6 | | | | | | |0 | | | |
RT4| | | | |8 | | | | | | | |0 | | | |
RT5| | | |8 | |6 |6 | | | | | | | | | |
RT6| | |8 | |7 | | | | |5 | | | | | | |
RT7| | | | |6 | | | | | | | | |0 | | |
* RT8| | | | | | | | | | | | | |0 | | |
* RT9| | | | | | | | | | | | | | | |0 |
T RT10| | | | | |7 | | | | | | | |0 |0 | |
O RT11| | | | | | | | | | | | | | |0 |0 |
* RT12| | | | | | | | | | | | | | | |0 |
* N1|3 | | | | | | | | | | | | | | | |
N2| |3 | | | | | | | | | | | | | | |
N3|1 |1 |1 |1 | | | | | | | | | | | | |
N4| | |2 | | | | | | | | | | | | | |
N6| | | | | | |1 |1 | |1 | | | | | | |
N7| | | | | | | |4 | | | | | | | | |
N8| | | | | | | | | |3 |2 | | | | | |
N9| | | | | | | | |1 | |1 |1 | | | | |
N10| | | | | | | | | | | |2 | | | | |
N11| | | | | | | | |3 | | | | | | | |
N12| | | | |8 | |2 | | | | | | | | | |
N13| | | | |8 | | | | | | | | | | | |
N14| | | | |8 | | | | | | | | | | | |
N15| | | | | | |9 | | | | | | | | | |
H1| | | | | | | | | | | |10| | | | |
Figure 3: The resulting directed graph
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
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RFC 1583 OSPF Version 2 March 1994
**FROM** **FROM**
|RT12|N9|N10|H1| |RT9|RT11|RT12|N9|
* -------------------- * ----------------------
* RT12| | | | | * RT9| | | |0 |
T N9|1 | | | | T RT11| | | |0 |
O N10|2 | | | | O RT12| | | |0 |
* H1|10 | | | | * N9| | | | |
* *
RT12's router links N9's network links
advertisement advertisement
Figure 4: Individual link state components
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
identical graphical representation. A router generates its
routing table from this graph by calculating a tree of shortest
paths with the router itself as root. Obviously, the shortest-
path tree depends on the router doing the calculation. The
shortest-path tree for Router RT6 in our example is depicted in
Figure 5.
The tree gives the entire route to any destination network or
host. However, only the next hop to the destination is used in
the forwarding process. Note also that the best route to any
router has also been calculated. For the processing of external
data, we note the next hop and distance to any router
advertising external routes. The resulting routing table for
Router RT6 is pictured in Table 2. Note that there is a
separate route for each end of a numbered serial line (in this
case, the serial line between Routers RT6 and RT10).
Routes to networks belonging to other AS'es (such as N12) appear
as dashed lines on the shortest path tree in Figure 5. Use of
this externally derived routing information is considered in the
next section.
2.2. Use of external routing information
After the tree is created the external routing information is
examined. This external routing information may originate from
another routing protocol such as EGP, or be statically
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RT6(origin)
RT5 o------------o-----------o Ib
/|\ 6 |\ 7
8/8|8\ | \
/ | \ | \
o | o | \7
N12 o N14 | \
N13 2 | \
N4 o-----o RT3 \
/ \ 5
1/ RT10 o-------o Ia
/ |\
RT4 o-----o N3 3| \1
/| | \ N6 RT7
/ | N8 o o---------o
/ | | | /|
RT2 o o RT1 | | 2/ |9
/ | | |RT8 / |
/3 |3 RT11 o o o o
/ | | | N12 N15
N2 o o N1 1| |4
| |
N9 o o N7
/|
/ |
N11 RT9 / |RT12
o--------o-------o o--------o H1
3 | 10
|2
|
o N10
Figure 5: The SPF tree for Router RT6
Edges that are not marked with a cost have a cost of
of zero (these are network-to-router links). Routes
to networks N12-N15 are external information that is
considered in Section 2.2
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Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8
Table 2: The portion of Router RT6's routing table listing local
destinations.
configured (static routes). Default routes can also be included
as part of the Autonomous System's external routing information.
External routing information is flooded unaltered throughout the
AS. In our example, all the routers in the Autonomous System
know that Router RT7 has two external routes, with metrics 2 and
9.
OSPF supports two types of external metrics. Type 1 external
metrics are equivalent to the link state metric. Type 2
external metrics are greater than the cost of any path internal
to the AS. Use of Type 2 external metrics assumes that routing
between AS'es is the major cost of routing a packet, and
eliminates the need for conversion of external costs to internal
link state metrics.
As an example of Type 1 external metric processing, suppose that
the Routers RT7 and RT5 in Figure 2 are advertising Type 1
external metrics. For each external route, the distance from
Router RT6 is calculated as the sum of the external route's cost
and the distance from Router RT6 to the advertising router. For
every external destination, the router advertising the shortest
route is discovered, and the next hop to the advertising router
becomes the next hop to the destination.
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Both Router RT5 and RT7 are advertising an external route to
destination Network N12. Router RT7 is preferred since it is
advertising N12 at a distance of 10 (8+2) to Router RT6, which
is better than Router RT5's 14 (6+8). Table 3 shows the entries
that are added to the routing table when external routes are
examined:
Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17
Table 3: The portion of Router RT6's routing table
listing external destinations.
Processing of Type 2 external metrics is simpler. The AS
boundary router advertising the smallest external metric is
chosen, regardless of the internal distance to the AS boundary
router. Suppose in our example both Router RT5 and Router RT7
were advertising Type 2 external routes. Then all traffic
destined for Network N12 would be forwarded to Router RT7, since
2 < 8. When several equal-cost Type 2 routes exist, the
internal distance to the advertising routers is used to break
the tie.
Both Type 1 and Type 2 external metrics can be present in the AS
at the same time. In that event, Type 1 external metrics always
take precedence.
This section has assumed that packets destined for external
destinations are always routed through the advertising AS
boundary router. This is not always desirable. For example,
suppose in Figure 2 there is an additional router attached to
Network N6, called Router RTX. Suppose further that RTX does
not participate in OSPF routing, but does exchange EGP
information with the AS boundary router RT7. Then, Router RT7
would end up advertising OSPF external routes for all
destinations that should be routed to RTX. An extra hop will
sometimes be introduced if packets for these destinations need
always be routed first to Router RT7 (the advertising router).
To deal with this situation, the OSPF protocol allows an AS
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boundary router to specify a "forwarding address" in its
external advertisements. In the above example, Router RT7 would
specify RTX's IP address as the "forwarding address" for all
those destinations whose packets should be routed directly to
RTX.
The "forwarding address" has one other application. It enables
routers in the Autonomous System's interior to function as
"route servers". For example, in Figure 2 the router RT6 could
become a route server, gaining external routing information
through a combination of static configuration and external
routing protocols. RT6 would then start advertising itself as
an AS boundary router, and would originate a collection of OSPF
external advertisements. In each external advertisement, Router
RT6 would specify the correct Autonomous System exit point to
use for the destination through appropriate setting of the
advertisement's "forwarding address" field.
2.3. Equal-cost multipath
The above discussion has been simplified by considering only a
single route to any destination. In reality, if multiple
equal-cost routes to a destination exist, they are all
discovered and used. This requires no conceptual changes to the
algorithm, and its discussion is postponed until we consider the
tree-building process in more detail.
With equal cost multipath, a router potentially has several
available next hops towards any given destination.
2.4. TOS-based routing
OSPF can calculate a separate set of routes for each IP Type of
Service. This means that, for any destination, there can
potentially be multiple routing table entries, one for each IP
TOS. The IP TOS values are represented in OSPF exactly as they
appear in the IP packet header.
Up to this point, all examples shown have assumed that routes do
not vary on TOS. In order to differentiate routes based on TOS,
separate interface costs can be configured for each TOS. For
example, in Figure 2 there could be multiple costs (one for each
TOS) listed for each interface. A cost for TOS 0 must always be
specified.
When interface costs vary based on TOS, a separate shortest path
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tree is calculated for each TOS (see Section 2.1). In addition,
external costs can vary based on TOS. For example, in Figure 2
Router RT7 could advertise a separate type 1 external metric for
each TOS. Then, when calculating the TOS X distance to Network
N15 the cost of the shortest TOS X path to RT7 would be added to
the TOS X cost advertised by RT7 for Network N15 (see Section
2.2).
All OSPF implementations must be capable of calculating routes
based on TOS. However, OSPF routers can be configured to route
all packets on the TOS 0 path (see Appendix C), eliminating the
need to calculate non-zero TOS paths. This can be used to
conserve routing table space and processing resources in the
router. These TOS-0-only routers can be mixed with routers that
do route based on TOS. TOS-0-only routers will be avoided as
much as possible when forwarding traffic requesting a non-zero
TOS.
It may be the case that no path exists for some non-zero TOS,
even if the router is calculating non-zero TOS paths. In that
case, packets requesting that non-zero TOS are routed along the
TOS 0 path (see Section 11.1).
3. Splitting the AS into Areas
OSPF allows collections of contiguous networks and hosts to be
grouped together. Such a group, together with the routers having
interfaces to any one of the included networks, is called an area.
Each area runs a separate copy of the basic link-state routing
algorithm. This means that each area has its own topological
database and corresponding graph, as explained in the previous
section.
The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic
as compared to treating the entire Autonomous System as a single
link-state domain.
With the introduction of areas, it is no longer true that all
routers in the AS have an identical topological database. A router
actually has a separate topological database for each area it is
connected to. (Routers connected to multiple areas are called area
border routers). Two routers belonging to the same area have, for
that area, identical area topological databases.
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Routing in the Autonomous System takes place on two levels,
depending on whether the source and destination of a packet reside
in the same area (intra-area routing is used) or different areas
(inter-area routing is used). In intra-area routing, the packet is
routed solely on information obtained within the area; no routing
information obtained from outside the area can be used. This
protects intra-area routing from the injection of bad routing
information. We discuss inter-area routing in Section 3.2.
3.1. The backbone of the Autonomous System
The backbone consists of those networks not contained in any
area, their attached routers, and those routers that belong to
multiple areas. The backbone must be contiguous.
It is possible to define areas in such a way that the backbone
is no longer contiguous. In this case the system administrator
must restore backbone connectivity by configuring virtual links.
Virtual links can be configured between any two backbone routers
that have an interface to a common non-backbone area. Virtual
links belong to the backbone. The protocol treats two routers
joined by a virtual link as if they were connected by an
unnumbered point-to-point network. On the graph of the
backbone, two such routers are joined by arcs whose costs are
the intra-area distances between the two routers. The routing
protocol traffic that flows along the virtual link uses intra-
area routing only.
The backbone is responsible for distributing routing information
between areas. The backbone itself has all of the properties of
an area. The topology of the backbone is invisible to each of
the areas, while the backbone itself knows nothing of the
topology of the areas.
3.2. Inter-area routing
When routing a packet between two areas the backbone is used.
The path that the packet will travel can be broken up into three
contiguous pieces: an intra-area path from the source to an area
border router, a backbone path between the source and
destination areas, and then another intra-area path to the
destination. The algorithm finds the set of such paths that
have the smallest cost.
Looking at this another way, inter-area routing can be pictured
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as forcing a star configuration on the Autonomous System, with
the backbone as hub and each of the areas as spokes.
The topology of the backbone dictates the backbone paths used
between areas. The topology of the backbone can be enhanced by
adding virtual links. This gives the system administrator some
control over the routes taken by inter-area traffic.
The correct area border router to use as the packet exits the
source area is chosen in exactly the same way routers
advertising external routes are chosen. Each area border router
in an area summarizes for the area its cost to all networks
external to the area. After the SPF tree is calculated for the
area, routes to all other networks are calculated by examining
the summaries of the area border routers.
3.3. Classification of routers
Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as Router RT5 in Figure 2. When the AS is
split into OSPF areas, the routers are further divided according
to function into the following four overlapping categories:
Internal routers
A router with all directly connected networks belonging to
the same area. Routers with only backbone interfaces also
belong to this category. These routers run a single copy of
the basic routing algorithm.
Area border routers
A router that attaches to multiple areas. Area border
routers run multiple copies of the basic algorithm, one copy
for each attached area and an additional copy for the
backbone. Area border routers condense the topological
information of their attached areas for distribution to the
backbone. The backbone in turn distributes the information
to the other areas.
Backbone routers
A router that has an interface to the backbone. This
includes all routers that interface to more than one area
(i.e., area border routers). However, backbone routers do
not have to be area border routers. Routers with all
interfaces connected to the backbone are considered to be
internal routers.
Moy [Page 23]
RFC 1583 OSPF Version 2 March 1994
AS boundary routers
A router that exchanges routing information with routers
belonging to other Autonomous Systems. Such a router has AS
external routes that are advertised throughout the
Autonomous System. The path to each AS boundary router is
known by every router in the AS. This classification is
completely independent of the previous classifications: AS
boundary routers may be internal or area border routers, and
may or may not participate in the backbone.
3.4. A sample area configuration
Figure 6 shows a sample area configuration. The first area
consists of networks N1-N4, along with their attached routers
RT1-RT4. The second area consists of networks N6-N8, along with
their attached routers RT7, RT8, RT10 and RT11. The third area
consists of networks N9-N11 and Host H1, along with their
attached routers RT9, RT11 and RT12. The third area has been
configured so that networks N9-N11 and Host H1 will all be
grouped into a single route, when advertised external to the
area (see Section 3.5 for more details).
In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area
border routers. Finally, as before, Routers RT5 and RT7 are AS
boundary routers.
Figure 7 shows the resulting topological database for the Area
1. The figure completely describes that area's intra-area
routing. It also shows the complete view of the internet for
the two internal routers RT1 and RT2. It is the job of the area
border routers, RT3 and RT4, to advertise into Area 1 the
distances to all destinations external to the area. These are
indicated in Figure 7 by the dashed stub routes. Also, RT3 and
RT4 must advertise into Area 1 the location of the AS boundary
routers RT5 and RT7. Finally, external advertisements from RT5
and RT7 are flooded throughout the entire AS, and in particular
throughout Area 1. These advertisements are included in Area
1's database, and yield routes to Networks N12-N15.
Routers RT3 and RT4 must also summarize Area 1's topology for
distribution to the backbone. Their backbone advertisements are
shown in Table 4. These summaries show which networks are
contained in Area 1 (i.e., Networks N1-N4), and the distance to
these networks from the routers RT3 and RT4 respectively.
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RFC 1583 OSPF Version 2 March 1994
...........................
. + .
. | 3+---+ . N12 N14
. N1|--|RT1|\ 1 . \ N13 /
. | +---+ \ . 8\ |8/8
. + \ ____ . \|/
. / \ 1+---+8 8+---+6
. * N3 *---|RT4|------|RT5|--------+
. \____/ +---+ +---+ |
. + / \ . |7 |
. | 3+---+ / \ . | |
. N2|--|RT2|/1 1\ . |6 |
. | +---+ +---+8 6+---+ |
. + |RT3|------|RT6| |
. +---+ +---+ |
. 2/ . Ia|7 |
. / . | |
. +---------+ . | |
.Area 1 N4 . | |
........................... | |
.......................... | |
. N11 . | |
. +---------+ . | |
. | . | | N12
. |3 . Ib|5 |6 2/
. +---+ . +----+ +---+/
. |RT9| . .........|RT10|.....|RT7|---N15.
. +---+ . . +----+ +---+ 9 .
. |1 . . + /3 1\ |1 .
. _|__ . . | / \ __|_ .
. / \ 1+----+2 |/ \ / \ .
. * N9 *------|RT11|----| * N6 * .
. \____/ +----+ | \____/ .
. | . . | | .
. |1 . . + |1 .
. +--+ 10+----+ . . N8 +---+ .
. |H1|-----|RT12| . . |RT8| .
. +--+SLIP +----+ . . +---+ .
. |2 . . |4 .
. | . . | .
. +---------+ . . +--------+ .
. N10 . . N7 .
. . .Area 2 .
.Area 3 . ................................
..........................
Figure 6: A sample OSPF area configuration
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RFC 1583 OSPF Version 2 March 1994
Network RT3 adv. RT4 adv.
_____________________________
N1 4 4
N2 4 4
N3 1 1
N4 2 3
Table 4: Networks advertised to the backbone
by Routers RT3 and RT4.
The topological database for the backbone is shown in Figure 8.
The set of routers pictured are the backbone routers. Router
RT11 is a backbone router because it belongs to two areas. In
order to make the backbone connected, a virtual link has been
configured between Routers R10 and R11.
Again, Routers RT3, RT4, RT7, RT10 and RT11 are area border
routers. As Routers RT3 and RT4 did above, they have condensed
the routing information of their attached areas for distribution
via the backbone; these are the dashed stubs that appear in
Figure 8. Remember that the third area has been configured to
condense Networks N9-N11 and Host H1 into a single route. This
yields a single dashed line for networks N9-N11 and Host H1 in
Figure 8. Routers RT5 and RT7 are AS boundary routers; their
externally derived information also appears on the graph in
Figure 8 as stubs.
The backbone enables the exchange of summary information between
area border routers. Every area border router hears the area
summaries from all other area border routers. It then forms a
picture of the distance to all networks outside of its area by
examining the collected advertisements, and adding in the
backbone distance to each advertising router.
Again using Routers RT3 and RT4 as an example, the procedure
goes as follows: They first calculate the SPF tree for the
backbone. This gives the distances to all other area border
routers. Also noted are the distances to networks (Ia and Ib)
and AS boundary routers (RT5 and RT7) that belong to the
backbone. This calculation is shown in Table 5.
Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised
internally to the area by RT3 and RT4. The advertisements that
Router RT3 and RT4 will make into Area 1 are shown in Table 6.
Moy [Page 26]
RFC 1583 OSPF Version 2 March 1994
**FROM**
|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |7 |N3|
----- -------------------
RT1| | | | | | |0 |
RT2| | | | | | |0 |
RT3| | | | | | |0 |
* RT4| | | | | | |0 |
* RT5| | |14|8 | | | |
T RT7| | |20|14| | | |
O N1|3 | | | | | | |
* N2| |3 | | | | | |
* N3|1 |1 |1 |1 | | | |
N4| | |2 | | | | |
Ia,Ib| | |15|22| | | |
N6| | |16|15| | | |
N7| | |20|19| | | |
N8| | |18|18| | | |
N9-N11,H1| | |19|16| | | |
N12| | | | |8 |2 | |
N13| | | | |8 | | |
N14| | | | |8 | | |
N15| | | | | |9 | |
Figure 7: Area 1's Database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
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RFC 1583 OSPF Version 2 March 1994
**FROM**
|RT|RT|RT|RT|RT|RT|RT
|3 |4 |5 |6 |7 |10|11|
------------------------
RT3| | | |6 | | | |
RT4| | |8 | | | | |
RT5| |8 | |6 |6 | | |
RT6|8 | |7 | | |5 | |
RT7| | |6 | | | | |
* RT10| | | |7 | | |2 |
* RT11| | | | | |3 | |
T N1|4 |4 | | | | | |
O N2|4 |4 | | | | | |
* N3|1 |1 | | | | | |
* N4|2 |3 | | | | | |
Ia| | | | | |5 | |
Ib| | | |7 | | | |
N6| | | | |1 |1 |3 |
N7| | | | |5 |5 |7 |
N8| | | | |4 |3 |2 |
N9-N11,H1| | | | | | |1 |
N12| | |8 | |2 | | |
N13| | |8 | | | | |
N14| | |8 | | | | |
N15| | | | |9 | | |
Figure 8: The backbone's database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
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Area border dist from dist from
router RT3 RT4
______________________________________
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
______________________________________
to Ia 20 27
to Ib 15 22
______________________________________
to RT5 14 8
to RT7 20 14
Table 5: Backbone distances calculated
by Routers RT3 and RT4.
Note that Table 6 assumes that an area range has been configured
for the backbone which groups Ia and Ib into a single
advertisement.
The information imported into Area 1 by Routers RT3 and RT4
enables an internal router, such as RT1, to choose an area
border router intelligently. Router RT1 would use RT4 for
traffic to Network N6, RT3 for traffic to Network N10, and would
load share between the two for traffic to Network N8.
Destination RT3 adv. RT4 adv.
_________________________________
Ia,Ib 15 22
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 19 26
_________________________________
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.
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RFC 1583 OSPF Version 2 March 1994
Router RT1 can also determine in this manner the shortest path
to the AS boundary routers RT5 and RT7. Then, by looking at RT5
and RT7's external advertisements, Router RT1 can decide between
RT5 or RT7 when sending to a destination in another Autonomous
System (one of the networks N12-N15).
Note that a failure of the line between Routers RT6 and RT10
will cause the backbone to become disconnected. Configuring a
virtual link between Routers RT7 and RT10 will give the backbone
more connectivity and more resistance to such failures. Also, a
virtual link between RT7 and RT10 would allow a much shorter
path between the third area (containing N9) and the router RT7,
which is advertising a good route to external network N12.
3.5. IP subnetting support
OSPF attaches an IP address mask to each advertised route. The
mask indicates the range of addresses being described by the
particular route. For example, a summary advertisement for the
destination 128.185.0.0 with a mask of 0xffff0000 actually is
describing a single route to the collection of destinations
128.185.0.0 - 128.185.255.255. Similarly, host routes are
always advertised with a mask of 0xffffffff, indicating the
presence of only a single destination.
Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-
length subnetting. This means that a single IP class A, B, or C
network number can be broken up into many subnets of various
sizes. For example, the network 128.185.0.0 could be broken up
into 62 variable-sized subnets: 15 subnets of size 4K, 15
subnets of size 256, and 32 subnets of size 8. Table 7 shows
some of the resulting network addresses together with their
masks:
Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 0xfffffff8 8
Table 7: Some sample subnet sizes.
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There are many possible ways of dividing up a class A, B, and C
network into variable sized subnets. The precise procedure for
doing so is beyond the scope of this specification. This
specification however establishes the following guideline: When
an IP packet is forwarded, it is always forwarded to the network
that is the best match for the packet's destination. Here best
match is synonymous with the longest or most specific match.
For example, the default route with destination of 0.0.0.0 and
mask 0x00000000 is always a match for every IP destination. Yet
it is always less specific than any other match. Subnet masks
must be assigned so that the best match for any IP destination
is unambiguous.
The OSPF area concept is modelled after an IP subnetted network.
OSPF areas have been loosely defined to be a collection of
networks. In actuality, an OSPF area is specified to be a list
of address ranges (see Section C.2 for more details). Each
address range is defined as an [address,mask] pair. Many
separate networks may then be contained in a single address
range, just as a subnetted network is composed of many separate
subnets. Area border routers then summarize the area contents
(for distribution to the backbone) by advertising a single route
for each address range. The cost of the route is the minimum
cost to any of the networks falling in the specified range.
For example, an IP subnetted network can be configured as a
single OSPF area. In that case, the area would be defined as a
single address range: a class A, B, or C network number along
with its natural IP mask. Inside the area, any number of
variable sized subnets could be defined. External to the area,
a single route for the entire subnetted network would be
distributed, hiding even the fact that the network is subnetted
at all. The cost of this route is the minimum of the set of
costs to the component subnets.
3.6. Supporting stub areas
In some Autonomous Systems, the majority of the topological
database may consist of AS external advertisements. An OSPF AS
external advertisement is usually flooded throughout the entire
AS. However, OSPF allows certain areas to be configured as
"stub areas". AS external advertisements are not flooded
into/throughout stub areas; routing to AS external destinations
in these areas is based on a (per-area) default only. This
reduces the topological database size, and therefore the memory
requirements, for a stub area's internal routers.
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In order to take advantage of the OSPF stub area support,
default routing must be used in the stub area. This is
accomplished as follows. One or more of the stub area's area
border routers must advertise a default route into the stub area
via summary link advertisements. These summary defaults are
flooded throughout the stub area, but no further. (For this
reason these defaults pertain only to the particular stub area).
These summary default routes will match any destination that is
not explicitly reachable by an intra-area or inter-area path
(i.e., AS external destinations).
An area can be configured as stub when there is a single exit
point from the area, or when the choice of exit point need not
be made on a per-external-destination basis. For example, Area
3 in Figure 6 could be configured as a stub area, because all
external traffic must travel though its single area border
router RT11. If Area 3 were configured as a stub, Router RT11
would advertise a default route for distribution inside Area 3
(in a summary link advertisement), instead of flooding the AS
external advertisements for Networks N12-N15 into/throughout the
area.
The OSPF protocol ensures that all routers belonging to an area
agree on whether the area has been configured as a stub. This
guarantees that no confusion will arise in the flooding of AS
external advertisements.
There are a couple of restrictions on the use of stub areas.
Virtual links cannot be configured through stub areas. In
addition, AS boundary routers cannot be placed internal to stub
areas.
3.7. Partitions of areas
OSPF does not actively attempt to repair area partitions. When
an area becomes partitioned, each component simply becomes a
separate area. The backbone then performs routing between the
new areas. Some destinations reachable via intra-area routing
before the partition will now require inter-area routing.
In the previous section, an area was described as a list of
address ranges. Any particular address range must still be
completely contained in a single component of the area
partition. This has to do with the way the area contents are
summarized to the backbone. Also, the backbone itself must not
partition. If it does, parts of the Autonomous System will
become unreachable. Backbone partitions can be repaired by
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configuring virtual links (see Section 15).
Another way to think about area partitions is to look at the
Autonomous System graph that was introduced in Section 2. Area
IDs can be viewed as colors for the graph's edges.[1] Each edge
of the graph connects to a network, or is itself a point-to-
point network. In either case, the edge is colored with the
network's Area ID.
A group of edges, all having the same color, and interconnected
by vertices, represents an area. If the topology of the
Autonomous System is intact, the graph will have several regions
of color, each color being a distinct Area ID.
When the AS topology changes, one of the areas may become
partitioned. The graph of the AS will then have multiple
regions of the same color (Area ID). The routing in the
Autonomous System will continue to function as long as these
regions of same color are connected by the single backbone
region.
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4. Functional Summary
A separate copy of OSPF's basic routing algorithm runs in each area.
Routers having interfaces to multiple areas run multiple copies of
the algorithm. A brief summary of the routing algorithm follows.
When a router starts, it first initializes the routing protocol data
structures. The router then waits for indications from the lower-
level protocols that its interfaces are functional.
A router then uses the OSPF's Hello Protocol to acquire neighbors.
The router sends Hello packets to its neighbors, and in turn
receives their Hello packets. On broadcast and point-to-point
networks, the router dynamically detects its neighboring routers by
sending its Hello packets to the multicast address AllSPFRouters.
On non-broadcast networks, some configuration information is
necessary in order to discover neighbors. On all multi-access
networks (broadcast or non-broadcast), the Hello Protocol also
elects a Designated router for the network.
The router will attempt to form adjacencies with some of its newly
acquired neighbors. Topological databases are synchronized between
pairs of adjacent routers. On multi-access networks, the Designated
Router determines which routers should become adjacent.
Adjacencies control the distribution of routing protocol packets.
Routing protocol packets are sent and received only on adjacencies.
In particular, distribution of topological database updates proceeds
along adjacencies.
A router periodically advertises its state, which is also called
link state. Link state is also advertised when a router's state
changes. A router's adjacencies are reflected in the contents of
its link state advertisements. This relationship between
adjacencies and link state allows the protocol to detect dead
routers in a timely fashion.
Link state advertisements are flooded throughout the area. The
flooding algorithm is reliable, ensuring that all routers in an area
have exactly the same topological database. This database consists
of the collection of link state advertisements received from each
router belonging to the area. From this database each router
calculates a shortest-path tree, with itself as root. This
shortest-path tree in turn yields a routing table for the protocol.
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4.1. Inter-area routing
The previous section described the operation of the protocol
within a single area. For intra-area routing, no other routing
information is pertinent. In order to be able to route to
destinations outside of the area, the area border routers inject
additional routing information into the area. This additional
information is a distillation of the rest of the Autonomous
System's topology.
This distillation is accomplished as follows: Each area border
router is by definition connected to the backbone. Each area
border router summarizes the topology of its attached areas for
transmission on the backbone, and hence to all other area border
routers. An area border router then has complete topological
information concerning the backbone, and the area summaries from
each of the other area border routers. From this information,
the router calculates paths to all destinations not contained in
its attached areas. The router then advertises these paths into
its attached areas. This enables the area's internal routers to
pick the best exit router when forwarding traffic to
destinations in other areas.
4.2. AS external routes
Routers that have information regarding other Autonomous Systems
can flood this information throughout the AS. This external
routing information is distributed verbatim to every
participating router. There is one exception: external routing
information is not flooded into "stub" areas (see Section 3.6).
To utilize external routing information, the path to all routers
advertising external information must be known throughout the AS
(excepting the stub areas). For that reason, the locations of
these AS boundary routers are summarized by the (non-stub) area
border routers.
4.3. Routing protocol packets
The OSPF protocol runs directly over IP, using IP protocol 89.
OSPF does not provide any explicit fragmentation/reassembly
support. When fragmentation is necessary, IP
fragmentation/reassembly is used. OSPF protocol packets have
been designed so that large protocol packets can generally be
split into several smaller protocol packets. This practice is
recommended; IP fragmentation should be avoided whenever
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possible.
Routing protocol packets should always be sent with the IP TOS
field set to 0. If at all possible, routing protocol packets
should be given preference over regular IP data traffic, both
when being sent and received. As an aid to accomplishing this,
OSPF protocol packets should have their IP precedence field set
to the value Internetwork Control (see [RFC 791]).
All OSPF protocol packets share a common protocol header that is
described in Appendix A. The OSPF packet types are listed below
in Table 8. Their formats are also described in Appendix A.
Type Packet name Protocol function
__________________________________________________________
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment
Table 8: OSPF packet types.
OSPF's Hello protocol uses Hello packets to discover and
maintain neighbor relationships. The Database Description and
Link State Request packets are used in the forming of
adjacencies. OSPF's reliable update mechanism is implemented by
the Link State Update and Link State Acknowledgment packets.
Each Link State Update packet carries a set of new link state
advertisements one hop further away from their point of
origination. A single Link State Update packet may contain the
link state advertisements of several routers. Each
advertisement is tagged with the ID of the originating router
and a checksum of its link state contents. The five different
types of OSPF link state advertisements are listed below in
Table 9.
As mentioned above, OSPF routing packets (with the exception of
Hellos) are sent only over adjacencies. Note that this means
that all OSPF protocol packets travel a single IP hop, except
those that are sent over virtual adjacencies. The IP source
address of an OSPF protocol packet is one end of a router
adjacency, and the IP destination address is either the other
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LS Advertisement Advertisement description
type name
_________________________________________________________
1 Router links Originated by all routers.
advertisements This advertisement describes
the collected states of the
router's interfaces to an
area. Flooded throughout a
single area only.
_________________________________________________________
2 Network links Originated for multi-access
advertisements networks by the Designated
Router. This advertisement
contains the list of routers
connected to the network.
Flooded throughout a single
area only.
_________________________________________________________
3,4 Summary link Originated by area border
advertisements routers, and flooded through-
out the advertisement's
associated area. Each summary
link advertisement describes
a route to a destination out-
side the area, yet still inside
the AS (i.e., an inter-area
route). Type 3 advertisements
describe routes to networks.
Type 4 advertisements describe
routes to AS boundary routers.
_________________________________________________________
5 AS external link Originated by AS boundary
advertisements routers, and flooded through-
out the AS. Each AS external
link advertisement describes
a route to a destination in
another Autonomous System.
Default routes for the AS can
also be described by AS
external link advertisements.
Table 9: OSPF link state advertisements.
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end of the adjacency or an IP multicast address.
4.4. Basic implementation requirements
An implementation of OSPF requires the following pieces of
system support:
Timers
Two different kind of timers are required. The first kind,
called single shot timers, fire once and cause a protocol
event to be processed. The second kind, called interval
timers, fire at continuous intervals. These are used for
the sending of packets at regular intervals. A good example
of this is the regular broadcast of Hello packets (on
broadcast networks). The granularity of both kinds of
timers is one second.
Interval timers should be implemented to avoid drift. In
some router implementations, packet processing can affect
timer execution. When multiple routers are attached to a
single network, all doing broadcasts, this can lead to the
synchronization of routing packets (which should be
avoided). If timers cannot be implemented to avoid drift,
small random amounts should be added to/subtracted from the
timer interval at each firing.
IP multicast
Certain OSPF packets take the form of IP multicast
datagrams. Support for receiving and sending IP multicast
datagrams, along with the appropriate lower-level protocol
support, is required. The IP multicast datagrams used by
OSPF never travel more than one hop. For this reason, the
ability to forward IP multicast datagrams is not required.
For information on IP multicast, see [RFC 1112].
Variable-length subnet support
The router's IP protocol support must include the ability to
divide a single IP class A, B, or C network number into many
subnets of various sizes. This is commonly called
variable-length subnetting; see Section 3.5 for details.
IP supernetting support
The router's IP protocol support must include the ability to
aggregate contiguous collections of IP class A, B, and C
networks into larger quantities called supernets.
Supernetting has been proposed as one way to improve the
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scaling of IP routing in the worldwide Internet. For more
information on IP supernetting, see [RFC 1519].
Lower-level protocol support
The lower level protocols referred to here are the network
access protocols, such as the Ethernet data link layer.
Indications must be passed from these protocols to OSPF as
the network interface goes up and down. For example, on an
ethernet it would be valuable to know when the ethernet
transceiver cable becomes unplugged.
Non-broadcast lower-level protocol support
Remember that non-broadcast networks are multi-access
networks such as a X.25 PDN. On these networks, the Hello
Protocol can be aided by providing an indication to OSPF
when an attempt is made to send a packet to a dead or non-
existent router. For example, on an X.25 PDN a dead
neighboring router may be indicated by the reception of a
X.25 clear with an appropriate cause and diagnostic, and
this information would be passed to OSPF.
List manipulation primitives
Much of the OSPF functionality is described in terms of its
operation on lists of link state advertisements. For
example, the collection of advertisements that will be
retransmitted to an adjacent router until acknowledged are
described as a list. Any particular advertisement may be on
many such lists. An OSPF implementation needs to be able to
manipulate these lists, adding and deleting constituent
advertisements as necessary.
Tasking support
Certain procedures described in this specification invoke
other procedures. At times, these other procedures should
be executed in-line, that is, before the current procedure
is finished. This is indicated in the text by instructions
to execute a procedure. At other times, the other
procedures are to be executed only when the current
procedure has finished. This is indicated by instructions
to schedule a task.
4.5. Optional OSPF capabilities
The OSPF protocol defines several optional capabilities. A
router indicates the optional capabilities that it supports in
its OSPF Hello packets, Database Description packets and in its
link state advertisements. This enables routers supporting a
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mix of optional capabilities to coexist in a single Autonomous
System.
Some capabilities must be supported by all routers attached to a
specific area. In this case, a router will not accept a
neighbor's Hello Packet unless there is a match in reported
capabilities (i.e., a capability mismatch prevents a neighbor
relationship from forming). An example of this is the
ExternalRoutingCapability (see below).
Other capabilities can be negotiated during the Database
Exchange process. This is accomplished by specifying the
optional capabilities in Database Description packets. A
capability mismatch with a neighbor in this case will result in
only a subset of link state advertisements being exchanged
between the two neighbors.
The routing table build process can also be affected by the
presence/absence of optional capabilities. For example, since
the optional capabilities are reported in link state
advertisements, routers incapable of certain functions can be
avoided when building the shortest path tree. An example of
this is the TOS routing capability (see below).
The current OSPF optional capabilities are listed below. See
Section A.2 for more information.
ExternalRoutingCapability
Entire OSPF areas can be configured as "stubs" (see Section
3.6). AS external advertisements will not be flooded into
stub areas. This capability is represented by the E-bit in
the OSPF options field (see Section A.2). In order to
ensure consistent configuration of stub areas, all routers
interfacing to such an area must have the E-bit clear in
their Hello packets (see Sections 9.5 and 10.5).
TOS capability
All OSPF implementations must be able to calculate separate
routes based on IP Type of Service. However, to save
routing table space and processing resources, an OSPF router
can be configured to ignore TOS when forwarding packets. In
this case, the router calculates routes for TOS 0 only.
This capability is represented by the T-bit in the OSPF
options field (see Section A.2). TOS-capable routers will
attempt to avoid non-TOS-capable routers when calculating
non-zero TOS paths.
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5. Protocol Data Structures
The OSPF protocol is described in this specification in terms of its
operation on various protocol data structures. The following list
comprises the top-level OSPF data structures. Any initialization
that needs to be done is noted. OSPF areas, interfaces and
neighbors also have associated data structures that are described
later in this specification.
Router ID
A 32-bit number that uniquely identifies this router in the AS.
One possible implementation strategy would be to use the
smallest IP interface address belonging to the router. If a
router's OSPF Router ID is changed, the router's OSPF software
should be restarted before the new Router ID takes effect.
Before restarting in order to change its Router ID, the router
should flush its self-originated link state advertisements from
the routing domain (see Section 14.1), or they will persist for
up to MaxAge minutes.
Area structures
Each one of the areas to which the router is connected has its
own data structure. This data structure describes the working
of the basic algorithm. Remember that each area runs a separate
copy of the basic algorithm.
Backbone (area) structure
The basic algorithm operates on the backbone as if it were an
area. For this reason the backbone is represented as an area
structure.
Virtual links configured
The virtual links configured with this router as one endpoint.
In order to have configured virtual links, the router itself
must be an area border router. Virtual links are identified by
the Router ID of the other endpoint -- which is another area
border router. These two endpoint routers must be attached to a
common area, called the virtual link's Transit area. Virtual
links are part of the backbone, and behave as if they were
unnumbered point-to-point networks between the two routers. A
virtual link uses the intra-area routing of its Transit area to
forward packets. Virtual links are brought up and down through
the building of the shortest-path trees for the Transit area.
List of external routes
These are routes to destinations external to the Autonomous
System, that have been gained either through direct experience
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with another routing protocol (such as EGP), or through
configuration information, or through a combination of the two
(e.g., dynamic external information to be advertised by OSPF
with configured metric). Any router having these external routes
is called an AS boundary router. These routes are advertised by
the router into the OSPF routing domain via AS external link
advertisements.
List of AS external link advertisements
Part of the topological database. These have originated from
the AS boundary routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary router, some of these AS external link
advertisements have been self-originated.
The routing table
Derived from the topological database. Each destination that
the router can forward to is represented by a cost and a set of
paths. A path is described by its type and next hop. For more
information, see Section 11.
TOS capability
This item indicates whether the router will calculate separate
routes based on TOS. This is a configurable parameter. For
more information, see Sections 4.5 and 16.9.
Figure 9 shows the collection of data structures present in a
typical router. The router pictured is RT10, from the map in Figure
6. Note that Router RT10 has a virtual link configured to Router
RT11, with Area 2 as the link's Transit area. This is indicated by
the dashed line in Figure 9. When the virtual link becomes active,
through the building of the shortest path tree for Area 2, it
becomes an interface to the backbone (see the two backbone
interfaces depicted in Figure 9).
6. The Area Data Structure
The area data structure contains all the information used to run the
basic routing algorithm. Each area maintains its own topological
database. A network belongs to a single area, and a router interface
connects to a single area. Each router adjacency also belongs to a
single area.
The OSPF backbone has all the properties of an area. For that
reason it is also represented by an area data structure. Note that
some items in the structure apply differently to the backbone than
to non-backbone areas.
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+----+
|RT10|------+
+----+ \+-------------+
/ \ |Routing Table|
/ \ +-------------+
/ \
+------+ / \ +--------+
|Area 2|---+ +---|Backbone|
+------+***********+ +--------+
/ \ * / \
/ \ * / \
+---------+ +---------+ +------------+ +------------+
|Interface| |Interface| |Virtual Link| |Interface Ib|
| to N6 | | to N8 | | to RT11 | +------------+
+---------+ +---------+ +------------+ |
/ \ | | |
/ \ | | |
+--------+ +--------+ | +-------------+ +------------+
|Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6|
| RT8 | | RT7 | | +-------------+ +------------+
+--------+ +--------+ |
|
+-------------+
|Neighbor RT11|
+-------------+
Figure 9: Router RT10's Data structures
The area topological (or link state) database consists of the
collection of router links, network links and summary link
advertisements that have originated from the area's routers. This
information is flooded throughout a single area only. The list of
AS external link advertisements (see Section 5) is also considered
to be part of each area's topological database.
Area ID
A 32-bit number identifying the area. 0.0.0.0 is reserved for
the Area ID of the backbone. If assigning subnetted networks as
separate areas, the IP network number could be used as the Area
ID.
List of component address ranges
The address ranges that define the area. Each address range is
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specified by an [address,mask] pair and a status indication of
either Advertise or DoNotAdvertise (see Section 12.4.3). Each
network is then assigned to an area depending on the address
range that it falls into (specified address ranges are not
allowed to overlap). As an example, if an IP subnetted network
is to be its own separate OSPF area, the area is defined to
consist of a single address range - an IP network number with
its natural (class A, B or C) mask.
Associated router interfaces
This router's interfaces connecting to the area. A router
interface belongs to one and only one area (or the backbone).
For the backbone structure this list includes all the virtual
links. A virtual link is identified by the Router ID of its
other endpoint; its cost is the cost of the shortest intra-area
path through the Transit area that exists between the two
routers.
List of router links advertisements
A router links advertisement is generated by each router in the
area. It describes the state of the router's interfaces to the
area.
List of network links advertisements
One network links advertisement is generated for each transit
multi-access network in the area. A network links advertisement
describes the set of routers currently connected to the network.
List of summary link advertisements
Summary link advertisements originate from the area's area
border routers. They describe routes to destinations internal
to the Autonomous System, yet external to the area.
Shortest-path tree
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router links and network links
advertisements by the Dijkstra algorithm (see Section 16.1).
AuType
The type of authentication used for this area. Authentication
types are defined in Appendix D. All OSPF packet exchanges are
authenticated. Different authentication schemes may be used in
different areas.
TransitCapability
Set to TRUE if and only if there are one or more active virtual
links using the area as a Transit area. Equivalently, this
parameter indicates whether the area can carry data traffic that
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neither originates nor terminates in the area itself. This
parameter is calculated when the area's shortest-path tree is
built (see Section 16.1, and is used as an input to a subsequent
step of the routing table build process (see Section 16.3).
ExternalRoutingCapability
Whether AS external advertisements will be flooded
into/throughout the area. This is a configurable parameter. If
AS external advertisements are excluded from the area, the area
is called a "stub". Internal to stub areas, routing to AS
external destinations will be based solely on a default summary
route. The backbone cannot be configured as a stub area. Also,
virtual links cannot be configured through stub areas. For more
information, see Section 3.6.
StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost
indicates the cost of the default summary link that the router
should advertise into the area. There can be a separate cost
configured for each IP TOS. See Section 12.4.3 for more
information.
Unless otherwise specified, the remaining sections of this document
refer to the operation of the protocol in a single area.
7. Bringing Up Adjacencies
OSPF creates adjacencies between neighboring routers for the purpose
of exchanging routing information. Not every two neighboring
routers will become adjacent. This section covers the generalities
involved in creating adjacencies. For further details consult
Section 10.
7.1. The Hello Protocol
The Hello Protocol is responsible for establishing and
maintaining neighbor relationships. It also ensures that
communication between neighbors is bidirectional. Hello packets
are sent periodically out all router interfaces. Bidirectional
communication is indicated when the router sees itself listed in
the neighbor's Hello Packet.
On multi-access networks, the Hello Protocol elects a Designated
Router for the network. Among other things, the Designated
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Router controls what adjacencies will be formed over the network
(see below).
The Hello Protocol works differently on broadcast networks, as
compared to non-broadcast networks. On broadcast networks, each
router advertises itself by periodically multicasting Hello
Packets. This allows neighbors to be discovered dynamically.
These Hello Packets contain the router's view of the Designated
Router's identity, and the list of routers whose Hello Packets
have been seen recently.
On non-broadcast networks some configuration information is
necessary for the operation of the Hello Protocol. Each router
that may potentially become Designated Router has a list of all
other routers attached to the network. A router, having
Designated Router potential, sends Hello Packets to all other
potential Designated Routers when its interface to the non-
broadcast network first becomes operational. This is an attempt
to find the Designated Router for the network. If the router
itself is elected Designated Router, it begins sending Hello
Packets to all other routers attached to the network.
After a neighbor has been discovered, bidirectional
communication ensured, and (if on a multi-access network) a
Designated Router elected, a decision is made regarding whether
or not an adjacency should be formed with the neighbor (see
Section 10.4). An attempt is always made to establish
adjacencies over point-to-point networks and virtual links. The
first step in bringing up an adjacency is to synchronize the
neighbors' topological databases. This is covered in the next
section.
7.2. The Synchronization of Databases
In a link-state routing algorithm, it is very important for all
routers' topological databases to stay synchronized. OSPF
simplifies this by requiring only adjacent routers to remain
synchronized. The synchronization process begins as soon as the
routers attempt to bring up the adjacency. Each router
describes its database by sending a sequence of Database
Description packets to its neighbor. Each Database Description
Packet describes a set of link state advertisements belonging to
the router's database. When the neighbor sees a link state
advertisement that is more recent than its own database copy, it
makes a note that this newer advertisement should be requested.
This sending and receiving of Database Description packets is
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called the "Database Exchange Process". During this process,
the two routers form a master/slave relationship. Each Database
Description Packet has a sequence number. Database Description
Packets sent by the master (polls) are acknowledged by the slave
through echoing of the sequence number. Both polls and their
responses contain summaries of link state data. The master is
the only one allowed to retransmit Database Description Packets.
It does so only at fixed intervals, the length of which is the
configured constant RxmtInterval.
Each Database Description contains an indication that there are
more packets to follow --- the M-bit. The Database Exchange
Process is over when a router has received and sent Database
Description Packets with the M-bit off.
During and after the Database Exchange Process, each router has
a list of those link state advertisements for which the neighbor
has more up-to-date instances. These advertisements are
requested in Link State Request Packets. Link State Request
packets that are not satisfied are retransmitted at fixed
intervals of time RxmtInterval. When the Database Description
Process has completed and all Link State Requests have been
satisfied, the databases are deemed synchronized and the routers
are marked fully adjacent. At this time the adjacency is fully
functional and is advertised in the two routers' link state
advertisements.
The adjacency is used by the flooding procedure as soon as the
Database Exchange Process begins. This simplifies database
synchronization, and guarantees that it finishes in a
predictable period of time.
7.3. The Designated Router
Every multi-access network has a Designated Router. The
Designated Router performs two main functions for the routing
protocol:
o The Designated Router originates a network links
advertisement on behalf of the network. This advertisement
lists the set of routers (including the Designated Router
itself) currently attached to the network. The Link State
ID for this advertisement (see Section 12.1.4) is the IP
interface address of the Designated Router. The IP network
number can then be obtained by using the subnet/network
mask.
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o The Designated Router becomes adjacent to all other routers
on the network. Since the link state databases are
synchronized across adjacencies (through adjacency bring-up
and then the flooding procedure), the Designated Router
plays a central part in the synchronization process.
The Designated Router is elected by the Hello Protocol. A
router's Hello Packet contains its Router Priority, which is
configurable on a per-interface basis. In general, when a
router's interface to a network first becomes functional, it
checks to see whether there is currently a Designated Router for
the network. If there is, it accepts that Designated Router,
regardless of its Router Priority. (This makes it harder to
predict the identity of the Designated Router, but ensures that
the Designated Router changes less often. See below.)
Otherwise, the router itself becomes Designated Router if it has
the highest Router Priority on the network. A more detailed
(and more accurate) description of Designated Router election is
presented in Section 9.4.
The Designated Router is the endpoint of many adjacencies. In
order to optimize the flooding procedure on broadcast networks,
the Designated Router multicasts its Link State Update Packets
to the address AllSPFRouters, rather than sending separate
packets over each adjacency.
Section 2 of this document discusses the directed graph
representation of an area. Router nodes are labelled with their
Router ID. Multi-access network nodes are actually labelled
with the IP address of their Designated Router. It follows that
when the Designated Router changes, it appears as if the network
node on the graph is replaced by an entirely new node. This
will cause the network and all its attached routers to originate
new link state advertisements. Until the topological databases
again converge, some temporary loss of connectivity may result.
This may result in ICMP unreachable messages being sent in
response to data traffic. For that reason, the Designated
Router should change only infrequently. Router Priorities
should be configured so that the most dependable router on a
network eventually becomes Designated Router.
7.4. The Backup Designated Router
In order to make the transition to a new Designated Router
smoother, there is a Backup Designated Router for each multi-
access network. The Backup Designated Router is also adjacent
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to all routers on the network, and becomes Designated Router
when the previous Designated Router fails. If there were no
Backup Designated Router, when a new Designated Router became
necessary, new adjacencies would have to be formed between the
new Designated Router and all other routers attached to the
network. Part of the adjacency forming process is the
synchronizing of topological databases, which can potentially
take quite a long time. During this time, the network would not
be available for transit data traffic. The Backup Designated
obviates the need to form these adjacencies, since they already
exist. This means the period of disruption in transit traffic
lasts only as long as it takes to flood the new link state
advertisements (which announce the new Designated Router).
The Backup Designated Router does not generate a network links
advertisement for the network. (If it did, the transition to a
new Designated Router would be even faster. However, this is a
tradeoff between database size and speed of convergence when the
Designated Router disappears.)
The Backup Designated Router is also elected by the Hello
Protocol. Each Hello Packet has a field that specifies the
Backup Designated Router for the network.
In some steps of the flooding procedure, the Backup Designated
Router plays a passive role, letting the Designated Router do
more of the work. This cuts down on the amount of local routing
traffic. See Section 13.3 for more information.
7.5. The graph of adjacencies
An adjacency is bound to the network that the two routers have
in common. If two routers have multiple networks in common,
they may have multiple adjacencies between them.
One can picture the collection of adjacencies on a network as
forming an undirected graph. The vertices consist of routers,
with an edge joining two routers if they are adjacent. The
graph of adjacencies describes the flow of routing protocol
packets, and in particular Link State Update Packets, through
the Autonomous System.
Two graphs are possible, depending on whether the common network
is multi-access. On physical point-to-point networks (and
virtual links), the two routers joined by the network will be
adjacent after their databases have been synchronized. On
multi-access networks, both the Designated Router and the Backup
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Designated Router are adjacent to all other routers attached to
the network, and these account for all adjacencies.
These graphs are shown in Figure 10. It is assumed that Router
RT7 has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding procedure
than the Designated Router (see Section 13.3). This is the
reason for the dashed lines connecting the Backup Designated
Router RT3.
8. Protocol Packet Processing
This section discusses the general processing of OSPF routing
protocol packets. It is very important that the router topological
databases remain synchronized. For this reason, routing protocol
packets should get preferential treatment over ordinary data
packets, both in sending and receiving.
Routing protocol packets are sent along adjacencies only (with the
+---+ +---+
|RT1|------------|RT2| o---------------o
+---+ N1 +---+ RT1 RT2
RT7
o---------+
+---+ +---+ +---+ /|\ |
|RT7| |RT3| |RT4| / | \ |
+---+ +---+ +---+ / | \ |
| | | / | \ |
+-----------------------+ RT5o RT6o oRT4 |
| | N2 * * * |
+---+ +---+ * * * |
|RT5| |RT6| * * * |
+---+ +---+ *** |
o---------+
RT3
Figure 10: The graph of adjacencies
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exception of Hello packets, which are used to discover the
adjacencies). This means that all routing protocol packets travel a
single IP hop, except those sent over virtual links.
All routing protocol packets begin with a standard header. The
sections below give the details on how to fill in and verify this
standard header. Then, for each packet type, the section is listed
that gives more details on that particular packet type's processing.
8.1. Sending protocol packets
When a router sends a routing protocol packet, it fills in the
fields of the standard OSPF packet header as follows. For more
details on the header format consult Section A.3.1:
Version #
Set to 2, the version number of the protocol as documented
in this specification.
Packet type
The type of OSPF packet, such as Link state Update or Hello
Packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the
packet).
Area ID
The OSPF area that the packet is being sent into.
Checksum
The standard IP 16-bit one's complement checksum of the
entire OSPF packet, excluding the 64-bit authentication
field. This checksum should be calculated before handing
the packet to the appropriate authentication procedure.
AuType and Authentication
Each OSPF packet exchange is authenticated. Authentication
types are assigned by the protocol and documented in
Appendix D. A different authentication scheme can be used
for each OSPF area. The 64-bit authentication field is set
by the appropriate authentication procedure (determined by
AuType). This procedure should be the last called when
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forming the packet to be sent. The setting of the
authentication field is determined by the packet contents
and the authentication key (which is configurable on a per-
interface basis).
The IP destination address for the packet is selected as
follows. On physical point-to-point networks, the IP
destination is always set to the address AllSPFRouters. On all
other network types (including virtual links), the majority of
OSPF packets are sent as unicasts, i.e., sent directly to the
other end of the adjacency. In this case, the IP destination is
just the Neighbor IP address associated with the other end of
the adjacency (see Section 10). The only packets not sent as
unicasts are on broadcast networks; on these networks Hello
packets are sent to the multicast destination AllSPFRouters, the
Designated Router and its Backup send both Link State Update
Packets and Link State Acknowledgment Packets to the multicast
address AllSPFRouters, while all other routers send both their
Link State Update and Link State Acknowledgment Packets to the
multicast address AllDRouters.
Retransmissions of Link State Update packets are ALWAYS sent as
unicasts.
The IP source address should be set to the IP address of the
sending interface. Interfaces to unnumbered point-to-point
networks have no associated IP address. On these interfaces,
the IP source should be set to any of the other IP addresses
belonging to the router. For this reason, there must be at
least one IP address assigned to the router.[2] Note that, for
most purposes, virtual links act precisely the same as
unnumbered point-to-point networks. However, each virtual link
does have an IP interface address (discovered during the routing
table build process) which is used as the IP source when sending
packets over the virtual link.
For more information on the format of specific OSPF packet
types, consult the sections listed in Table 10.
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Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing OSPF protocol packet transmission.
8.2. Receiving protocol packets
Whenever a protocol packet is received by the router it is
marked with the interface it was received on. For routers that
have virtual links configured, it may not be immediately obvious
which interface to associate the packet with. For example,
consider the Router RT11 depicted in Figure 6. If RT11 receives
an OSPF protocol packet on its interface to Network N8, it may
want to associate the packet with the interface to Area 2, or
with the virtual link to Router RT10 (which is part of the
backbone). In the following, we assume that the packet is
initially associated with the non-virtual link.[3]
In order for the packet to be accepted at the IP level, it must
pass a number of tests, even before the packet is passed to OSPF
for processing:
o The IP checksum must be correct.
o The packet's IP destination address must be the IP address
of the receiving interface, or one of the IP multicast
addresses AllSPFRouters or AllDRouters.
o The IP protocol specified must be OSPF (89).
o Locally originated packets should not be passed on to OSPF.
That is, the source IP address should be examined to make
sure this is not a multicast packet that the router itself
generated.
Next, the OSPF packet header is verified. The fields specified
in the header must match those configured for the receiving
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interface. If they do not, the packet should be discarded:
o The version number field must specify protocol version 2.
o The 16-bit one's complement checksum of the OSPF packet's
contents must be verified. Remember that the 64-bit
authentication field must be excluded from the checksum
calculation.
o The Area ID found in the OSPF header must be verified. If
both of the following cases fail, the packet should be
discarded. The Area ID specified in the header must either:
(1) Match the Area ID of the receiving interface. In this
case, the packet has been sent over a single hop.
Therefore, the packet's IP source address must be on the
same network as the receiving interface. This can be
determined by comparing the packet's IP source address
to the interface's IP address, after masking both
addresses with the interface mask. This comparison
should not be performed on point-to-point networks. On
point-to-point networks, the interface addresses of each
end of the link are assigned independently, if they are
assigned at all.
(2) Indicate the backbone. In this case, the packet has
been sent over a virtual link. The receiving router
must be an area border router, and the Router ID
specified in the packet (the source router) must be the
other end of a configured virtual link. The receiving
interface must also attach to the virtual link's
configured Transit area. If all of these checks
succeed, the packet is accepted and is from now on
associated with the virtual link (and the backbone
area).
o Packets whose IP destination is AllDRouters should only be
accepted if the state of the receiving interface is DR or
Backup (see Section 9.1).
o The AuType specified in the packet must match the AuType
specified for the associated area.
Next, the packet must be authenticated. This depends on the
AuType specified (see Appendix D). The authentication procedure
may use an Authentication key, which can be configured on a
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per-interface basis. If the authentication fails, the packet
should be discarded.
If the packet type is Hello, it should then be further processed
by the Hello Protocol (see Section 10.5). All other packet
types are sent/received only on adjacencies. This means that
the packet must have been sent by one of the router's active
neighbors. If the receiving interface is a multi-access network
(either broadcast or non-broadcast) the sender is identified by
the IP source address found in the packet's IP header. If the
receiving interface is a point-to-point link or a virtual link,
the sender is identified by the Router ID (source router) found
in the packet's OSPF header. The data structure associated with
the receiving interface contains the list of active neighbors.
Packets not matching any active neighbor are discarded.
At this point all received protocol packets are associated with
an active neighbor. For the further input processing of
specific packet types, consult the sections listed in Table 11.
Type Packet name detailed section (receive)
________________________________________________________
1 Hello Section 10.5
2 Database description Section 10.6
3 Link state request Section 10.7
4 Link state update Section 13
5 Link state ack Section 13.7
Table 11: Sections describing OSPF protocol packet reception.
9. The Interface Data Structure
An OSPF interface is the connection between a router and a network.
There is a single OSPF interface structure for each attached
network; each interface structure has at most one IP interface
address (see below). The support for multiple addresses on a single
network is a matter for future consideration.
An OSPF interface can be considered to belong to the area that
contains the attached network. All routing protocol packets
originated by the router over this interface are labelled with the
interface's Area ID. One or more router adjacencies may develop
over an interface. A router's link state advertisements reflect the
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state of its interfaces and their associated adjacencies.
The following data items are associated with an interface. Note
that a number of these items are actually configuration for the
attached network; those items must be the same for all routers
connected to the network.
Type
The kind of network to which the interface attaches. Its value
is either broadcast, non-broadcast yet still multi-access,
point-to-point or virtual link.
State
The functional level of an interface. State determines whether
or not full adjacencies are allowed to form over the interface.
State is also reflected in the router's link state
advertisements.
IP interface address
The IP address associated with the interface. This appears as
the IP source address in all routing protocol packets originated
over this interface. Interfaces to unnumbered point-to-point
networks do not have an associated IP address.
IP interface mask
Also referred to as the subnet mask, this indicates the portion
of the IP interface address that identifies the attached
network. Masking the IP interface address with the IP interface
mask yields the IP network number of the attached network. On
point-to-point networks and virtual links, the IP interface mask
is not defined. On these networks, the link itself is not
assigned an IP network number, and so the addresses of each side
of the link are assigned independently, if they are assigned at
all.
Area ID
The Area ID of the area to which the attached network belongs.
All routing protocol packets originating from the interface are
labelled with this Area ID.
HelloInterval
The length of time, in seconds, between the Hello packets that
the router sends on the interface. Advertised in Hello packets
sent out this interface.
RouterDeadInterval
The number of seconds before the router's neighbors will declare
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it down, when they stop hearing the router's Hello Packets.
Advertised in Hello packets sent out this interface.
InfTransDelay
The estimated number of seconds it takes to transmit a Link
State Update Packet over this interface. Link state
advertisements contained in the Link State Update packet will
have their age incremented by this amount before transmission.
This value should take into account transmission and propagation
delays; it must be greater than zero.
Router Priority
An 8-bit unsigned integer. When two routers attached to a
network both attempt to become Designated Router, the one with
the highest Router Priority takes precedence. A router whose
Router Priority is set to 0 is ineligible to become Designated
Router on the attached network. Advertised in Hello packets
sent out this interface.
Hello Timer
An interval timer that causes the interface to send a Hello
packet. This timer fires every HelloInterval seconds. Note
that on non-broadcast networks a separate Hello packet is sent
to each qualified neighbor.
Wait Timer
A single shot timer that causes the interface to exit the
Waiting state, and as a consequence select a Designated Router
on the network. The length of the timer is RouterDeadInterval
seconds.
List of neighboring routers
The other routers attached to this network. On multi-access
networks, this list is formed by the Hello Protocol.
Adjacencies will be formed to some of these neighbors. The set
of adjacent neighbors can be determined by an examination of all
of the neighbors' states.
Designated Router
The Designated Router selected for the attached network. The
Designated Router is selected on all multi-access networks by
the Hello Protocol. Two pieces of identification are kept for
the Designated Router: its Router ID and its IP interface
address on the network. The Designated Router advertises link
state for the network; this network link state advertisement is
labelled with the Designated Router's IP address. The
Designated Router is initialized to 0.0.0.0, which indicates the
lack of a Designated Router.
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Backup Designated Router
The Backup Designated Router is also selected on all multi-
access networks by the Hello Protocol. All routers on the
attached network become adjacent to both the Designated Router
and the Backup Designated Router. The Backup Designated Router
becomes Designated Router when the current Designated Router
fails. The Backup Designated Router is initialized to 0.0.0.0,
indicating the lack of a Backup Designated Router.
Interface output cost(s)
The cost of sending a data packet on the interface, expressed in
the link state metric. This is advertised as the link cost for
this interface in the router links advertisement. There may be
a separate cost for each IP Type of Service. The cost of an
interface must be greater than zero.
RxmtInterval
The number of seconds between link state advertisement
retransmissions, for adjacencies belonging to this interface.
Also used when retransmitting Database Description and Link
State Request Packets.
Authentication key
This configured data allows the authentication procedure to
generate and/or verify the Authentication field in the OSPF
header. The Authentication key can be configured on a per-
interface basis. For example, if the AuType indicates simple
password, the Authentication key would be a 64-bit password.
This key would be inserted directly into the OSPF header when
originating routing protocol packets, and there could be a
separate password for each network.
9.1. Interface states
The various states that router interfaces may attain is
documented in this section. The states are listed in order of
progressing functionality. For example, the inoperative state
is listed first, followed by a list of intermediate states
before the final, fully functional state is achieved. The
specification makes use of this ordering by sometimes making
references such as "those interfaces in state greater than X".
Figure 11 shows the graph of interface state changes. The arcs
of the graph are labelled with the event causing the state
change. These events are documented in Section 9.2. The
interface state machine is described in more detail in Section
9.3.
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+----+ UnloopInd +--------+
|Down|<--------------|Loopback|
+----+ +--------+
|
|InterfaceUp
+-------+ | +--------------+
|Waiting|<-+-------------->|Point-to-point|
+-------+ +--------------+
|
WaitTimer|BackupSeen
|
|
| NeighborChange
+------+ +-+<---------------- +-------+
|Backup|<----------|?|----------------->|DROther|
+------+---------->+-+<-----+ +-------+
Neighbor | |
Change | |Neighbor
| |Change
| +--+
+---->|DR|
+--+
Figure 11: Interface State changes
In addition to the state transitions pictured,
Event InterfaceDown always forces Down State, and
Event LoopInd always forces Loopback State
Down
This is the initial interface state. In this state, the
lower-level protocols have indicated that the interface is
unusable. No protocol traffic at all will be sent or
received on such a interface. In this state, interface
parameters should be set to their initial values. All
interface timers should be disabled, and there should be no
adjacencies associated with the interface.
Loopback
In this state, the router's interface to the network is
looped back. The interface may be looped back in hardware
or software. The interface will be unavailable for regular
data traffic. However, it may still be desirable to gain
information on the quality of this interface, either through
sending ICMP pings to the interface or through something
like a bit error test. For this reason, IP packets may
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still be addressed to an interface in Loopback state. To
facilitate this, such interfaces are advertised in router
links advertisements as single host routes, whose
destination is the IP interface address.[4]
Waiting
In this state, the router is trying to determine the
identity of the (Backup) Designated Router for the network.
To do this, the router monitors the Hello Packets it
receives. The router is not allowed to elect a Backup
Designated Router nor a Designated Router until it
transitions out of Waiting state. This prevents unnecessary
changes of (Backup) Designated Router.
Point-to-point
In this state, the interface is operational, and connects
either to a physical point-to-point network or to a virtual
link. Upon entering this state, the router attempts to form
an adjacency with the neighboring router. Hello Packets are
sent to the neighbor every HelloInterval seconds.
DR Other
The interface is to a multi-access network on which another
router has been selected to be the Designated Router. In
this state, the router itself has not been selected Backup
Designated Router either. The router forms adjacencies to
both the Designated Router and the Backup Designated Router
(if they exist).
Backup
In this state, the router itself is the Backup Designated
Router on the attached network. It will be promoted to
Designated Router when the present Designated Router fails.
The router establishes adjacencies to all other routers
attached to the network. The Backup Designated Router
performs slightly different functions during the Flooding
Procedure, as compared to the Designated Router (see Section
13.3). See Section 7.4 for more details on the functions
performed by the Backup Designated Router.
DR In this state, this router itself is the Designated Router
on the attached network. Adjacencies are established to all
other routers attached to the network. The router must also
originate a network links advertisement for the network
node. The advertisement will contain links to all routers
(including the Designated Router itself) attached to the
network. See Section 7.3 for more details on the functions
performed by the Designated Router.
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9.2. Events causing interface state changes
State changes can be effected by a number of events. These
events are pictured as the labelled arcs in Figure 11. The
label definitions are listed below. For a detailed explanation
of the effect of these events on OSPF protocol operation,
consult Section 9.3.
InterfaceUp
Lower-level protocols have indicated that the network
interface is operational. This enables the interface to
transition out of Down state. On virtual links, the
interface operational indication is actually a result of the
shortest path calculation (see Section 16.7).
WaitTimer
The Wait Timer has fired, indicating the end of the waiting
period that is required before electing a (Backup)
Designated Router.
BackupSeen
The router has detected the existence or non-existence of a
Backup Designated Router for the network. This is done in
one of two ways. First, an Hello Packet may be received
from a neighbor claiming to be itself the Backup Designated
Router. Alternatively, an Hello Packet may be received from
a neighbor claiming to be itself the Designated Router, and
indicating that there is no Backup Designated Router. In
either case there must be bidirectional communication with
the neighbor, i.e., the router must also appear in the
neighbor's Hello Packet. This event signals an end to the
Waiting state.
NeighborChange
There has been a change in the set of bidirectional
neighbors associated with the interface. The (Backup)
Designated Router needs to be recalculated. The following
neighbor changes lead to the NeighborChange event. For an
explanation of neighbor states, see Section 10.1.
o Bidirectional communication has been established to a
neighbor. In other words, the state of the neighbor has
transitioned to 2-Way or higher.
o There is no longer bidirectional communication with a
neighbor. In other words, the state of the neighbor has
transitioned to Init or lower.
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o One of the bidirectional neighbors is newly declaring
itself as either Designated Router or Backup Designated
Router. This is detected through examination of that
neighbor's Hello Packets.
o One of the bidirectional neighbors is no longer
declaring itself as Designated Router, or is no longer
declaring itself as Backup Designated Router. This is
again detected through examination of that neighbor's
Hello Packets.
o The advertised Router Priority for a bidirectional
neighbor has changed. This is again detected through
examination of that neighbor's Hello Packets.
LoopInd
An indication has been received that the interface is now
looped back to itself. This indication can be received
either from network management or from the lower level
protocols.
UnloopInd
An indication has been received that the interface is no
longer looped back. As with the LoopInd event, this
indication can be received either from network management or
from the lower level protocols.
InterfaceDown
Lower-level protocols indicate that this interface is no
longer functional. No matter what the current interface
state is, the new interface state will be Down.
9.3. The Interface state machine
A detailed description of the interface state changes follows.
Each state change is invoked by an event (Section 9.2). This
event may produce different effects, depending on the current
state of the interface. For this reason, the state machine
below is organized by current interface state and received
event. Each entry in the state machine describes the resulting
new interface state and the required set of additional actions.
When an interface's state changes, it may be necessary to
originate a new router links advertisement. See Section 12.4
for more details.
Some of the required actions below involve generating events for
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the neighbor state machine. For example, when an interface
becomes inoperative, all neighbor connections associated with
the interface must be destroyed. For more information on the
neighbor state machine, see Section 10.3.
State(s): Down
Event: InterfaceUp
New state: Depends upon action routine
Action: Start the interval Hello Timer, enabling the
periodic sending of Hello packets out the interface.
If the attached network is a physical point-to-point
network or virtual link, the interface state
transitions to Point-to-Point. Else, if the router
is not eligible to become Designated Router the
interface state transitions to DR Other.
Otherwise, the attached network is multi-access and
the router is eligible to become Designated Router.
In this case, in an attempt to discover the attached
network's Designated Router the interface state is
set to Waiting and the single shot Wait Timer is
started. If in addition the attached network is
non-broadcast, examine the configured list of
neighbors for this interface and generate the
neighbor event Start for each neighbor that is also
eligible to become Designated Router.
State(s): Waiting
Event: BackupSeen
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Waiting
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Event: WaitTimer
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): DR Other, Backup or DR
Event: NeighborChange
New state: Depends upon action routine.
Action: Recalculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Any State
Event: InterfaceDown
New state: Down
Action: All interface variables are reset, and interface
timers disabled. Also, all neighbor connections
associated with the interface are destroyed. This
is done by generating the event KillNbr on all
associated neighbors (see Section 10.2).
State(s): Any State
Event: LoopInd
New state: Loopback
Action: Since this interface is no longer connected to the
attached network the actions associated with the
above InterfaceDown event are executed.
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State(s): Loopback
Event: UnloopInd
New state: Down
Action: No actions are necessary. For example, the
interface variables have already been reset upon
entering the Loopback state. Note that reception of
an InterfaceUp event is necessary before the
interface again becomes fully functional.
9.4. Electing the Designated Router
This section describes the algorithm used for calculating a
network's Designated Router and Backup Designated Router. This
algorithm is invoked by the Interface state machine. The
initial time a router runs the election algorithm for a network,
the network's Designated Router and Backup Designated Router are
initialized to 0.0.0.0. This indicates the lack of both a
Designated Router and a Backup Designated Router.
The Designated Router election algorithm proceeds as follows:
Call the router doing the calculation Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router X is examined. This
list is precisely the collection of Router X's neighbors (on
this network) whose state is greater than or equal to 2-Way (see
Section 10.1). Router X itself is also considered to be on the
list. Discard all routers from the list that are ineligible to
become Designated Router. (Routers having Router Priority of 0
are ineligible to become Designated Router.) The following
steps are then executed, considering only those routers that
remain on the list:
(1) Note the current values for the network's Designated Router
and Backup Designated Router. This is used later for
comparison purposes.
(2) Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
declared themselves to be Designated Router are eligible to
become Backup Designated Router. If one or more of these
routers have declared themselves Backup Designated Router
(i.e., they are currently listing themselves as Backup
Designated Router, but not as Designated Router, in their
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Hello Packets) the one having highest Router Priority is
declared to be Backup Designated Router. In case of a tie,
the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router,
choose the router having highest Router Priority, (again
excluding those routers who have declared themselves
Designated Router), and again use the Router ID to break
ties.
(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
themselves Designated Router (i.e., they are currently
listing themselves as Designated Router in their Hello
Packets) the one having highest Router Priority is declared
to be Designated Router. In case of a tie, the one having
the highest Router ID is chosen. If no routers have
declared themselves Designated Router, assign the Designated
Router to be the same as the newly elected Backup Designated
Router.
(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated
Router or no longer the Backup Designated Router, repeat
steps 2 and 3, and then proceed to step 5. For example, if
Router X is now the Designated Router, when step 2 is
repeated X will no longer be eligible for Backup Designated
Router election. Among other things, this will ensure that
no router will declare itself both Backup Designated Router
and Designated Router.[5]
(5) As a result of these calculations, the router itself may now
be Designated Router or Backup Designated Router. See
Sections 7.3 and 7.4 for the additional duties this would
entail. The router's interface state should be set
accordingly. If the router itself is now Designated Router,
the new interface state is DR. If the router itself is now
Backup Designated Router, the new interface state is Backup.
Otherwise, the new interface state is DR Other.
(6) If the attached network is non-broadcast, and the router
itself has just become either Designated Router or Backup
Designated Router, it must start sending Hello Packets to
those neighbors that are not eligible to become Designated
Router (see Section 9.5.1). This is done by invoking the
neighbor event Start for each neighbor having a Router
Priority of 0.
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(7) If the above calculations have caused the identity of either
the Designated Router or Backup Designated Router to change,
the set of adjacencies associated with this interface will
need to be modified. Some adjacencies may need to be
formed, and others may need to be broken. To accomplish
this, invoke the event AdjOK? on all neighbors whose state
is at least 2-Way. This will cause their eligibility for
adjacency to be reexamined (see Sections 10.3 and 10.4).
The reason behind the election algorithm's complexity is the
desire for an orderly transition from Backup Designated Router
to Designated Router, when the current Designated Router fails.
This orderly transition is ensured through the introduction of
hysteresis: no new Backup Designated Router can be chosen until
the old Backup accepts its new Designated Router
responsibilities.
The above procedure may elect the same router to be both
Designated Router and Backup Designated Router, although that
router will never be the calculating router (Router X) itself.
The elected Designated Router may not be the router having the
highest Router Priority, nor will the Backup Designated Router
necessarily have the second highest Router Priority. If Router
X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a
Designated Router will be selected in the above procedure. Note
also that if Router X is the only attached router that is
eligible to become Designated Router, it will select itself as
Designated Router and there will be no Backup Designated Router
for the network.
9.5. Sending Hello packets
Hello packets are sent out each functioning router interface.
They are used to discover and maintain neighbor
relationships.[6] On multi-access networks, Hello Packets are
also used to elect the Designated Router and Backup Designated
Router, and in that way determine what adjacencies should be
formed.
The format of an Hello packet is detailed in Section A.3.2. The
Hello Packet contains the router's Router Priority (used in
choosing the Designated Router), and the interval between Hello
Packets sent out the interface (HelloInterval). The Hello
Packet also indicates how often a neighbor must be heard from to
remain active (RouterDeadInterval). Both HelloInterval and
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RouterDeadInterval must be the same for all routers attached to
a common network. The Hello packet also contains the IP address
mask of the attached network (Network Mask). On unnumbered
point-to-point networks and on virtual links this field should
be set to 0.0.0.0.
The Hello packet's Options field describes the router's optional
OSPF capabilities. There are currently two optional
capabilities defined (see Sections 4.5 and A.2). The T-bit of
the Options field should be set if the router is capable of
calculating separate routes for each IP TOS. The E-bit should
be set if and only if the attached area is capable of processing
AS external advertisements (i.e., it is not a stub area). If
the E-bit is set incorrectly the neighboring routers will refuse
to accept the Hello Packet (see Section 10.5). The rest of the
Hello Packet's Options field should be set to zero.
In order to ensure two-way communication between adjacent
routers, the Hello packet contains the list of all routers from
which Hello Packets have been seen recently. The Hello packet
also contains the router's current choice for Designated Router
and Backup Designated Router. A value of 0.0.0.0 in these
fields means that one has not yet been selected.
On broadcast networks and physical point-to-point networks,
Hello packets are sent every HelloInterval seconds to the IP
multicast address AllSPFRouters. On virtual links, Hello
packets are sent as unicasts (addressed directly to the other
end of the virtual link) every HelloInterval seconds. On non-
broadcast networks, the sending of Hello packets is more
complicated. This will be covered in the next section.
9.5.1. Sending Hello packets on non-broadcast networks
Static configuration information is necessary in order for
the Hello Protocol to function on non-broadcast networks
(see Section C.5). Every attached router which is eligible
to become Designated Router has a configured list of all of
its neighbors on the network. Each listed neighbor is
labelled with its Designated Router eligibility.
The interface state must be at least Waiting for any Hello
Packets to be sent. Hello Packets are then sent directly
(as unicasts) to some subset of a router's neighbors.
Sometimes an Hello Packet is sent periodically on a timer;
at other times it is sent as a response to a received Hello
Packet. A router's hello-sending behavior varies depending
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on whether the router itself is eligible to become
Designated Router.
If the router is eligible to become Designated Router, it
must periodically send Hello Packets to all neighbors that
are also eligible. In addition, if the router is itself the
Designated Router or Backup Designated Router, it must also
send periodic Hello Packets to all other neighbors. This
means that any two eligible routers are always exchanging
Hello Packets, which is necessary for the correct operation
of the Designated Router election algorithm. To minimize
the number of Hello Packets sent, the number of eligible
routers on a non-broadcast network should be kept small.
If the router is not eligible to become Designated Router,
it must periodically send Hello Packets to both the
Designated Router and the Backup Designated Router (if they
exist). It must also send an Hello Packet in reply to an
Hello Packet received from any eligible neighbor (other than
the current Designated Router and Backup Designated Router).
This is needed to establish an initial bidirectional
relationship with any potential Designated Router.
When sending Hello packets periodically to any neighbor, the
interval between Hello Packets is determined by the
neighbor's state. If the neighbor is in state Down, Hello
Packets are sent every PollInterval seconds. Otherwise,
Hello Packets are sent every HelloInterval seconds.
10. The Neighbor Data Structure
An OSPF router converses with its neighboring routers. Each
separate conversation is described by a "neighbor data structure".
Each conversation is bound to a particular OSPF router interface,
and is identified either by the neighboring router's OSPF Router ID
or by its Neighbor IP address (see below). Thus if the OSPF router
and another router have multiple attached networks in common,
multiple conversations ensue, each described by a unique neighbor
data structure. Each separate conversation is loosely referred to
in the text as being a separate "neighbor".
The neighbor data structure contains all information pertinent to
the forming or formed adjacency between the two neighbors.
(However, remember that not all neighbors become adjacent.) An
adjacency can be viewed as a highly developed conversation between
two routers.
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State
The functional level of the neighbor conversation. This is
described in more detail in Section 10.1.
Inactivity Timer
A single shot timer whose firing indicates that no Hello Packet
has been seen from this neighbor recently. The length of the
timer is RouterDeadInterval seconds.
Master/Slave
When the two neighbors are exchanging databases, they form a
master/slave relationship. The master sends the first Database
Description Packet, and is the only part that is allowed to
retransmit. The slave can only respond to the master's Database
Description Packets. The master/slave relationship is
negotiated in state ExStart.
DD Sequence Number
A 32-bit number identifying individual Database Description
packets. When the neighbor state ExStart is entered, the DD
sequence number should be set to a value not previously seen by
the neighboring router. One possible scheme is to use the
machine's time of day counter. The DD sequence number is then
incremented by the master with each new Database Description
packet sent. The slave's DD sequence number indicates the last
packet received from the master. Only one packet is allowed
outstanding at a time.
Neighbor ID
The OSPF Router ID of the neighboring router. The Neighbor ID
is learned when Hello packets are received from the neighbor, or
is configured if this is a virtual adjacency (see Section C.4).
Neighbor Priority
The Router Priority of the neighboring router. Contained in the
neighbor's Hello packets, this item is used when selecting the
Designated Router for the attached network.
Neighbor IP address
The IP address of the neighboring router's interface to the
attached network. Used as the Destination IP address when
protocol packets are sent as unicasts along this adjacency.
Also used in router links advertisements as the Link ID for the
attached network if the neighboring router is selected to be
Designated Router (see Section 12.4.1). The Neighbor IP address
is learned when Hello packets are received from the neighbor.
For virtual links, the Neighbor IP address is learned during the
routing table build process (see Section 15).
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Neighbor Options
The optional OSPF capabilities supported by the neighbor.
Learned during the Database Exchange process (see Section 10.6).
The neighbor's optional OSPF capabilities are also listed in its
Hello packets. This enables received Hello Packets to be
rejected (i.e., neighbor relationships will not even start to
form) if there is a mismatch in certain crucial OSPF
capabilities (see Section 10.5). The optional OSPF capabilities
are documented in Section 4.5.
Neighbor's Designated Router
The neighbor's idea of the Designated Router. If this is the
neighbor itself, this is important in the local calculation of
the Designated Router. Defined only on multi-access networks.
Neighbor's Backup Designated Router
The neighbor's idea of the Backup Designated Router. If this is
the neighbor itself, this is important in the local calculation
of the Backup Designated Router. Defined only on multi-access
networks.
The next set of variables are lists of link state advertisements.
These lists describe subsets of the area topological database.
There can be five distinct types of link state advertisements in an
area topological database: router links, network links, and Type 3
and 4 summary links (all stored in the area data structure), and AS
external links (stored in the global data structure).
Link state retransmission list
The list of link state advertisements that have been flooded but
not acknowledged on this adjacency. These will be retransmitted
at intervals until they are acknowledged, or until the adjacency
is destroyed.
Database summary list
The complete list of link state advertisements that make up the
area topological database, at the moment the neighbor goes into
Database Exchange state. This list is sent to the neighbor in
Database Description packets.
Link state request list
The list of link state advertisements that need to be received
from this neighbor in order to synchronize the two neighbors'
topological databases. This list is created as Database
Description packets are received, and is then sent to the
neighbor in Link State Request packets. The list is depleted as
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appropriate Link State Update packets are received.
10.1. Neighbor states
The state of a neighbor (really, the state of a conversation
being held with a neighboring router) is documented in the
following sections. The states are listed in order of
progressing functionality. For example, the inoperative state
is listed first, followed by a list of intermediate states
before the final, fully functional state is achieved. The
specification makes use of this ordering by sometimes making
references such as "those neighbors/adjacencies in state greater
than X". Figures 12 and 13 show the graph of neighbor state
changes. The arcs of the graphs are labelled with the event
causing the state change. The neighbor events are documented in
Section 10.2.
The graph in Figure 12 shows the state changes effected by the
Hello Protocol. The Hello Protocol is responsible for neighbor
+----+
|Down|
+----+
| | Start
| +-------+
Hello | +---->|Attempt|
Received | +-------+
| |
+----+<-+ |HelloReceived
|Init|<---------------+
+----+<--------+
| |
|2-Way |1-Way
|Received |Received
| |
+-------+ | +-----+
|ExStart|<--------+------->|2-Way|
+-------+ +-----+
Figure 12: Neighbor state changes (Hello Protocol)
In addition to the state transitions pictured,
Event KillNbr always forces Down State,
Event InactivityTimer always forces Down State,
Event LLDown always forces Down State
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acquisition and maintenance, and for ensuring two way
communication between neighbors.
The graph in Figure 13 shows the forming of an adjacency. Not
every two neighboring routers become adjacent (see Section
10.4). The adjacency starts to form when the neighbor is in
state ExStart. After the two routers discover their
master/slave status, the state transitions to Exchange. At this
point the neighbor starts to be used in the flooding procedure,
and the two neighboring routers begin synchronizing their
databases. When this synchronization is finished, the neighbor
is in state Full and we say that the two routers are fully
adjacent. At this point the adjacency is listed in link state
advertisements.
For a more detailed description of neighbor state changes,
together with the additional actions involved in each change,
see Section 10.3.
+-------+
|ExStart|
+-------+
|
NegotiationDone|
+->+--------+
|Exchange|
+--+--------+
|
Exchange|
Done |
+----+ | +-------+
|Full|<---------+----->|Loading|
+----+<-+ +-------+
| LoadingDone |
+------------------+
Figure 13: Neighbor state changes (Database Exchange)
In addition to the state transitions pictured,
Event SeqNumberMismatch forces ExStart state,
Event BadLSReq forces ExStart state,
Event 1-Way forces Init state,
Event KillNbr always forces Down State,
Event InactivityTimer always forces Down State,
Event LLDown always forces Down State,
Event AdjOK? leads to adjacency forming/breaking
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Down
This is the initial state of a neighbor conversation. It
indicates that there has been no recent information received
from the neighbor. On non-broadcast networks, Hello packets
may still be sent to "Down" neighbors, although at a reduced
frequency (see Section 9.5.1).
Attempt
This state is only valid for neighbors attached to non-
broadcast networks. It indicates that no recent information
has been received from the neighbor, but that a more
concerted effort should be made to contact the neighbor.
This is done by sending the neighbor Hello packets at
intervals of HelloInterval (see Section 9.5.1).
Init
In this state, an Hello packet has recently been seen from
the neighbor. However, bidirectional communication has not
yet been established with the neighbor (i.e., the router
itself did not appear in the neighbor's Hello packet). All
neighbors in this state (or higher) are listed in the Hello
packets sent from the associated interface.
2-Way
In this state, communication between the two routers is
bidirectional. This has been assured by the operation of
the Hello Protocol. This is the most advanced state short
of beginning adjacency establishment. The (Backup)
Designated Router is selected from the set of neighbors in
state 2-Way or greater.
ExStart
This is the first step in creating an adjacency between the
two neighboring routers. The goal of this step is to decide
which router is the master, and to decide upon the initial
DD sequence number. Neighbor conversations in this state or
greater are called adjacencies.
Exchange
In this state the router is describing its entire link state
database by sending Database Description packets to the
neighbor. Each Database Description Packet has a DD
sequence number, and is explicitly acknowledged. Only one
Database Description Packet is allowed outstanding at any
one time. In this state, Link State Request Packets may
also be sent asking for the neighbor's more recent
advertisements. All adjacencies in Exchange state or
greater are used by the flooding procedure. In fact, these
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adjacencies are fully capable of transmitting and receiving
all types of OSPF routing protocol packets.
Loading
In this state, Link State Request packets are sent to the
neighbor asking for the more recent advertisements that have
been discovered (but not yet received) in the Exchange
state.
Full
In this state, the neighboring routers are fully adjacent.
These adjacencies will now appear in router links and
network links advertisements.
10.2. Events causing neighbor state changes
State changes can be effected by a number of events. These
events are shown in the labels of the arcs in Figures 12 and 13.
The label definitions are as follows:
HelloReceived
A Hello packet has been received from a neighbor.
Start
This is an indication that Hello Packets should now be sent
to the neighbor at intervals of HelloInterval seconds. This
event is generated only for neighbors associated with non-
broadcast networks.
2-WayReceived
Bidirectional communication has been realized between the
two neighboring routers. This is indicated by this router
seeing itself in the other's Hello packet.
NegotiationDone
The Master/Slave relationship has been negotiated, and DD
sequence numbers have been exchanged. This signals the
start of the sending/receiving of Database Description
packets. For more information on the generation of this
event, consult Section 10.8.
ExchangeDone
Both routers have successfully transmitted a full sequence
of Database Description packets. Each router now knows what
parts of its link state database are out of date. For more
information on the generation of this event, consult Section
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10.8.
BadLSReq
A Link State Request has been received for a link state
advertisement not contained in the database. This indicates
an error in the Database Exchange process.
Loading Done
Link State Updates have been received for all out-of-date
portions of the database. This is indicated by the Link
state request list becoming empty after the Database
Exchange process has completed.
AdjOK?
A decision must be made (again) as to whether an adjacency
should be established/maintained with the neighbor. This
event will start some adjacencies forming, and destroy
others.
The following events cause well developed neighbors to revert to
lesser states. Unlike the above events, these events may occur
when the neighbor conversation is in any of a number of states.
SeqNumberMismatch
A Database Description packet has been received that either
a) has an unexpected DD sequence number, b) unexpectedly has
the Init bit set or c) has an Options field differing from
the last Options field received in a Database Description
packet. Any of these conditions indicate that some error
has occurred during adjacency establishment.
1-Way
An Hello packet has been received from the neighbor, in
which this router is not mentioned. This indicates that
communication with the neighbor is not bidirectional.
KillNbr
This is an indication that all communication with the
neighbor is now impossible, forcing the neighbor to
revert to Down state.
InactivityTimer
The inactivity Timer has fired. This means that no Hello
packets have been seen recently from the neighbor. The
neighbor reverts to Down state.
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LLDown
This is an indication from the lower level protocols that
the neighbor is now unreachable. For example, on an X.25
network this could be indicated by an X.25 clear indication
with appropriate cause and diagnostic fields. This event
forces the neighbor into Down state.
10.3. The Neighbor state machine
A detailed description of the neighbor state changes follows.
Each state change is invoked by an event (Section 10.2). This
event may produce different effects, depending on the current
state of the neighbor. For this reason, the state machine below
is organized by current neighbor state and received event. Each
entry in the state machine describes the resulting new neighbor
state and the required set of additional actions.
When a neighbor's state changes, it may be necessary to rerun
the Designated Router election algorithm. This is determined by
whether the interface NeighborChange event is generated (see
Section 9.2). Also, if the Interface is in DR state (the router
is itself Designated Router), changes in neighbor state may
cause a new network links advertisement to be originated (see
Section 12.4).
When the neighbor state machine needs to invoke the interface
state machine, it should be done as a scheduled task (see
Section 4.4). This simplifies things, by ensuring that neither
state machine will be executed recursively.
State(s): Down
Event: Start
New state: Attempt
Action: Send an Hello Packet to the neighbor (this neighbor
is always associated with a non-broadcast network)
and start the Inactivity Timer for the neighbor.
The timer's later firing would indicate that
communication with the neighbor was not attained.
State(s): Attempt
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Event: HelloReceived
New state: Init
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has now been heard from.
State(s): Down
Event: HelloReceived
New state: Init
Action: Start the Inactivity Timer for the neighbor. The
timer's later firing would indicate that the
neighbor is dead.
State(s): Init or greater
Event: HelloReceived
New state: No state change.
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has again been heard from.
State(s): Init
Event: 2-WayReceived
New state: Depends upon action routine.
Action: Determine whether an adjacency should be established
with the neighbor (see Section 10.4). If not, the
new neighbor state is 2-Way.
Otherwise (an adjacency should be established) the
neighbor state transitions to ExStart. Upon
entering this state, the router increments the DD
sequence number for this neighbor. If this is the
first time that an adjacency has been attempted, the
DD sequence number should be assigned some unique
value (like the time of day clock). It then
declares itself master (sets the master/slave bit to
master), and starts sending Database Description
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Packets, with the initialize (I), more (M) and
master (MS) bits set. This Database Description
Packet should be otherwise empty. This Database
Description Packet should be retransmitted at
intervals of RxmtInterval until the next state is
entered (see Section 10.8).
State(s): ExStart
Event: NegotiationDone
New state: Exchange
Action: The router must list the contents of its entire area
link state database in the neighbor Database summary
list. The area link state database consists of the
router links, network links and summary links
contained in the area structure, along with the AS
external links contained in the global structure.
AS external link advertisements are omitted from a
virtual neighbor's Database summary list. AS
external advertisements are omitted from the
Database summary list if the area has been
configured as a stub (see Section 3.6).
Advertisements whose age is equal to MaxAge are
instead added to the neighbor's Link state
retransmission list. A summary of the Database
summary list will be sent to the neighbor in
Database Description packets. Each Database
Description Packet has a DD sequence number, and is
explicitly acknowledged. Only one Database
Description Packet is allowed outstanding at any one
time. For more detail on the sending and receiving
of Database Description packets, see Sections 10.8
and 10.6.
State(s): Exchange
Event: ExchangeDone
New state: Depends upon action routine.
Action: If the neighbor Link state request list is empty,
the new neighbor state is Full. No other action is
required. This is an adjacency's final state.
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Otherwise, the new neighbor state is Loading. Start
(or continue) sending Link State Request packets to
the neighbor (see Section 10.9). These are requests
for the neighbor's more recent advertisements (which
were discovered but not yet received in the Exchange
state). These advertisements are listed in the Link
state request list associated with the neighbor.
State(s): Loading
Event: Loading Done
New state: Full
Action: No action required. This is an adjacency's final
state.
State(s): 2-Way
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether an adjacency should be formed with
the neighboring router (see Section 10.4). If not,
the neighbor state remains at 2-Way. Otherwise,
transition the neighbor state to ExStart and perform
the actions associated with the above state machine
entry for state Init and event 2-WayReceived.
State(s): ExStart or greater
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether the neighboring router should
still be adjacent. If yes, there is no state change
and no further action is necessary.
Otherwise, the (possibly partially formed) adjacency
must be destroyed. The neighbor state transitions
to 2-Way. The Link state retransmission list,
Database summary list and Link state request list
are cleared of link state advertisements.
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State(s): Exchange or greater
Event: SeqNumberMismatch
New state: ExStart
Action: The (possibly partially formed) adjacency is torn
down, and then an attempt is made at
reestablishment. The neighbor state first
transitions to ExStart. The Link state
retransmission list, Database summary list and Link
state request list are cleared of link state
advertisements. Then the router increments the DD
sequence number for this neighbor, declares itself
master (sets the master/slave bit to master), and
starts sending Database Description Packets, with
the initialize (I), more (M) and master (MS) bits
set. This Database Description Packet should be
otherwise empty (see Section 10.8).
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action: The action for event BadLSReq is exactly the same as
for the neighbor event SeqNumberMismatch. The
(possibly partially formed) adjacency is torn down,
and then an attempt is made at reestablishment. For
more information, see the neighbor state machine
entry that is invoked when event SeqNumberMismatch
is generated in state Exchange or greater.
State(s): Any state
Event: KillNbr
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements. Also, the Inactivity Timer is
disabled.
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State(s): Any state
Event: LLDown
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements. Also, the Inactivity Timer is
disabled.
State(s): Any state
Event: InactivityTimer
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements.
State(s): 2-Way or greater
Event: 1-WayReceived
New state: Init
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements.
State(s): 2-Way or greater
Event: 2-WayReceived
New state: No state change.
Action: No action required.
State(s): Init
Event: 1-WayReceived
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New state: No state change.
Action: No action required.
10.4. Whether to become adjacent
Adjacencies are established with some subset of the router's
neighbors. Routers connected by point-to-point networks and
virtual links always become adjacent. On multi-access networks,
all routers become adjacent to both the Designated Router and
the Backup Designated Router.
The adjacency-forming decision occurs in two places in the
neighbor state machine. First, when bidirectional communication
is initially established with the neighbor, and secondly, when
the identity of the attached network's (Backup) Designated
Router changes. If the decision is made to not attempt an
adjacency, the state of the neighbor communication stops at 2-
Way.
An adjacency should be established with a bidirectional neighbor
when at least one of the following conditions holds:
o The underlying network type is point-to-point
o The underlying network type is virtual link
o The router itself is the Designated Router
o The router itself is the Backup Designated Router
o The neighboring router is the Designated Router
o The neighboring router is the Backup Designated Router
10.5. Receiving Hello Packets
This section explains the detailed processing of a received
Hello Packet. (See Section A.3.2 for the format of Hello
packets.) The generic input processing of OSPF packets will
have checked the validity of the IP header and the OSPF packet
header. Next, the values of the Network Mask, HelloInterval,
and RouterDeadInterval fields in the received Hello packet must
be checked against the values configured for the receiving
interface. Any mismatch causes processing to stop and the
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packet to be dropped. In other words, the above fields are
really describing the attached network's configuration. However,
there is one exception to the above rule: on point-to-point
networks and on virtual links, the Network Mask in the received
Hello Packet should be ignored.
The receiving interface attaches to a single OSPF area (this
could be the backbone). The setting of the E-bit found in the
Hello Packet's Options field must match this area's
ExternalRoutingCapability. If AS external advertisements are
not flooded into/throughout the area (i.e, the area is a "stub")
the E-bit must be clear in received Hello Packets, otherwise the
E-bit must be set. A mismatch causes processing to stop and the
packet to be dropped. The setting of the rest of the bits in
the Hello Packet's Options field should be ignored.
At this point, an attempt is made to match the source of the
Hello Packet to one of the receiving interface's neighbors. If
the receiving interface is a multi-access network (either
broadcast or non-broadcast) the source is identified by the IP
source address found in the Hello's IP header. If the receiving
interface is a point-to-point link or a virtual link, the source
is identified by the Router ID found in the Hello's OSPF packet
header. The interface's current list of neighbors is contained
in the interface's data structure. If a matching neighbor
structure cannot be found, (i.e., this is the first time the
neighbor has been detected), one is created. The initial state
of a newly created neighbor is set to Down.
When receiving an Hello Packet from a neighbor on a multi-access
network (broadcast or non-broadcast), set the neighbor
structure's Neighbor ID equal to the Router ID found in the
packet's OSPF header. When receiving an Hello on a point-to-
point network (but not on a virtual link) set the neighbor
structure's Neighbor IP address to the packet's IP source
address.
Now the rest of the Hello Packet is examined, generating events
to be given to the neighbor and interface state machines. These
state machines are specified either to be executed or scheduled
(see Section 4.4). For example, by specifying below that the
neighbor state machine be executed in line, several neighbor
state transitions may be effected by a single received Hello:
o Each Hello Packet causes the neighbor state machine to be
executed with the event HelloReceived.
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o Then the list of neighbors contained in the Hello Packet is
examined. If the router itself appears in this list, the
neighbor state machine should be executed with the event 2-
WayReceived. Otherwise, the neighbor state machine should
be executed with the event 1-WayReceived, and the processing
of the packet stops.
o Next, the Hello Packet's Router Priority field is examined.
If this field is different than the one previously received
from the neighbor, the receiving interface's state machine
is scheduled with the event NeighborChange. In any case,
the Router Priority field in the neighbor data structure
should be updated accordingly.
o Next the Designated Router field in the Hello Packet is
examined. If the neighbor is both declaring itself to be
Designated Router (Designated Router field = Neighbor IP
address) and the Backup Designated Router field in the
packet is equal to 0.0.0.0 and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Designated Router and it
had not previously, or the neighbor is not declaring itself
Designated Router where it had previously, the receiving
interface's state machine is scheduled with the event
NeighborChange. In any case, the Neighbors' Designated
Router item in the neighbor structure is updated
accordingly.
o Finally, the Backup Designated Router field in the Hello
Packet is examined. If the neighbor is declaring itself to
be Backup Designated Router (Backup Designated Router field
= Neighbor IP address) and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Backup Designated Router
and it had not previously, or the neighbor is not declaring
itself Backup Designated Router where it had previously, the
receiving interface's state machine is scheduled with the
event NeighborChange. In any case, the Neighbor's Backup
Designated Router item in the neighbor structure is updated
accordingly.
On non-broadcast multi-access networks, receipt of an Hello
Packet may also cause an Hello Packet to be sent back to the
neighbor in response. See Section 9.5.1 for more details.
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10.6. Receiving Database Description Packets
This section explains the detailed processing of a received
Database Description Packet. The incoming Database Description
Packet has already been associated with a neighbor and receiving
interface by the generic input packet processing (Section 8.2).
The further processing of the Database Description Packet
depends on the neighbor state. If the neighbor's state is Down
or Attempt the packet should be ignored. Otherwise, if the
state is:
Init
The neighbor state machine should be executed with the event
2-WayReceived. This causes an immediate state change to
either state 2-Way or state ExStart. If the new state is
ExStart, the processing of the current packet should then
continue in this new state by falling through to case
ExStart below.
2-Way
The packet should be ignored. Database Description Packets
are used only for the purpose of bringing up adjacencies.[7]
ExStart
If the received packet matches one of the following cases,
then the neighbor state machine should be executed with the
event NegotiationDone (causing the state to transition to
Exchange), the packet's Options field should be recorded in
the neighbor structure's Neighbor Options field and the
packet should be accepted as next in sequence and processed
further (see below). Otherwise, the packet should be
ignored.
o The initialize(I), more (M) and master(MS) bits are set,
the contents of the packet are empty, and the neighbor's
Router ID is larger than the router's own. In this case
the router is now Slave. Set the master/slave bit to
slave, and set the DD sequence number to that specified
by the master.
o The initialize(I) and master(MS) bits are off, the
packet's DD sequence number equals the router's own DD
sequence number (indicating acknowledgment) and the
neighbor's Router ID is smaller than the router's own.
In this case the router is Master.
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Exchange
If the state of the MS-bit is inconsistent with the
master/slave state of the connection, generate the neighbor
event SeqNumberMismatch and stop processing the packet.
Otherwise:
o If the initialize(I) bit is set, generate the neighbor
event SeqNumberMismatch and stop processing the packet.
o If the packet's Options field indicates a different set
of optional OSPF capabilities than were previously
received from the neighbor (recorded in the Neighbor
Options field of the neighbor structure), generate the
neighbor event SeqNumberMismatch and stop processing the
packet.
o If the router is master, and the packet's DD sequence
number equals the router's own DD sequence number (this
packet is the next in sequence) the packet should be
accepted and its contents processed (below).
o If the router is master, and the packet's DD sequence
number is one less than the router's DD sequence number,
the packet is a duplicate. Duplicates should be
discarded by the master.
o If the router is slave, and the packet's DD sequence
number is one more than the router's own DD sequence
number (this packet is the next in sequence) the packet
should be accepted and its contents processed (below).
o If the router is slave, and the packet's DD sequence
number is equal to the router's DD sequence number, the
packet is a duplicate. The slave must respond to
duplicates by repeating the last Database Description
packet that it had sent.
o Else, generate the neighbor event SeqNumberMismatch and
stop processing the packet.
Loading or Full
In this state, the router has sent and received an entire
sequence of Database Description Packets. The only packets
received should be duplicates (see above). In particular,
the packet's Options field should match the set of optional
OSPF capabilities previously indicated by the neighbor
(stored in the neighbor structure's Neighbor Options field).
Any other packets received, including the reception of a
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packet with the Initialize(I) bit set, should generate the
neighbor event SeqNumberMismatch.[8] Duplicates should be
discarded by the master. The slave must respond to
duplicates by repeating the last Database Description packet
that it had sent.
When the router accepts a received Database Description Packet
as the next in sequence the packet contents are processed as
follows. For each link state advertisement listed, the
advertisement's LS type is checked for validity. If the LS type
is unknown (e.g., not one of the LS types 1-5 defined by this
specification), or if this is a AS external advertisement (LS
type = 5) and the neighbor is associated with a stub area,
generate the neighbor event SeqNumberMismatch and stop
processing the packet. Otherwise, the router looks up the
advertisement in its database to see whether it also has an
instance of the link state advertisement. If it does not, or if
the database copy is less recent (see Section 13.1), the link
state advertisement is put on the Link state request list so
that it can be requested (immediately or at some later time) in
Link State Request Packets.
When the router accepts a received Database Description Packet
as the next in sequence, it also performs the following actions,
depending on whether it is master or slave:
Master
Increments the DD sequence number. If the router has
already sent its entire sequence of Database Description
Packets, and the just accepted packet has the more bit (M)
set to 0, the neighbor event ExchangeDone is generated.
Otherwise, it should send a new Database Description to the
slave.
Slave
Sets the DD sequence number to the DD sequence number
appearing in the received packet. The slave must send a
Database Description Packet in reply. If the received
packet has the more bit (M) set to 0, and the packet to be
sent by the slave will also have the M-bit set to 0, the
neighbor event ExchangeDone is generated. Note that the
slave always generates this event before the master.
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10.7. Receiving Link State Request Packets
This section explains the detailed processing of received Link
State Request packets. Received Link State Request Packets
specify a list of link state advertisements that the neighbor
wishes to receive. Link State Request Packets should be
accepted when the neighbor is in states Exchange, Loading, or
Full. In all other states Link State Request Packets should be
ignored.
Each link state advertisement specified in the Link State
Request packet should be located in the router's database, and
copied into Link State Update packets for transmission to the
neighbor. These link state advertisements should NOT be placed
on the Link state retransmission list for the neighbor. If a
link state advertisement cannot be found in the database,
something has gone wrong with the Database Exchange process, and
neighbor event BadLSReq should be generated.
10.8. Sending Database Description Packets
This section describes how Database Description Packets are sent
to a neighbor. The router's optional OSPF capabilities (see
Section 4.5) are transmitted to the neighbor in the Options
field of the Database Description packet. The router should
maintain the same set of optional capabilities throughout the
Database Exchange and flooding procedures. If for some reason
the router's optional capabilities change, the Database Exchange
procedure should be restarted by reverting to neighbor state
ExStart. There are currently two optional capabilities defined.
The T-bit should be set if and only if the router is capable of
calculating separate routes for each IP TOS. The E-bit should
be set if and only if the attached network belongs to a non-stub
area. The rest of the Options field should be set to zero.
The sending of Database Description packets depends on the
neighbor's state. In state ExStart the router sends empty
Database Description packets, with the initialize (I), more (M)
and master (MS) bits set. These packets are retransmitted every
RxmtInterval seconds.
In state Exchange the Database Description Packets actually
contain summaries of the link state information contained in the
router's database. Each link state advertisement in the area's
topological database (at the time the neighbor transitions into
Exchange state) is listed in the neighbor Database summary list.
When a new Database Description Packet is to be sent, the
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packet's DD sequence number is incremented, and the (new) top of
the Database summary list is described by the packet. Items are
removed from the Database summary list when the previous packet
is acknowledged.
In state Exchange, the determination of when to send a Database
Description packet depends on whether the router is master or
slave:
Master
Database Description packets are sent when either a) the
slave acknowledges the previous Database Description packet
by echoing the DD sequence number or b) RxmtInterval seconds
elapse without an acknowledgment, in which case the previous
Database Description packet is retransmitted.
Slave
Database Description packets are sent only in response to
Database Description packets received from the master. If
the Database Description packet received from the master is
new, a new Database Description packet is sent, otherwise
the previous Database Description packet is resent.
In states Loading and Full the slave must resend its last
Database Description packet in response to duplicate Database
Description packets received from the master. For this reason
the slave must wait RouterDeadInterval seconds before freeing
the last Database Description packet. Reception of a Database
Description packet from the master after this interval will
generate a SeqNumberMismatch neighbor event.
10.9. Sending Link State Request Packets
In neighbor states Exchange or Loading, the Link state request
list contains a list of those link state advertisements that
need to be obtained from the neighbor. To request these
advertisements, a router sends the neighbor the beginning of the
Link state request list, packaged in a Link State Request
packet.
When the neighbor responds to these requests with the proper
Link State Update packet(s), the Link state request list is
truncated and a new Link State Request packet is sent. This
process continues until the Link state request list becomes
empty. Unsatisfied Link State Request packets are retransmitted
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at intervals of RxmtInterval. There should be at most one Link
State Request packet outstanding at any one time.
When the Link state request list becomes empty, and the neighbor
state is Loading (i.e., a complete sequence of Database
Description packets has been sent to and received from the
neighbor), the Loading Done neighbor event is generated.
10.10. An Example
Figure 14 shows an example of an adjacency forming. Routers RT1
and RT2 are both connected to a broadcast network. It is
assumed that RT2 is the Designated Router for the network, and
that RT2 has a higher Router ID than Router RT1.
The neighbor state changes realized by each router are listed on
the sides of the figure.
At the beginning of Figure 14, Router RT1's interface to the
network becomes operational. It begins sending Hello Packets,
although it doesn't know the identity of the Designated Router
or of any other neighboring routers. Router RT2 hears this
hello (moving the neighbor to Init state), and in its next Hello
Packet indicates that it is itself the Designated Router and
that it has heard Hello Packets from RT1. This in turn causes
RT1 to go to state ExStart, as it starts to bring up the
adjacency.
RT1 begins by asserting itself as the master. When it sees that
RT2 is indeed the master (because of RT2's higher Router ID),
RT1 transitions to slave state and adopts its neighbor's DD
sequence number. Database Description packets are then
exchanged, with polls coming from the master (RT2) and responses
from the slave (RT1). This sequence of Database Description
Packets ends when both the poll and associated response has the
M-bit off.
In this example, it is assumed that RT2 has a completely up to
date database. In that case, RT2 goes immediately into Full
state. RT1 will go into Full state after updating the necessary
parts of its database. This is done by sending Link State
Request Packets, and receiving Link State Update Packets in
response. Note that, while RT1 has waited until a complete set
of Database Description Packets has been received (from RT2)
before sending any Link State Request Packets, this need not be
the case. RT1 could have interleaved the sending of Link State
Request Packets with the reception of Database Description
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+---+ +---+
|RT1| |RT2|
+---+ +---+
Down Down
Hello(DR=0,seen=0)
------------------------------>
Hello (DR=RT2,seen=RT1,...) Init
<------------------------------
ExStart D-D (Seq=x,I,M,Master)
------------------------------>
D-D (Seq=y,I,M,Master) ExStart
<------------------------------
Exchange D-D (Seq=y,M,Slave)
------------------------------>
D-D (Seq=y+1,M,Master) Exchange
<------------------------------
D-D (Seq=y+1,M,Slave)
------------------------------>
...
...
...
D-D (Seq=y+n, Master)
<------------------------------
D-D (Seq=y+n, Slave)
Loading ------------------------------>
LS Request Full
------------------------------>
LS Update
<------------------------------
LS Request
------------------------------>
LS Update
<------------------------------
Full
Figure 14: An adjacency bring-up example
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Packets.
11. The Routing Table Structure
The routing table data structure contains all the information
necessary to forward an IP data packet toward its destination. Each
routing table entry describes the collection of best paths to a
particular destination. When forwarding an IP data packet, the
routing table entry providing the best match for the packet's IP
destination is located. The matching routing table entry then
provides the next hop towards the packet's destination. OSPF also
provides for the existence of a default route (Destination ID =
DefaultDestination, Address Mask = 0x00000000). When the default
route exists, it matches all IP destinations (although any other
matching entry is a better match). Finding the routing table entry
that best matches an IP destination is further described in Section
11.1.
There is a single routing table in each router. Two sample routing
tables are described in Sections 11.2 and 11.3. The building of the
routing table is discussed in Section 16.
The rest of this section defines the fields found in a routing table
entry. The first set of fields describes the routing table entry's
destination.
Destination Type
The destination can be one of three types. Only the first type,
Network, is actually used when forwarding IP data traffic. The
other destinations are used solely as intermediate steps in the
routing table build process.
Network
A range of IP addresses, to which IP data traffic may be
forwarded. This includes IP networks (class A, B, or C), IP
subnets, IP supernets and single IP hosts. The default
route also falls in this category.
Area border router
Routers that are connected to multiple OSPF areas. Such
routers originate summary link advertisements. These
routing table entries are used when calculating the inter-
area routes (see Section 16.2). These routing table entries
may also be associated with configured virtual links.
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AS boundary router
Routers that originate AS external link advertisements.
These routing table entries are used when calculating the AS
external routes (see Section 16.4).
Destination ID
The destination's identifier or name. This depends on the
Destination Type. For networks, the identifier is their
associated IP address. For all other types, the identifier is
the OSPF Router ID.[9]
Address Mask
Only defined for networks. The network's IP address together
with its address mask defines a range of IP addresses. For IP
subnets, the address mask is referred to as the subnet mask.
For host routes, the mask is "all ones" (0xffffffff).
Optional Capabilities
When the destination is a router (either an area border router
or an AS boundary router) this field indicates the optional OSPF
capabilities supported by the destination router. The two
optional capabilities currently defined by this specification
are the ability to route based on IP TOS and the ability to
process AS external link advertisements. For a further
discussion of OSPF's optional capabilities, see Section 4.5.
The set of paths to use for a destination may vary based on IP Type
of Service and the OSPF area to which the paths belong. This means
that there may be multiple routing table entries for the same
destination, depending on the values of the next two fields.
Type of Service
There can be a separate set of routes for each IP Type of
Service. The encoding of TOS in OSPF link state advertisements
is described in Section 12.3.
Area
This field indicates the area whose link state information has
led to the routing table entry's collection of paths. This is
called the entry's associated area. For sets of AS external
paths, this field is not defined. For destinations of type
"area border router", there may be separate sets of paths (and
therefore separate routing table entries) associated with each
of several areas. This will happen when two area border routers
share multiple areas in common. For all other destination
types, only the set of paths associated with the best area (the
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one providing the shortest route) is kept.
The rest of the routing table entry describes the set of paths to
the destination. The following fields pertain to the set of paths
as a whole. In other words, each one of the paths contained in a
routing table entry is of the same path-type and cost (see below).
Path-type
There are four possible types of paths used to route traffic to
the destination, listed here in order of preference: intra-area,
inter-area, type 1 external or type 2 external. Intra-area
paths indicate destinations belonging to one of the router's
attached areas. Inter-area paths are paths to destinations in
other OSPF areas. These are discovered through the examination
of received summary link advertisements. AS external paths are
paths to destinations external to the AS. These are detected
through the examination of received AS external link
advertisements.
Cost
The link state cost of the path to the destination. For all
paths except type 2 external paths this describes the entire
path's cost. For Type 2 external paths, this field describes
the cost of the portion of the path internal to the AS. This
cost is calculated as the sum of the costs of the path's
constituent links.
Type 2 cost
Only valid for type 2 external paths. For these paths, this
field indicates the cost of the path's external portion. This
cost has been advertised by an AS boundary router, and is the
most significant part of the total path cost. For example, a
type 2 external path with type 2 cost of 5 is always preferred
over a path with type 2 cost of 10, regardless of the cost of
the two paths' internal components.
Link State Origin
Valid only for intra-area paths, this field indicates the link
state advertisement (router links or network links) that
directly references the destination. For example, if the
destination is a transit network, this is the transit network's
network links advertisement. If the destination is a stub
network, this is the router links advertisement for the attached
router. The advertisement is discovered during the shortest-
path tree calculation (see Section 16.1). Multiple
advertisements may reference the destination, however a tie-
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breaking scheme always reduces the choice to a single
advertisement. The Link State Origin field is not used by the
OSPF protocol, but it is used by the routing table calculation
in OSPF's Multicast routing extensions (MOSPF).
When multiple paths of equal path-type and cost exist to a
destination (called elsewhere "equal-cost" paths), they are stored
in a single routing table entry. Each one of the "equal-cost" paths
is distinguished by the following fields:
Next hop
The outgoing router interface to use when forwarding traffic to
the destination. On multi-access networks, the next hop also
includes the IP address of the next router (if any) in the path
towards the destination. This next router will always be one of
the adjacent neighbors.
Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary
link or AS external link that led to this path.
11.1. Routing table lookup
When an IP data packet is received, an OSPF router finds the
routing table entry that best matches the packet's destination.
This routing table entry then provides the outgoing interface
and next hop router to use in forwarding the packet. This
section describes the process of finding the best matching
routing table entry. The process consists of a number of steps,
wherein the collection of routing table entries is progressively
pruned. In the end, the single routing table entry remaining is
the called best match.
Note that the steps described below may fail to produce a best
match routing table entry (i.e., all existing routing table
entries are pruned for some reason or another). In this case,
the packet's IP destination is considered unreachable. Instead
of being forwarded, the packet should be dropped and an ICMP
destination unreachable message should be returned to the
packet's source.
(1) Select the complete set of "matching" routing table entries
from the routing table. Each routing table entry describes
a (set of) path(s) to a range of IP addresses. If the data
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packet's IP destination falls into an entry's range of IP
addresses, the routing table entry is called a match. (It is
quite likely that multiple entries will match the data
packet. For example, a default route will match all
packets.)
(2) Suppose that the packet's IP destination falls into one of
the router's configured area address ranges (see Section
3.5), and that the particular area address range is active.
This means that there are one or more reachable (by intra-
area paths) networks contained in the area address range.
The packet's IP destination is then required to belong to
one of these constituent networks. For this reason, only
matching routing table entries with path-type of intra-area
are considered (all others are pruned). If no such matching
entries exist, the destination is unreachable (see above).
Otherwise, skip to step 4.
(3) Reduce the set of matching entries to those having the most
preferential path-type (see Section 11). OSPF has a four
level hierarchy of paths. Intra-area paths are the most
preferred, followed in order by inter-area, type 1 external
and type 2 external paths.
(4) Select the remaining routing table entry that provides the
longest (most specific) match. Another way of saying this is
to choose the remaining entry that specifies the narrowest
range of IP addresses.[10] For example, the entry for the
address/mask pair of (128.185.1.0, 0xffffff00) is more
specific than an entry for the pair (128.185.0.0,
0xffff0000). The default route is the least specific match,
since it matches all destinations.
(5) At this point, there may still be multiple routing table
entries remaining. Each routing entry will specify the same
range of IP addresses, but a different IP Type of Service.
Select the routing table entry whose TOS value matches the
TOS found in the packet header. If there is no routing table
entry for this TOS, select the routing table entry for TOS
0. In other words, packets requesting TOS X are routed along
the TOS 0 path if a TOS X path does not exist.
11.2. Sample routing table, without areas
Consider the Autonomous System pictured in Figure 2. No OSPF
areas have been configured. A single metric is shown per
outbound interface, indicating that routes will not vary based
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on TOS. The calculation of Router RT6's routing table proceeds
as described in Section 2.1. The resulting routing table is
shown in Table 12. Destination types are abbreviated: Network
as "N", area border router as "BR" and AS boundary router as
"ASBR".
There are no instances of multiple equal-cost shortest paths in
this example. Also, since there are no areas, there are no
inter-area paths.
Routers RT5 and RT7 are AS boundary routers. Intra-area routes
have been calculated to Routers RT5 and RT7. This allows
external routes to be calculated to the destinations advertised
by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15). It is
assumed all AS external advertisements originated by RT5 and RT7
are advertising type 1 external metrics. This results in type 1
external paths being calculated to destinations N12-N15.
11.3. Sample routing table, with areas
Consider the previous example, this time split into OSPF areas.
An OSPF area configuration is pictured in Figure 6. Router
RT4's routing table will be described for this area
configuration. Router RT4 has a connection to Area 1 and a
backbone connection. This causes Router RT4 to view the AS as
the concatenation of the two graphs shown in Figures 7 and 8.
The resulting routing table is displayed in Table 13.
Again, Routers RT5 and RT7 are AS boundary routers. Routers
RT3, RT4, RT7, RT10 and RT11 are area border routers. Note that
there are two routing table entries (in this case having
identical paths) for Router RT7, in its dual capacities as an
area border router and an AS boundary router. Note also that
there are two routing entries for the area border router RT3,
since it has two areas in common with RT4 (Area 1 and the
backbone).
Backbone paths have been calculated to all area border routers
(BR). These are used when determining the inter-area routes.
Note that all of the inter-area routes are associated with the
backbone; this is always the case when the calculating router is
itself an area border router. Routing information is condensed
at area boundaries. In this example, we assume that Area 3 has
been defined so that networks N9-N11 and the host route to H1
are all condensed to a single route when advertised into the
backbone (by Router RT11). Note that the cost of this route is
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Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
____________________________________________________________
N N1 0 intra-area 10 RT3 *
N N2 0 intra-area 10 RT3 *
N N3 0 intra-area 7 RT3 *
N N4 0 intra-area 8 RT3 *
N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 *
ASBR RT5 0 intra-area 6 RT5 *
ASBR RT7 0 intra-area 8 RT10 *
____________________________________________________________
N N12 * type 1 ext. 10 RT10 RT7
N N13 * type 1 ext. 14 RT5 RT5
N N14 * type 1 ext. 14 RT5 RT5
N N15 * type 1 ext. 17 RT10 RT7
Table 12: The routing table for Router RT6
(no configured areas).
the minimum of the set of costs to its individual components.
There is a virtual link configured between Routers RT10 and
RT11. Without this configured virtual link, RT11 would be
unable to advertise a route for networks N9-N11 and Host H1 into
the backbone, and there would not be an entry for these networks
in Router RT4's routing table.
In this example there are two equal-cost paths to Network N12.
However, they both use the same next hop (Router RT5).
Router RT4's routing table would improve (i.e., some of the
paths in the routing table would become shorter) if an
additional virtual link were configured between Router RT4 and
Router RT3. The new virtual link would itself be associated
with the first entry for area border router RT3 in Table 13 (an
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Type Dest Area Path Type Cost Next Adv.
Hops(s) Router(s)
__________________________________________________________________
N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 *
BR RT3 1 intra-area 1 * *
__________________________________________________________________
N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 *
BR RT3 0 intra-area 21 RT5 *
BR RT7 0 intra-area 14 RT5 *
BR RT10 0 intra-area 22 RT5 *
BR RT11 0 intra-area 25 RT5 *
ASBR RT5 0 intra-area 8 * *
ASBR RT7 0 intra-area 14 RT5 *
__________________________________________________________________
N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 26 RT5 RT11
__________________________________________________________________
N N12 * type 1 ext. 16 RT5 RT5,RT7
N N13 * type 1 ext. 16 RT5 RT5
N N14 * type 1 ext. 16 RT5 RT5
N N15 * type 1 ext. 23 RT5 RT7
Table 13: Router RT4's routing table
in the presence of areas.
intra-area path through Area 1). This would yield a cost of 1
for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link are shown
in Table 14.
12. Link State Advertisements
Each router in the Autonomous System originates one or more link
state advertisements. There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection
of link state advertisements forms the link state or topological
database. Each separate type of advertisement has a separate
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Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
BR RT3 0 intra-area 1 * *
BR RT10 0 intra-area 16 RT3 *
BR RT11 0 intra-area 19 RT3 *
________________________________________________________________
N N9-N11,H1 0 inter-area 20 RT3 RT11
Table 14: Changes resulting from an
additional virtual link.
function. Router links and network links advertisements describe
how an area's routers and networks are interconnected. Summary link
advertisements provide a way of condensing an area's routing
information. AS external advertisements provide a way of
transparently advertising externally-derived routing information
throughout the Autonomous System.
Each link state advertisement begins with a standard 20-byte header.
This link state advertisement header is discussed below.
12.1. The Link State Advertisement Header
The link state advertisement header contains the LS type, Link
State ID and Advertising Router fields. The combination of
these three fields uniquely identifies the link state
advertisement.
There may be several instances of an advertisement present in
the Autonomous System, all at the same time. It must then be
determined which instance is more recent. This determination is
made by examining the LS sequence, LS checksum and LS age
fields. These fields are also contained in the 20-byte link
state advertisement header.
Several of the OSPF packet types list link state advertisements.
When the instance is not important, an advertisement is referred
to by its LS type, Link State ID and Advertising Router (see
Link State Request Packets). Otherwise, the LS sequence number,
LS age and LS checksum fields must also be referenced.
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A detailed explanation of the fields contained in the link state
advertisement header follows.
12.1.1. LS age
This field is the age of the link state advertisement in
seconds. It should be processed as an unsigned 16-bit
integer. It is set to 0 when the link state advertisement
is originated. It must be incremented by InfTransDelay on
every hop of the flooding procedure. Link state
advertisements are also aged as they are held in each
router's database.
The age of a link state advertisement is never incremented
past MaxAge. Advertisements having age MaxAge are not used
in the routing table calculation. When an advertisement's
age first reaches MaxAge, it is reflooded. A link state
advertisement of age MaxAge is finally flushed from the
database when it is no longer needed to ensure database
synchronization. For more information on the aging of link
state advertisements, consult Section 14.
The LS age field is examined when a router receives two
instances of a link state advertisement, both having
identical LS sequence numbers and LS checksums. An instance
of age MaxAge is then always accepted as most recent; this
allows old advertisements to be flushed quickly from the
routing domain. Otherwise, if the ages differ by more than
MaxAgeDiff, the instance having the smaller age is accepted
as most recent.[11] See Section 13.1 for more details.
12.1.2. Options
The Options field in the link state advertisement header
indicates which optional capabilities are associated with
the advertisement. OSPF's optional capabilities are
described in Section 4.5. There are currently two optional
capabilities defined; they are represented by the T-bit and
E-bit found in the Options field. The rest of the Options
field should be set to zero.
The E-bit represents OSPF's ExternalRoutingCapability. This
bit should be set in all advertisements associated with the
backbone, and all advertisements associated with non-stub
areas (see Section 3.6). It should also be set in all AS
external link advertisements. It should be reset in all
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router links, network links and summary link advertisements
associated with a stub area. For all link state
advertisements, the setting of the E-bit is for
informational purposes only; it does not affect the routing
table calculation.
The T-bit represents OSPF's TOS routing capability. This
bit should be set in a router links advertisement if and
only if the router is capable of calculating separate routes
for each IP TOS (see Section 2.4). The T-bit should always
be set in network links advertisements. It should be set in
summary link and AS external link advertisements if and only
if the advertisement describes paths for all TOS values,
instead of just the TOS 0 path. Note that, with the T-bit
set, there may still be only a single metric in the
advertisement (the TOS 0 metric). This would mean that
paths for non-zero TOS exist, but are equivalent to the TOS
0 path. A link state advertisement's T-bit is examined when
calculating the routing table's non-zero TOS paths (see
Section 16.9).
12.1.3. LS type
The LS type field dictates the format and function of the
link state advertisement. Advertisements of different types
have different names (e.g., router links or network links).
All advertisement types, except the AS external link
advertisements (LS type = 5), are flooded throughout a
single area only. AS external link advertisements are
flooded throughout the entire Autonomous System, excepting
stub areas (see Section 3.6). Each separate advertisement
type is briefly described below in Table 15.
12.1.4. Link State ID
This field identifies the piece of the routing domain that
is being described by the advertisement. Depending on the
advertisement's LS type, the Link State ID takes on the
values listed in Table 16.
Actually, for Type 3 summary link (LS type = 3)
advertisements and AS external link (LS type = 5)
advertisements, the Link State ID may additionally have one
or more of the destination network's "host" bits set. For
example, when originating an AS external link for the
network 10.0.0.0 with mask of 255.0.0.0, the Link State ID
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LS Type Advertisement description
__________________________________________________
1 These are the router links
advertisements. They describe the
collected states of the router's
interfaces. For more information,
consult Section 12.4.1.
__________________________________________________
2 These are the network links
advertisements. They describe the set
of routers attached to the network. For
more information, consult
Section 12.4.2.
__________________________________________________
3 or 4 These are the summary link
advertisements. They describe
inter-area routes, and enable the
condensation of routing information at
area borders. Originated by area border
routers, the Type 3 advertisements
describe routes to networks while the
Type 4 advertisements describe routes to
AS boundary routers.
__________________________________________________
5 These are the AS external link
advertisements. Originated by AS
boundary routers, they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS external link
advertisement.
Table 15: OSPF link state advertisements.
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LS Type Link State ID
_______________________________________________
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.
Table 16: The advertisement's Link State ID.
can be set to anything in the range 10.0.0.0 through
10.255.255.255 inclusive (although 10.0.0.0 should be used
whenever possible). The freedom to set certain host bits
allows a router to originate separate advertisements for two
networks having the same address but different masks. See
Appendix F for details.
When the link state advertisement is describing a network
(LS type = 2, 3 or 5), the network's IP address is easily
derived by masking the Link State ID with the network/subnet
mask contained in the body of the link state advertisement.
When the link state advertisement is describing a router (LS
type = 1 or 4), the Link State ID is always the described
router's OSPF Router ID.
When an AS external advertisement (LS Type = 5) is
describing a default route, its Link State ID is set to
DefaultDestination (0.0.0.0).
12.1.5. Advertising Router
This field specifies the OSPF Router ID of the
advertisement's originator. For router links
advertisements, this field is identical to the Link State ID
field. Network link advertisements are originated by the
network's Designated Router. Summary link advertisements
are originated by area border routers. AS external link
advertisements are originated by AS boundary routers.
12.1.6. LS sequence number
The sequence number field is a signed 32-bit integer. It is
used to detect old and duplicate link state advertisements.
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The space of sequence numbers is linearly ordered. The
larger the sequence number (when compared as signed 32-bit
integers) the more recent the advertisement. To describe to
sequence number space more precisely, let N refer in the
discussion below to the constant 2**31.
The sequence number -N (0x80000000) is reserved (and
unused). This leaves -N + 1 (0x80000001) as the smallest
(and therefore oldest) sequence number. A router uses this
sequence number the first time it originates any link state
advertisement. Afterwards, the advertisement's sequence
number is incremented each time the router originates a new
instance of the advertisement. When an attempt is made to
increment the sequence number past the maximum value of N -
1 (0x7fffffff), the current instance of the advertisement
must first be flushed from the routing domain. This is done
by prematurely aging the advertisement (see Section 14.1)
and reflooding it. As soon as this flood has been
acknowledged by all adjacent neighbors, a new instance can
be originated with sequence number of -N + 1 (0x80000001).
The router may be forced to promote the sequence number of
one of its advertisements when a more recent instance of the
advertisement is unexpectedly received during the flooding
process. This should be a rare event. This may indicate
that an out-of-date advertisement, originated by the router
itself before its last restart/reload, still exists in the
Autonomous System. For more information see Section 13.4.
12.1.7. LS checksum
This field is the checksum of the complete contents of the
advertisement, excepting the LS age field. The LS age field
is excepted so that an advertisement's age can be
incremented without updating the checksum. The checksum
used is the same that is used for ISO connectionless
datagrams; it is commonly referred to as the Fletcher
checksum. It is documented in Annex B of [RFC 905]. The
link state advertisement header also contains the length of
the advertisement in bytes; subtracting the size of the LS
age field (two bytes) yields the amount of data to checksum.
The checksum is used to detect data corruption of an
advertisement. This corruption can occur while an
advertisement is being flooded, or while it is being held in
a router's memory. The LS checksum field cannot take on the
value of zero; the occurrence of such a value should be
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considered a checksum failure. In other words, calculation
of the checksum is not optional.
The checksum of a link state advertisement is verified in
two cases: a) when it is received in a Link State Update
Packet and b) at times during the aging of the link state
database. The detection of a checksum failure leads to
separate actions in each case. See Sections 13 and 14 for
more details.
Whenever the LS sequence number field indicates that two
instances of an advertisement are the same, the LS checksum
field is examined. If there is a difference, the instance
with the larger LS checksum is considered to be most
recent.[12] See Section 13.1 for more details.
12.2. The link state database
A router has a separate link state database for every area to
which it belongs. The link state database has been referred to
elsewhere in the text as the topological database. All routers
belonging to the same area have identical topological databases
for the area.
The databases for each individual area are always dealt with
separately. The shortest path calculation is performed
separately for each area (see Section 16). Components of the
area topological database are flooded throughout the area only.
Finally, when an adjacency (belonging to Area A) is being
brought up, only the database for Area A is synchronized between
the two routers.
The area database is composed of router links advertisements,
network links advertisements, and summary link advertisements
(all listed in the area data structure). In addition, external
routes (AS external advertisements) are included in all non-stub
area databases (see Section 3.6).
An implementation of OSPF must be able to access individual
pieces of an area database. This lookup function is based on an
advertisement's LS type, Link State ID and Advertising
Router.[13] There will be a single instance (the most up-to-
date) of each link state advertisement in the database. The
database lookup function is invoked during the link state
flooding procedure (Section 13) and the routing table
calculation (Section 16). In addition, using this lookup
function the router can determine whether it has itself ever
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originated a particular link state advertisement, and if so,
with what LS sequence number.
A link state advertisement is added to a router's database when
either a) it is received during the flooding process (Section
13) or b) it is originated by the router itself (Section 12.4).
A link state advertisement is deleted from a router's database
when either a) it has been overwritten by a newer instance
during the flooding process (Section 13) or b) the router
originates a newer instance of one of its self-originated
advertisements (Section 12.4) or c) the advertisement ages out
and is flushed from the routing domain (Section 14). Whenever a
link state advertisement is deleted from the database it must
also be removed from all neighbors' Link state retransmission
lists (see Section 10).
12.3. Representation of TOS
All OSPF link state advertisements (with the exception of
network links advertisements) specify metrics. In router links
advertisements, the metrics indicate the costs of the described
interfaces. In summary link and AS external link
advertisements, the metric indicates the cost of the described
path. In all of these advertisements, a separate metric can be
specified for each IP TOS. The encoding of TOS in OSPF link
state advertisements is specified in Table 17. That table
relates the OSPF encoding to the IP packet header's TOS field
(defined in [RFC 1349]). The OSPF encoding is expressed as a
decimal integer, and the IP packet header's TOS field is
expressed in the binary TOS values used in [RFC 1349].
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OSPF encoding RFC 1349 TOS values
___________________________________________
0 0000 normal service
2 0001 minimize monetary cost
4 0010 maximize reliability
6 0011
8 0100 maximize throughput
10 0101
12 0110
14 0111
16 1000 minimize delay
18 1001
20 1010
22 1011
24 1100
26 1101
28 1110
30 1111
Table 17: Representing TOS in OSPF.
Each OSPF link state advertisement must specify the TOS 0
metric. Other TOS metrics, if they appear, must appear in order
of increasing TOS encoding. For example, the TOS 8 (maximize
throughput) metric must always appear before the TOS 16
(minimize delay) metric when both are specified. If a metric
for some non-zero TOS is not specified, its cost defaults to the
cost for TOS 0, unless the T-bit is reset in the advertisement's
Options field (see Section 12.1.2 for more details).
12.4. Originating link state advertisements
Into any given OSPF area, a router will originate several link
state advertisements. Each router originates a router links
advertisement. If the router is also the Designated Router for
any of the area's networks, it will originate network links
advertisements for those networks.
Area border routers originate a single summary link
advertisement for each known inter-area destination. AS
boundary routers originate a single AS external link
advertisement for each known AS external destination.
Destinations are advertised one at a time so that the change in
any single route can be flooded without reflooding the entire
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collection of routes. During the flooding procedure, many link
state advertisements can be carried by a single Link State
Update packet.
As an example, consider Router RT4 in Figure 6. It is an area
border router, having a connection to Area 1 and the backbone.
Router RT4 originates 5 distinct link state advertisements into
the backbone (one router links, and one summary link for each of
the networks N1-N4). Router RT4 will also originate 8 distinct
link state advertisements into Area 1 (one router links and
seven summary link advertisements as pictured in Figure 7). If
RT4 has been selected as Designated Router for Network N3, it
will also originate a network links advertisement for N3 into
Area 1.
In this same figure, Router RT5 will be originating 3 distinct
AS external link advertisements (one for each of the networks
N12-N14). These will be flooded throughout the entire AS,
assuming that none of the areas have been configured as stubs.
However, if area 3 has been configured as a stub area, the
external advertisements for networks N12-N14 will not be flooded
into area 3 (see Section 3.6). Instead, Router RT11 would
originate a default summary link advertisement that would be
flooded throughout area 3 (see Section 12.4.3). This instructs
all of area 3's internal routers to send their AS external
traffic to RT11.
Whenever a new instance of a link state advertisement is
originated, its LS sequence number is incremented, its LS age is
set to 0, its LS checksum is calculated, and the advertisement
is added to the link state database and flooded out the
appropriate interfaces. See Section 13.2 for details concerning
the installation of the advertisement into the link state
database. See Section 13.3 for details concerning the flooding
of newly originated advertisements.
The ten events that can cause a new instance of a link state
advertisement to be originated are:
(1) The LS age field of one of the router's self-originated
advertisements reaches the value LSRefreshTime. In this
case, a new instance of the link state advertisement is
originated, even though the contents of the advertisement
(apart from the link state advertisement header) will be the
same. This guarantees periodic originations of all link
state advertisements. This periodic updating of link state
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advertisements adds robustness to the link state algorithm.
Link state advertisements that solely describe unreachable
destinations should not be refreshed, but should instead be
flushed from the routing domain (see Section 14.1).
When whatever is being described by a link state advertisement
changes, a new advertisement is originated. However, two
instances of the same link state advertisement may not be
originated within the time period MinLSInterval. This may
require that the generation of the next instance be delayed by
up to MinLSInterval. The following events may cause the
contents of a link state advertisement to change. These events
should cause new originations if and only if the contents of the
new advertisement would be different:
(2) An interface's state changes (see Section 9.1). This may
mean that it is necessary to produce a new instance of the
router links advertisement.
(3) An attached network's Designated Router changes. A new
router links advertisement should be originated. Also, if
the router itself is now the Designated Router, a new
network links advertisement should be produced. If the
router itself is no longer the Designated Router, any
network links advertisement that it might have originated
for the network should be flushed from the routing domain
(see Section 14.1).
(4) One of the neighboring routers changes to/from the FULL
state. This may mean that it is necessary to produce a new
instance of the router links advertisement. Also, if the
router is itself the Designated Router for the attached
network, a new network links advertisement should be
produced.
The next four events concern area border routers only:
(5) An intra-area route has been added/deleted/modified in the
routing table. This may cause a new instance of a summary
links advertisement (for this route) to be originated in
each attached area (possibly including the backbone).
(6) An inter-area route has been added/deleted/modified in the
routing table. This may cause a new instance of a summary
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links advertisement (for this route) to be originated in
each attached area (but NEVER for the backbone).
(7) The router becomes newly attached to an area. The router
must then originate summary link advertisements into the
newly attached area for all pertinent intra-area and inter-
area routes in the router's routing table. See Section
12.4.3 for more details.
(8) When the state of one of the router's configured virtual
links changes, it may be necessary to originate a new router
links advertisement into the virtual link's transit area
(see the discussion of the router links advertisement's bit
V in Section 12.4.1), as well as originating a new router
links advertisement into the backbone.
The last two events concern AS boundary routers (and former AS
boundary routers) only:
(9) An external route gained through direct experience with an
external routing protocol (like EGP) changes. This will
cause an AS boundary router to originate a new instance of
an AS external link advertisement.
(10)
A router ceases to be an AS boundary router, perhaps after
restarting. In this situation the router should flush all AS
external link advertisements that it had previously
originated. These advertisements can be flushed via the
premature aging procedure specified in Section 14.1.
The construction of each type of link state advertisement is
explained in detail below. In general, these sections describe
the contents of the advertisement body (i.e., the part coming
after the 20-byte advertisement header). For information
concerning the building of the link state advertisement header,
see Section 12.1.
12.4.1. Router links
A router originates a router links advertisement for each
area that it belongs to. Such an advertisement describes
the collected states of the router's links to the area. The
advertisement is flooded throughout the particular area, and
no further.
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....................................
. 192.1.2 Area 1 .
. + .
. | .
. | 3+---+1 .
. N1 |--|RT1|-----+ .
. | +---+ .
. | _______N3 .
. + / . 1+---+
. * 192.1.1 *------|RT4|
. + /_______/ . +---+
. | / | .
. | 3+---+1 / | .
. N2 |--|RT2|-----+ 1| .
. | +---+ +---+8 . 6+---+
. | |RT3|----------------|RT6|
. + +---+ . +---+
. 192.1.3 |2 . 18.10.0.6|7
. | . |
. +------------+ .
. 192.1.4 (N4) .
....................................
Figure 15: Area 1 with IP addresses shown
The format of a router links advertisement is shown in
Appendix A (Section A.4.2). The first 20 bytes of the
advertisement consist of the generic link state
advertisement header that was discussed in Section 12.1.
Router links advertisements have LS type = 1. The router
indicates whether it is willing to calculate separate routes
for each IP TOS by setting (or resetting) the T-bit of the
link state advertisement's Options field.
A router also indicates whether it is an area border router,
or an AS boundary router, by setting the appropriate bits
(bit B and bit E, respectively) in its router links
advertisements. This enables paths to those types of routers
to be saved in the routing table, for later processing of
summary link advertisements and AS external link
advertisements. Bit B should be set whenever the router is
actively attached to two or more areas, even if the router
is not currently attached to the OSPF backbone area. Bit E
should never be set in a router links advertisement for a
stub area (stub areas cannot contain AS boundary routers).
In addition, the router sets bit V in its router links
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advertisement for Area A if and only if it is the endpoint
of an active virtual link using Area A as its Transit area.
This enables the other routers attached to Area A to
discover whether the area supports any virtual links (i.e.,
is a transit area).
The router links advertisement then describes the router's
working connections (i.e., interfaces or links) to the area.
Each link is typed according to the kind of attached
network. Each link is also labelled with its Link ID. This
Link ID gives a name to the entity that is on the other end
of the link. Table 18 summarizes the values used for the
Type and Link ID fields.
Link type Description Link ID
__________________________________________________
1 Point-to-point Neighbor Router ID
link
2 Link to transit Interface address of
network Designated Router
3 Link to stub IP network number
network
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the
router links advertisement.
In addition, the Link Data field is specified for each link.
This field gives 32 bits of extra information for the link.
For links to transit networks, numbered links to routers and
virtual links, this field specifies the IP interface address
of the associated router interface (this is needed by the
routing table calculation, see Section 16.1.1). For links
to stub networks, this field specifies the network's IP
address mask. For unnumbered point-to-point networks, the
Link Data field should be set to the unnumbered interface's
MIB-II [RFC 1213] ifIndex value.
Finally, the cost of using the link for output (possibly
specifying a different cost for each Type of Service) is
specified. The output cost of a link is configurable. It
must always be non-zero.
To further describe the process of building the list of link
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descriptions, suppose a router wishes to build a router
links advertisement for Area A. The router examines its
collection of interface data structures. For each
interface, the following steps are taken:
o If the attached network does not belong to Area A, no
links are added to the advertisement, and the next
interface should be examined.
o Else, if the state of the interface is Down, no links
are added.
o Else, if the state of the interface is Point-to-Point,
then add links according to the following:
- If the neighboring router is fully adjacent, add a
Type 1 link (point-to-point) if this is an interface
to a point-to-point network, or add a Type 4 link
(virtual link) if this is a virtual link. The Link
ID should be set to the Router ID of the neighboring
router. For virtual links and numbered point-to-
point networks, the Link Data should specify the IP
interface address. For unnumbered point-to-point
networks, the Link Data field should specify the
interface's MIB-II [RFC 1213] ifIndex value.
- If this is a numbered point-to-point network (i.e,
not a virtual link and not an unnumbered point-to-
point network) and the neighboring router's IP
address is known, add a Type 3 link (stub network)
whose Link ID is the neighbor's IP address, whose
Link Data is the mask 0xffffffff indicating a host
route, and whose cost is the interface's configured
output cost.
o Else if the state of the interface is Loopback, add a
Type 3 link (stub network) as long as this is not an
interface to an unnumbered serial line. The Link ID
should be set to the IP interface address, the Link Data
set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.
o Else if the state of the interface is Waiting, add a
Type 3 link (stub network) whose Link ID is the IP
network number of the attached network and whose Link
Data is the attached network's address mask.
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o Else, there has been a Designated Router selected for
the attached network. If the router is fully adjacent
to the Designated Router, or if the router itself is
Designated Router and is fully adjacent to at least one
other router, add a single Type 2 link (transit network)
whose Link ID is the IP interface address of the
attached network's Designated Router (which may be the
router itself) and whose Link Data is the router's own
IP interface address. Otherwise, add a link as if the
interface state were Waiting (see above).
Unless otherwise specified, the cost of each link generated
by the above procedure is equal to the output cost of the
associated interface. Note that in the case of serial
lines, multiple links may be generated by a single
interface.
After consideration of all the router interfaces, host links
are added to the advertisement by examining the list of
attached hosts. A host route is represented as a Type 3
link (stub network) whose Link ID is the host's IP address
and whose Link Data is the mask of all ones (0xffffffff).
As an example, consider the router links advertisements
generated by Router RT3, as pictured in Figure 6. The area
containing Router RT3 (Area 1) has been redrawn, with actual
network addresses, in Figure 15. Assume that the last byte
of all of RT3's interface addresses is 3, giving it the
interface addresses 192.1.1.3 and 192.1.4.3, and that the
other routers have similar addressing schemes. In addition,
assume that all links are functional, and that Router IDs
are assigned as the smallest IP interface address.
RT3 originates two router links advertisements, one for Area
1 and one for the backbone. Assume that Router RT4 has been
selected as the Designated router for network 192.1.1.0.
RT3's router links advertisement for Area 1 is then shown
below. It indicates that RT3 has two connections to Area 1,
the first a link to the transit network 192.1.1.0 and the
second a link to the stub network 192.1.4.0. Note that the
transit network is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is
the Designated Router RT4's IP interface to 192.1.1.0).
Note also that RT3 has indicated that it is capable of
calculating separate routes based on IP TOS, through setting
the T-bit in the Options field. It has also indicated that
it is an area border router.
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; RT3's router links advertisement for Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# other metrics = 0
TOS 0 metric = 1
Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# other metrics = 0
TOS 0 metric = 2
Next RT3's router links advertisement for the backbone is
shown. It indicates that RT3 has a single attachment to the
backbone. This attachment is via an unnumbered point-to-
point link to Router RT6. RT3 has again indicated that it
is TOS-capable, and that it is an area border router.
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's router ID
Advertising Router = 192.1.1.3 ;RT3's router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# other metrics = 0
TOS 0 metric = 8
Even though Router RT3 has indicated that it is TOS-capable
in the above examples, only a single metric (the TOS 0
metric) has been specified for each interface. Different
metrics can be specified for each TOS. The encoding of TOS
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in OSPF link state advertisements is described in Section
12.3.
As an example, suppose the point-to-point link between
Routers RT3 and RT6 in Figure 15 is a satellite link. The
AS administrator may want to encourage the use of the line
for high bandwidth traffic. This would be done by setting
the metric artificially low for the appropriate TOS value.
Router RT3 would then originate the following router links
advertisement for the backbone (TOS 8 = maximize
throughput):
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# other metrics = 1
TOS 0 metric = 8
TOS = 8 ;maximize throughput
metric = 1 ;traffic preferred
12.4.2. Network links
A network links advertisement is generated for every transit
multi-access network. (A transit network is a network
having two or more attached routers). The network links
advertisement describes all the routers that are attached to
the network.
The Designated Router for the network originates the
advertisement. The Designated Router originates the
advertisement only if it is fully adjacent to at least one
other router on the network. The network links
advertisement is flooded throughout the area that contains
the transit network, and no further. The networks links
advertisement lists those routers that are fully adjacent to
the Designated Router; each fully adjacent router is
identified by its OSPF Router ID. The Designated Router
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includes itself in this list.
The Link State ID for a network links advertisement is the
IP interface address of the Designated Router. This value,
masked by the network's address mask (which is also
contained in the network links advertisement) yields the
network's IP address.
A router that has formerly been the Designated Router for a
network, but is no longer, should flush the network links
advertisement that it had previously originated. This
advertisement is no longer used in the routing table
calculation. It is flushed by prematurely incrementing the
advertisement's age to MaxAge and reflooding (see Section
14.1). In addition, in those rare cases where a router's
Router ID has changed, any network links advertisements that
were originated with the router's previous Router ID must be
flushed. Since the router may have no idea what it's
previous Router ID might have been, these network links
advertisements are indicated by having their Link State ID
equal to one of the router's IP interface addresses and
their Advertising Router not equal to the router's current
Router ID (see Section 13.4 for more details).
As an example of a network links advertisement, again
consider the area configuration in Figure 6. Network links
advertisements are originated for Network N3 in Area 1,
Networks N6 and N8 in Area 2, and Network N9 in Area 3.
Assuming that Router RT4 has been selected as the Designated
Router for Network N3, the following network links
advertisement is generated by RT4 on behalf of Network N3
(see Figure 15 for the address assignments):
; network links advertisement for Network N3
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 2 ;indicates network links
Link State ID = 192.1.1.4 ;IP address of Desig. Rtr.
Advertising Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID
Attached Router = 192.1.1.1 ;Router ID
Attached Router = 192.1.1.2 ;Router ID
Attached Router = 192.1.1.3 ;Router ID
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12.4.3. Summary links
Each summary link advertisement describes a route to a
single destination. Summary link advertisements are flooded
throughout a single area only. The destination described is
one that is external to the area, yet still belonging to the
Autonomous System.
Summary link advertisements are originated by area border
routers. The precise summary routes to advertise into an
area are determined by examining the routing table structure
(see Section 11) in accordance with the algorithm described
below. Note that only intra-area routes are advertised into
the backbone, while both intra-area and inter-area routes
are advertised into the other areas.
To determine which routes to advertise into an attached Area
A, each routing table entry is processed as follows.
Remember that each routing table entry describes a set of
equal-cost best paths to a particular destination:
o Only Destination Types of network and AS boundary router
are advertised in summary link advertisements. If the
routing table entry's Destination Type is area border
router, examine the next routing table entry.
o AS external routes are never advertised in summary link
advertisements. If the routing table entry has Path-
type of type 1 external or type 2 external, examine the
next routing table entry.
o Else, if the area associated with this set of paths is
the Area A itself, do not generate a summary link
advertisement for the route.[14]
o Else, if the next hops associated with this set of paths
belong to Area A itself, do not generate a summary link
advertisement for the route.[15] This is the logical
equivalent of a Distance Vector protocol's split horizon
logic.
o Else, if the routing table cost equals or exceeds the
value LSInfinity, a summary link advertisement cannot be
generated for this route.
o Else, if the destination of this route is an AS boundary
router, generate a Type 4 link state advertisement for
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the destination, with Link State ID equal to the AS
boundary router's Router ID and metric equal to the
routing table entry's cost. These advertisements should
not be generated if Area A has been configured as a stub
area.
o Else, the Destination type is network. If this is an
inter-area route, generate a Type 3 advertisement for
the destination, with Link State ID equal to the
network's address (if necessary, the Link State ID can
also have one or more of the network's host bits set;
see Appendix F for details) and metric equal to the
routing table cost.
o The one remaining case is an intra-area route to a
network. This means that the network is contained in
one of the router's directly attached areas. In
general, this information must be condensed before
appearing in summary link advertisements. Remember that
an area has been defined as a list of address ranges,
each range consisting of an [address,mask] pair and a
status indication of either Advertise or DoNotAdvertise.
At most a single Type 3 advertisement is made for each
range. When the range's status indicates Advertise, a
Type 3 advertisement is generated with Link State ID
equal to the range's address (if necessary, the Link
State ID can also have one or more of the range's "host"
bits set; see Appendix F for details) and cost equal to
the smallest cost of any of the component networks. When
the range's status indicates DoNotAdvertise, the Type 3
advertisement is suppressed and the component networks
remain hidden from other areas.
By default, if a network is not contained in any
explicitly configured address range, a Type 3
advertisement is generated with Link State ID equal to
the network's address (if necessary, the Link State ID
can also have one or more of the network's "host" bits
set; see Appendix F for details) and metric equal to the
network's routing table cost.
If virtual links are being used to provide/increase
connectivity of the backbone, routing information
concerning the backbone networks should not be condensed
before being summarized into the virtual links' Transit
areas. Nor should the advertisement of backbone networks
into Transit areas be suppressed. In other words, the
backbone's configured ranges should be ignored when
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originating summary links into Transit areas. The
existence of virtual links is determined during the
shortest path calculation for the Transit areas (see
Section 16.1).
If a router advertises a summary advertisement for a
destination which then becomes unreachable, the router must
then flush the advertisement from the routing domain by
setting its age to MaxAge and reflooding (see Section 14.1).
Also, if the destination is still reachable, yet can no
longer be advertised according to the above procedure (e.g.,
it is now an inter-area route, when it used to be an intra-
area route associated with some non-backbone area; it would
thus no longer be advertisable to the backbone), the
advertisement should also be flushed from the routing
domain.
For an example of summary link advertisements, consider
again the area configuration in Figure 6. Routers RT3, RT4,
RT7, RT10 and RT11 are all area border routers, and
therefore are originating summary link advertisements.
Consider in particular Router RT4. Its routing table was
calculated as the example in Section 11.3. RT4 originates
summary link advertisements into both the backbone and Area
1. Into the backbone, Router RT4 originates separate
advertisements for each of the networks N1-N4. Into Area 1,
Router RT4 originates separate advertisements for networks
N6-N8 and the AS boundary routers RT5,RT7. It also
condenses host routes Ia and Ib into a single summary link
advertisement. Finally, the routes to networks N9,N10,N11
and Host H1 are advertised by a single summary link
advertisement. This condensation was originally performed
by the router RT11.
These advertisements are illustrated graphically in Figures
7 and 8. Two of the summary link advertisements originated
by Router RT4 follow. The actual IP addresses for the
networks and routers in question have been assigned in
Figure 15.
; summary link advertisement for Network N1,
; originated by Router RT4 into the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 3 ;summary link to IP net
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
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TOS = 0
metric = 4
; summary link advertisement for AS boundary router RT7
; originated by Router RT4 into Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 4 ;summary link to ASBR
Link State ID = Router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 14
Summary link advertisements pertain to a single destination
(IP network or AS boundary router). However, for a single
destination there may be separate sets of paths, and
therefore separate routing table entries, for each Type of
Service. All these entries must be considered when building
the summary link advertisement for the destination; a single
advertisement must specify the separate costs (if they
exist) for each TOS. The encoding of TOS in OSPF link state
advertisements is described in Section 12.3.
Clearing the T-bit in the Options field of a summary link
advertisement indicates that there is a TOS 0 path to the
destination, but no paths for non-zero TOS. This can happen
when non-TOS-capable routers exist in the routing domain
(see Section 2.4).
12.4.4. Originating summary links into stub areas
The algorithm in Section 12.4.3 is optional when Area A is
an OSPF stub area. Area border routers connecting to a stub
area can originate summary link advertisements into the area
according to the above Section's algorithm, or can choose to
originate only a subset of the advertisements, possibly
under configuration control. The fewer advertisements
originated, the smaller the stub area's link state database,
further reducing the demands on its routers' resources.
However, omitting advertisements may also lead to sub-
optimal inter-area routing, although routing will continue
to function.
As specified in Section 12.4.3, Type 4 link state
advertisements (ASBR summary links) are never originated
into stub areas.
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In a stub area, instead of importing external routes each
area border router originates a "default summary link" into
the area. The Link State ID for the default summary link is
set to DefaultDestination, and the metric set to the (per-
area) configurable parameter StubDefaultCost. Note that
StubDefaultCost need not be configured identically in all of
the stub area's area border routers.
12.4.5. AS external links
AS external link advertisements describe routes to
destinations external to the Autonomous System. Most AS
external link advertisements describe routes to specific
external destinations; in these cases the advertisement's
Link State ID is set to the destination network's IP address
(if necessary, the Link State ID can also have one or more
of the network's "host" bits set; see Appendix F for
details). However, a default route for the Autonomous
System can be described in an AS external link advertisement
by setting the advertisement's Link State ID to
DefaultDestination (0.0.0.0). AS external link
advertisements are originated by AS boundary routers. An AS
boundary router originates a single AS external link
advertisement for each external route that it has learned,
either through another routing protocol (such as EGP), or
through configuration information.
In general, AS external link advertisements are the only
type of link state advertisements that are flooded
throughout the entire Autonomous System; all other types of
link state advertisements are specific to a single area.
However, AS external link advertisements are not flooded
into/throughout stub areas (see Section 3.6). This enables
a reduction in link state database size for routers internal
to stub areas.
The metric that is advertised for an external route can be
one of two types. Type 1 metrics are comparable to the link
state metric. Type 2 metrics are assumed to be larger than
the cost of any intra-AS path. As with summary link
advertisements, if separate paths exist based on TOS,
separate TOS costs can be included in the AS external link
advertisement. The encoding of TOS in OSPF link state
advertisements is described in Section 12.3. If the T-bit
of the advertisement's Options field is clear, no non-zero
TOS paths to the destination exist.
If a router advertises an AS external link advertisement for
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a destination which then becomes unreachable, the router
must then flush the advertisement from the routing domain by
setting its age to MaxAge and reflooding (see Section 14.1).
For an example of AS external link advertisements, consider
once again the AS pictured in Figure 6. There are two AS
boundary routers: RT5 and RT7. Router RT5 originates three
external link advertisements, for networks N12-N14. Router
RT7 originates two external link advertisements, for
networks N12 and N15. Assume that RT7 has learned its route
to N12 via EGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the following
advertisement for N12:
; AS external link advertisement for Network N12,
; originated by Router RT7
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 2
Forwarding address = 0.0.0.0
In the above example, the forwarding address field has been
set to 0.0.0.0, indicating that packets for the external
destination should be forwarded to the advertising OSPF
router (RT7). This is not always desirable. Consider the
example pictured in Figure 16. There are three OSPF routers
(RTA, RTB and RTC) connected to a common network. Only one
of these routers, RTA, is exchanging EGP information with
the non-OSPF router RTX. RTA must then originate AS
external link advertisements for those destinations it has
learned from RTX. By using the AS external link
advertisement's forwarding address field, RTA can specify
that packets for these destinations be forwarded directly to
RTX. Without this feature, Routers RTB and RTC would take
an extra hop to get to these destinations.
Note that when the forwarding address field is non-zero, it
should point to a router belonging to another Autonomous
System.
A forwarding address can also be specified for the default
route. For example, in figure 16 RTA may want to specify
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that all externally-destined packets should by default be
forwarded to its EGP peer RTX. The resulting AS external
link advertisement is pictured below. Note that the Link
State ID is set to DefaultDestination.
; Default route, originated by Router RTA
; Packets forwarded through RTX
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange
EGP information with RTX. In this case, RTA and RTB would
originate the same set of AS external link advertisements.
These advertisements, if they specify the same metric, would
be functionally equivalent since they would specify the same
destination and forwarding address (RTX). This leads to a
clear duplication of effort. If only one of RTA or RTB
originated the set of external advertisements, the routing
would remain the same, and the size of the link state
database would decrease. However, it must be unambiguously
defined as to which router originates the advertisements
(otherwise neither may, or the identity of the originator
may oscillate). The following rule is thereby established:
if two routers, both reachable from one another, originate
functionally equivalent AS external advertisements (i.e.,
same destination, cost and non-zero forwarding address),
then the advertisement originated by the router having the
highest OSPF Router ID is used. The router having the lower
OSPF Router ID can then flush its advertisement. Flushing a
link state advertisement is discussed in Section 14.1.
13. The Flooding Procedure
Link State Update packets provide the mechanism for flooding link
state advertisements. A Link State Update packet may contain
several distinct advertisements, and floods each advertisement one
hop further from its point of origination. To make the flooding
procedure reliable, each advertisement must be acknowledged
separately. Acknowledgments are transmitted in Link State
Acknowledgment packets. Many separate acknowledgments can also be
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+
|
+---+.....|.EGP
|RTA|-----|.....+---+
+---+ |-----|RTX|
| +---+
+---+ |
|RTB|-----|
+---+ |
|
+---+ |
|RTC|-----|
+---+ |
|
+
Figure 16: Forwarding address example
grouped together into a single packet.
The flooding procedure starts when a Link State Update packet has
been received. Many consistency checks have been made on the
received packet before being handed to the flooding procedure (see
Section 8.2). In particular, the Link State Update packet has been
associated with a particular neighbor, and a particular area. If
the neighbor is in a lesser state than Exchange, the packet should
be dropped without further processing.
All types of link state advertisements, other than AS external link
advertisements, are associated with a specific area. However, link
state advertisements do not contain an area field. A link state
advertisement's area must be deduced from the Link State Update
packet header.
For each link state advertisement contained in the packet, the
following steps are taken:
(1) Validate the advertisement's LS checksum. If the checksum turns
out to be invalid, discard the advertisement and get the next
one from the Link State Update packet.
(2) Examine the link state advertisement's LS type. If the LS type
is unknown, discard the advertisement and get the next one from
the Link State Update Packet. This specification defines LS
types 1-5 (see Section 4.3).
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(3) Else if this is a AS external link advertisement (LS type = 5),
and the area has been configured as a stub area, discard the
advertisement and get the next one from the Link State Update
Packet. AS external link advertisements are not flooded
into/throughout stub areas (see Section 3.6).
(4) Else if the advertisement's LS age is equal to MaxAge, and there
is currently no instance of the advertisement in the router's
link state database, then take the following actions:
(a) Acknowledge the receipt of the advertisement by sending a
Link State Acknowledgment packet back to the sending
neighbor (see Section 13.5).
(b) Purge all outstanding requests for equal or previous
instances of the advertisement from the sending neighbor's
Link State Request list (see Section 10).
(c) If the sending neighbor is in state Exchange or in state
Loading, then install the MaxAge advertisement in the link
state database. Otherwise, simply discard the
advertisement. In either case, examine the next
advertisement (if any) listed in the Link State Update
packet.
(5) Otherwise, find the instance of this advertisement that is
currently contained in the router's link state database. If
there is no database copy, or the received advertisement is more
recent than the database copy (see Section 13.1 below for the
determination of which advertisement is more recent) the
following steps must be performed:
(a) If there is already a database copy, and if the database
copy was installed less than MinLSInterval seconds ago,
discard the new advertisement (without acknowledging it) and
examine the next advertisement (if any) listed in the Link
State Update packet.
(b) Otherwise immediately flood the new advertisement out some
subset of the router's interfaces (see Section 13.3). In
some cases (e.g., the state of the receiving interface is DR
and the advertisement was received from a router other than
the Backup DR) the advertisement will be flooded back out
the receiving interface. This occurrence should be noted
for later use by the acknowledgment process (Section 13.5).
(c) Remove the current database copy from all neighbors' Link
state retransmission lists.
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(d) Install the new advertisement in the link state database
(replacing the current database copy). This may cause the
routing table calculation to be scheduled. In addition,
timestamp the new advertisement with the current time (i.e.,
the time it was received). The flooding procedure cannot
overwrite the newly installed advertisement until
MinLSInterval seconds have elapsed. The advertisement
installation process is discussed further in Section 13.2.
(e) Possibly acknowledge the receipt of the advertisement by
sending a Link State Acknowledgment packet back out the
receiving interface. This is explained below in Section
13.5.
(f) If this new link state advertisement indicates that it was
originated by the receiving router itself (i.e., is
considered a self-originated advertisement), the router must
take special action, either updating the advertisement or in
some cases flushing it from the routing domain. For a
description of how self-originated advertisements are
detected and subsequently handled, see Section 13.4.
(6) Else, if there is an instance of the advertisement on the
sending neighbor's Link state request list, an error has
occurred in the Database Exchange process. In this case,
restart the Database Exchange process by generating the neighbor
event BadLSReq for the sending neighbor and stop processing the
Link State Update packet.
(7) Else, if the received advertisement is the same instance as the
database copy (i.e., neither one is more recent) the following
two steps should be performed:
(a) If the advertisement is listed in the Link state
retransmission list for the receiving adjacency, the router
itself is expecting an acknowledgment for this
advertisement. The router should treat the received
advertisement as an acknowledgment, by removing the
advertisement from the Link state retransmission list. This
is termed an "implied acknowledgment". Its occurrence
should be noted for later use by the acknowledgment process
(Section 13.5).
(b) Possibly acknowledge the receipt of the advertisement by
sending a Link State Acknowledgment packet back out the
receiving interface. This is explained below in Section
13.5.
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(8) Else, the database copy is more recent. Note an unusual event
to network management, discard the advertisement and process the
next link state advertisement contained in the Link State Update
packet.
13.1. Determining which link state is newer
When a router encounters two instances of a link state
advertisement, it must determine which is more recent. This
occurred above when comparing a received advertisement to its
database copy. This comparison must also be done during the
Database Exchange procedure which occurs during adjacency
bring-up.
A link state advertisement is identified by its LS type, Link
State ID and Advertising Router. For two instances of the same
advertisement, the LS sequence number, LS age, and LS checksum
fields are used to determine which instance is more recent:
o The advertisement having the newer LS sequence number is
more recent. See Section 12.1.6 for an explanation of the
LS sequence number space. If both instances have the same
LS sequence number, then:
o If the two instances have different LS checksums, then the
instance having the larger LS checksum (when considered as a
16-bit unsigned integer) is considered more recent.
o Else, if only one of the instances has its LS age field set
to MaxAge, the instance of age MaxAge is considered to be
more recent.
o Else, if the LS age fields of the two instances differ by
more than MaxAgeDiff, the instance having the smaller
(younger) LS age is considered to be more recent.
o Else, the two instances are considered to be identical.
13.2. Installing link state advertisements in the database
Installing a new link state advertisement in the database,
either as the result of flooding or a newly self-originated
advertisement, may cause the OSPF routing table structure to be
recalculated. The contents of the new advertisement should be
compared to the old instance, if present. If there is no
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difference, there is no need to recalculate the routing table.
(Note that even if the contents are the same, the LS checksum
will probably be different, since the checksum covers the LS
sequence number.)
If the contents are different, the following pieces of the
routing table must be recalculated, depending on the new
advertisement's LS type field:
Router links and network links advertisements
The entire routing table must be recalculated, starting with
the shortest path calculations for each area (not just the
area whose topological database has changed). The reason
that the shortest path calculation cannot be restricted to
the single changed area has to do with the fact that AS
boundary routers may belong to multiple areas. A change in
the area currently providing the best route may force the
router to use an intra-area route provided by a different
area.[16]
Summary link advertisements
The best route to the destination described by the summary
link advertisement must be recalculated (see Section 16.5).
If this destination is an AS boundary router, it may also be
necessary to re-examine all the AS external link
advertisements.
AS external link advertisements
The best route to the destination described by the AS
external link advertisement must be recalculated (see
Section 16.6).
Also, any old instance of the advertisement must be removed from
the database when the new advertisement is installed. This old
instance must also be removed from all neighbors' Link state
retransmission lists (see Section 10).
13.3. Next step in the flooding procedure
When a new (and more recent) advertisement has been received, it
must be flooded out some set of the router's interfaces. This
section describes the second part of flooding procedure (the
first part being the processing that occurred in Section 13),
namely, selecting the outgoing interfaces and adding the
advertisement to the appropriate neighbors' Link state
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retransmission lists. Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link
state request lists.
This section is equally applicable to the flooding of an
advertisement that the router itself has just originated (see
Section 12.4). For these advertisements, this section provides
the entirety of the flooding procedure (i.e., the processing of
Section 13 is not performed, since, for example, the
advertisement has not been received from a neighbor and
therefore does not need to be acknowledged).
Depending upon the advertisement's LS type, the advertisement
can be flooded out only certain interfaces. These interfaces,
defined by the following, are called the eligible interfaces:
AS external link advertisements (LS Type = 5)
AS external link advertisements are flooded throughout the
entire AS, with the exception of stub areas (see Section
3.6). The eligible interfaces are all the router's
interfaces, excluding virtual links and those interfaces
attaching to stub areas.
All other LS types
All other types are specific to a single area (Area A). The
eligible interfaces are all those interfaces attaching to
the Area A. If Area A is the backbone, this includes all
the virtual links.
Link state databases must remain synchronized over all
adjacencies associated with the above eligible interfaces. This
is accomplished by executing the following steps on each
eligible interface. It should be noted that this procedure may
decide not to flood a link state advertisement out a particular
interface, if there is a high probability that the attached
neighbors have already received the advertisement. However, in
these cases the flooding procedure must be absolutely sure that
the neighbors eventually do receive the advertisement, so the
advertisement is still added to each adjacency's Link state
retransmission list. For each eligible interface:
(1) Each of the neighbors attached to this interface are
examined, to determine whether they must receive the new
advertisement. The following steps are executed for each
neighbor:
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(a) If the neighbor is in a lesser state than Exchange, it
does not participate in flooding, and the next neighbor
should be examined.
(b) Else, if the adjacency is not yet full (neighbor state
is Exchange or Loading), examine the Link state request
list associated with this adjacency. If there is an
instance of the new advertisement on the list, it
indicates that the neighboring router has an instance of
the advertisement already. Compare the new
advertisement to the neighbor's copy:
o If the new advertisement is less recent, then
examine the next neighbor.
o If the two copies are the same instance, then delete
the advertisement from the Link state request list,
and examine the next neighbor.[17]
o Else, the new advertisement is more recent. Delete
the advertisement from the Link state request list.
(c) If the new advertisement was received from this
neighbor, examine the next neighbor.
(d) At this point we are not positive that the neighbor has
an up-to-date instance of this new advertisement. Add
the new advertisement to the Link state retransmission
list for the adjacency. This ensures that the flooding
procedure is reliable; the advertisement will be
retransmitted at intervals until an acknowledgment is
seen from the neighbor.
(2) The router must now decide whether to flood the new link
state advertisement out this interface. If in the previous
step, the link state advertisement was NOT added to any of
the Link state retransmission lists, there is no need to
flood the advertisement out the interface and the next
interface should be examined.
(3) If the new advertisement was received on this interface, and
it was received from either the Designated Router or the
Backup Designated Router, chances are that all the neighbors
have received the advertisement already. Therefore, examine
the next interface.
(4) If the new advertisement was received on this interface, and
the interface state is Backup (i.e., the router itself is
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the Backup Designated Router), examine the next interface.
The Designated Router will do the flooding on this
interface. If the Designated Router fails, this router will
end up retransmitting the updates.
(5) If this step is reached, the advertisement must be flooded
out the interface. Send a Link State Update packet (with
the new advertisement as contents) out the interface. The
advertisement's LS age must be incremented by InfTransDelay
(which must be > 0) when copied into the outgoing Link State
Update packet (until the LS age field reaches its maximum
value of MaxAge).
On broadcast networks, the Link State Update packets are
multicast. The destination IP address specified for the
Link State Update Packet depends on the state of the
interface. If the interface state is DR or Backup, the
address AllSPFRouters should be used. Otherwise, the
address AllDRouters should be used.
On non-broadcast, multi-access networks, separate Link State
Update packets must be sent, as unicasts, to each adjacent
neighbor (i.e., those in state Exchange or greater). The
destination IP addresses for these packets are the
neighbors' IP addresses.
13.4. Receiving self-originated link state
It is a common occurrence for a router to receive self-
originated link state advertisements via the flooding procedure.
A self-originated advertisement is detected when either 1) the
advertisement's Advertising Router is equal to the router's own
Router ID or 2) the advertisement is a network links
advertisement and its Link State ID is equal to one of the
router's own IP interface addresses.
However, if the received self-originated advertisement is newer
than the last instance that the router actually originated, the
router must take special action. The reception of such an
advertisement indicates that there are link state advertisements
in the routing domain that were originated before the last time
the router was restarted. In most cases, the router must then
advance the advertisement's LS sequence number one past the
received LS sequence number, and originate a new instance of the
advertisement.
It may be the case the router no longer wishes to originate the
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received advertisement. Possible examples include: 1) the
advertisement is a summary link or AS external link and the
router no longer has an (advertisable) route to the destination,
2) the advertisement is a network links advertisement but the
router is no longer Designated Router for the network or 3) the
advertisement is a network links advertisement whose Link State
ID is one of the router's own IP interface addresses but whose
Advertising Router is not equal to the router's own Router ID
(this latter case should be rare, and it indicates that the
router's Router ID has changed since originating the
advertisement). In all these cases, instead of updating the
advertisement, the advertisement should be flushed from the
routing domain by incrementing the received advertisement's LS
age to MaxAge and reflooding (see Section 14.1).
13.5. Sending Link State Acknowledgment packets
Each newly received link state advertisement must be
acknowledged. This is usually done by sending Link State
Acknowledgment packets. However, acknowledgments can also be
accomplished implicitly by sending Link State Update packets
(see step 7a of Section 13).
Many acknowledgments may be grouped together into a single Link
State Acknowledgment packet. Such a packet is sent back out the
interface that has received the advertisements. The packet can
be sent in one of two ways: delayed and sent on an interval
timer, or sent directly (as a unicast) to a particular neighbor.
The particular acknowledgment strategy used depends on the
circumstances surrounding the receipt of the advertisement.
Sending delayed acknowledgments accomplishes several things: it
facilitates the packaging of multiple acknowledgments in a
single Link State Acknowledgment packet; it enables a single
Link State Acknowledgment packet to indicate acknowledgments to
several neighbors at once (through multicasting); and it
randomizes the Link State Acknowledgment packets sent by the
various routers attached to a multi-access network. The fixed
interval between a router's delayed transmissions must be short
(less than RxmtInterval) or needless retransmissions will ensue.
Direct acknowledgments are sent to a particular neighbor in
response to the receipt of duplicate link state advertisements.
These acknowledgments are sent as unicasts, and are sent
immediately when the duplicate is received.
The precise procedure for sending Link State Acknowledgment
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packets is described in Table 19. The circumstances surrounding
the receipt of the advertisement are listed in the left column.
The acknowledgment action then taken is listed in one of the two
right columns. This action depends on the state of the
concerned interface; interfaces in state Backup behave
differently from interfaces in all other states. Delayed
acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment
packets as multicasts. The Destination IP address used depends
on the state of the interface. If the state is DR or Backup,
the destination AllSPFRouters is used. In other states, the
destination AllDRouters is used. On non-broadcast networks,
delayed Link State Acknowledgment packets must be unicast
separately over each adjacency (i.e., neighbor whose state is >=
Exchange).
The reasoning behind sending the above packets as multicasts is
best explained by an example. Consider the network
configuration depicted in Figure 15. Suppose RT4 has been
elected as Designated Router, and RT3 as Backup Designated
Router for the network N3. When Router RT4 floods a new
advertisement to Network N3, it is received by routers RT1, RT2,
and RT3. These routers will not flood the advertisement back
onto net N3, but they still must ensure that their topological
databases remain synchronized with their adjacent neighbors. So
RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3.
Likewise, RT4 and RT3 are both waiting to see acknowledgments
from RT1 and RT2. This is best achieved by sending the
acknowledgments as multicasts.
The reason that the acknowledgment logic for Backup DRs is
slightly different is because they perform differently during
the flooding of link state advertisements (see Section 13.3,
step 4).
13.6. Retransmitting link state advertisements
Advertisements flooded out an adjacency are placed on the
adjacency's Link state retransmission list. In order to ensure
that flooding is reliable, these advertisements are
retransmitted until they are acknowledged. The length of time
between retransmissions is a configurable per-interface value,
RxmtInterval. If this is set too low for an interface, needless
retransmissions will ensue. If the value is set too high, the
speed of the flooding, in the face of lost packets, may be
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Action taken in state
Circumstances Backup All other states
_______________________________________________________________
Advertisement has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
_______________________________________________________________
Advertisement is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received
was not flooded from Designated
back out receiving Router, otherwise
interface do nothing
_______________________________________________________________
Advertisement is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent.
treated as an im- tisement received
plied acknowledg- from Designated
ment (see Section Router, otherwise
13, step 7a). do nothing
_______________________________________________________________
Advertisement is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
_______________________________________________________________
Advertisement's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent.
MaxAge, and there is
no current instance
of the advertisement
in the link state
database (see
Section 13, step 4).
Table 19: Sending link state acknowledgements.
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affected.
Several retransmitted advertisements may fit into a single Link
State Update packet. When advertisements are to be
retransmitted, only the number fitting in a single Link State
Update packet should be transmitted. Another packet of
retransmissions can be sent when some of the advertisements are
acknowledged, or on the next firing of the retransmission timer.
Link State Update Packets carrying retransmissions are always
sent as unicasts (directly to the physical address of the
neighbor). They are never sent as multicasts. Each
advertisement's LS age must be incremented by InfTransDelay
(which must be > 0) when copied into the outgoing Link State
Update packet (until the LS age field reaches its maximum value
of MaxAge).
If the adjacent router goes down, retransmissions may occur
until the adjacency is destroyed by OSPF's Hello Protocol. When
the adjacency is destroyed, the Link state retransmission list
is cleared.
13.7. Receiving link state acknowledgments
Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding
procedure. In particular, it has been associated with a
particular neighbor. If this neighbor is in a lesser state than
Exchange, the Link State Acknowledgment packet is discarded.
Otherwise, for each acknowledgment in the Link State
Acknowledgment packet, the following steps are performed:
o Does the advertisement acknowledged have an instance on the
Link state retransmission list for the neighbor? If not,
examine the next acknowledgment. Otherwise:
o If the acknowledgment is for the same instance that is
contained on the list, remove the item from the list and
examine the next acknowledgment. Otherwise:
o Log the questionable acknowledgment, and examine the next
one.
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14. Aging The Link State Database
Each link state advertisement has an LS age field. The LS age is
expressed in seconds. An advertisement's LS age field is
incremented while it is contained in a router's database. Also,
when copied into a Link State Update Packet for flooding out a
particular interface, the advertisement's LS age is incremented by
InfTransDelay.
An advertisement's LS age is never incremented past the value
MaxAge. Advertisements having age MaxAge are not used in the
routing table calculation. As a router ages its link state
database, an advertisement's LS age may reach MaxAge.[18] At this
time, the router must attempt to flush the advertisement from the
routing domain. This is done simply by reflooding the MaxAge
advertisement just as if it was a newly originated advertisement
(see Section 13.3).
When creating a Database summary list for a newly forming adjacency,
any MaxAge advertisements present in the link state database are
added to the neighbor's Link state retransmission list instead of
the neighbor's Database summary list. See Section 10.3 for more
details.
A MaxAge advertisement must be removed immediately from the router's
link state database as soon as both a) it is no longer contained on
any neighbor Link state retransmission lists and b) none of the
router's neighbors are in states Exchange or Loading.
When, in the process of aging the link state database, an
advertisement's LS age hits a multiple of CheckAge, its LS checksum
should be verified. If the LS checksum is incorrect, a program or
memory error has been detected, and at the very least the router
itself should be restarted.
14.1. Premature aging of advertisements
A link state advertisement can be flushed from the routing
domain by setting its LS age to MaxAge and reflooding the
advertisement. This procedure follows the same course as
flushing an advertisement whose LS age has naturally reached the
value MaxAge (see Section 14). In particular, the MaxAge
advertisement is removed from the router's link state database
as soon as a) it is no longer contained on any neighbor Link
state retransmission lists and b) none of the router's neighbors
are in states Exchange or Loading. We call the setting of an
advertisement's LS age to MaxAge premature aging.
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Premature aging is used when it is time for a self-originated
advertisement's sequence number field to wrap. At this point,
the current advertisement instance (having LS sequence number of
0x7fffffff) must be prematurely aged and flushed from the
routing domain before a new instance with sequence number
0x80000001 can be originated. See Section 12.1.6 for more
information.
Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its
external advertisement from the routing domain via premature
aging. This procedure is preferable to the alternative, which is
to originate a new advertisement for the destination specifying
a metric of LSInfinity. Premature aging is also be used when
unexpectedly receiving self-originated advertisements during the
flooding procedure (see Section 13.4).
A router may only prematurely age its own self-originated link
state advertisements. The router may not prematurely age
advertisements that have been originated by other routers. An
advertisement is considered self-originated when either 1) the
advertisement's Advertising Router is equal to the router's own
Router ID or 2) the advertisement is a network links
advertisement and its Link State ID is equal to one of the
router's own IP interface addresses.
15. Virtual Links
The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
or some areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can
be configured through non-backbone areas. Virtual links serve to
connect physically separate components of the backbone. The two
endpoints of a virtual link are area border routers. The virtual
link must be configured in both routers. The configuration
information in each router consists of the other virtual endpoint
(the other area border router), and the non-backbone area the two
routers have in common (called the transit area). Virtual links
cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-
point network (belonging to the backbone) joining the two area
border routers. An attempt is made to establish an adjacency over
the virtual link. When this adjacency is established, the virtual
link will be included in backbone router links advertisements, and
OSPF packets pertaining to the backbone area will flow over the
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adjacency. Such an adjacency has been referred to in this document
as a "virtual adjacency".
In each endpoint router, the cost and viability of the virtual link
is discovered by examining the routing table entry for the other
endpoint router. (The entry's associated area must be the
configured transit area). Actually, there may be a separate routing
table entry for each Type of Service. These are called the virtual
link's corresponding routing table entries. The InterfaceUp event
occurs for a virtual link when its corresponding TOS 0 routing table
entry becomes reachable. Conversely, the InterfaceDown event occurs
when its TOS 0 routing table entry becomes unreachable.[19] In other
words, the virtual link's viability is determined by the existence
of an intra-area path, through the transit area, between the two
endpoints. Note that a virtual link whose underlying path has cost
greater than hexadecimal 0xffff (the maximum size of an interface
cost in a router links advertisement) should be considered
inoperational (i.e., treated the same as if the path did not exist).
The other details concerning virtual links are as follows:
o AS external links are NEVER flooded over virtual adjacencies.
This would be duplication of effort, since the same AS external
links are already flooded throughout the virtual link's transit
area. For this same reason, AS external link advertisements are
not summarized over virtual adjacencies during the Database
Exchange process.
o The cost of a virtual link is NOT configured. It is defined to
be the cost of the intra-area path between the two defining area
border routers. This cost appears in the virtual link's
corresponding routing table entry. When the cost of a virtual
link changes, a new router links advertisement should be
originated for the backbone area.
o Just as the virtual link's cost and viability are determined by
the routing table build process (through construction of the
routing table entry for the other endpoint), so are the IP
interface address for the virtual interface and the virtual
neighbor's IP address. These are used when sending OSPF
protocol packets over the virtual link. Note that when one (or
both) of the virtual link endpoints connect to the transit area
via an unnumbered point-to-point link, it may be impossible to
calculate either the virtual interface's IP address and/or the
virtual neighbor's IP address, thereby causing the virtual link
to fail.
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o In each endpoint's router links advertisement for the backbone,
the virtual link is represented as a Type 4 link whose Link ID
is set to the virtual neighbor's OSPF Router ID and whose Link
Data is set to the virtual interface's IP address. See Section
12.4.1 for more information. Note that it may be the case that
there is a TOS 0 path, but no non-zero TOS paths, between the
two endpoint routers. In this case, both routers must revert to
being non-TOS-capable, clearing the T-bit in the Options field
of their backbone router links advertisements.
o When virtual links are configured for the backbone, information
concerning backbone networks should not be condensed before
being summarized for the transit areas. In other words, each
backbone network should be advertised into the transit areas in
a separate summary link advertisement, regardless of the
backbone's configured area address ranges. See Section 12.4.3
for more information.
o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers. This may be
hard to estimate for a virtual link; it is better to err on the
side of making it too large.
16. Calculation Of The Routing Table
This section details the OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step. At
each step, the router must access individual pieces of the link
state databases (e.g., a router links advertisement originated by a
certain router). This access is performed by the lookup function
discussed in Section 12.2. The lookup process may return a link
state advertisement whose LS age is equal to MaxAge. Such an
advertisement should not be used in the routing table calculation,
and is treated just as if the lookup process had failed.
The OSPF routing table's organization is explained in Section 11.
Two examples of the routing table build process are presented in
Sections 11.2 and 11.3. This process can be broken into the
following steps:
(1) The present routing table is invalidated. The routing table is
built again from scratch. The old routing table is saved so
that changes in routing table entries can be identified.
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(2) The intra-area routes are calculated by building the shortest-
path tree for each attached area. In particular, all routing
table entries whose Destination Type is "area border router" are
calculated in this step. This step is described in two parts.
At first the tree is constructed by only considering those links
between routers and transit networks. Then the stub networks
are incorporated into the tree. During the area's shortest-path
tree calculation, the area's TransitCapability is also
calculated for later use in Step 4.
(3) The inter-area routes are calculated, through examination of
summary link advertisements. If the router is attached to
multiple areas (i.e., it is an area border router), only
backbone summary link advertisements are examined.
(4) In area border routers connecting to one or more transit areas
(i.e, non-backbone areas whose TransitCapability is found to be
TRUE), the transit areas' summary link advertisements are
examined to see whether better paths exist using the transit
areas than were found in Steps 2-3 above.
(5) Routes to external destinations are calculated, through
examination of AS external link advertisements. The locations
of the AS boundary routers (which originate the AS external link
advertisements) have been determined in steps 2-4.
Steps 2-5 are explained in further detail below. The explanations
describe the calculations for TOS 0 only. It may also be necessary
to perform each step (separately) for each of the non-zero TOS
values.[20] For more information concerning the building of non-zero
TOS routes see Section 16.9.
Changes made to routing table entries as a result of these
calculations can cause the OSPF protocol to take further actions.
For example, a change to an intra-area route will cause an area
border router to originate new summary link advertisements (see
Section 12.4). See Section 16.7 for a complete list of the OSPF
protocol actions resulting from routing table changes.
16.1. Calculating the shortest-path tree for an area
This calculation yields the set of intra-area routes associated
with an area (called hereafter Area A). A router calculates the
shortest-path tree using itself as the root.[21] The formation
of the shortest path tree is done here in two stages. In the
first stage, only links between routers and transit networks are
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considered. Using the Dijkstra algorithm, a tree is formed from
this subset of the link state database. In the second stage,
leaves are added to the tree by considering the links to stub
networks.
The procedure will be explained using the graph terminology that
was introduced in Section 2. The area's link state database is
represented as a directed graph. The graph's vertices are
routers, transit networks and stub networks. The first stage of
the procedure concerns only the transit vertices (routers and
transit networks) and their connecting links. Throughout the
shortest path calculation, the following data is also associated
with each transit vertex:
Vertex (node) ID
A 32-bit number uniquely identifying the vertex. For router
vertices this is the router's OSPF Router ID. For network
vertices, this is the IP address of the network's Designated
Router.
A link state advertisement
Each transit vertex has an associated link state
advertisement. For router vertices, this is a router links
advertisement. For transit networks, this is a network
links advertisement (which is actually originated by the
network's Designated Router). In any case, the
advertisement's Link State ID is always equal to the above
Vertex ID.
List of next hops
The list of next hops for the current set of shortest paths
from the root to this vertex. There can be multiple
shortest paths due to the equal-cost multipath capability.
Each next hop indicates the outgoing router interface to use
when forwarding traffic to the destination. On multi-access
networks, the next hop also includes the IP address of the
next router (if any) in the path towards the destination.
Distance from root
The link state cost of the current set of shortest paths
from the root to the vertex. The link state cost of a path
is calculated as the sum of the costs of the path's
constituent links (as advertised in router links and network
links advertisements). One path is said to be "shorter"
than another if it has a smaller link state cost.
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The first stage of the procedure (i.e., the Dijkstra algorithm)
can now be summarized as follows. At each iteration of the
algorithm, there is a list of candidate vertices. Paths from
the root to these vertices have been found, but not necessarily
the shortest ones. However, the paths to the candidate vertex
that is closest to the root are guaranteed to be shortest; this
vertex is added to the shortest-path tree, removed from the
candidate list, and its adjacent vertices are examined for
possible addition to/modification of the candidate list. The
algorithm then iterates again. It terminates when the candidate
list becomes empty.
The following steps describe the algorithm in detail. Remember
that we are computing the shortest path tree for Area A. All
references to link state database lookup below are from Area A's
database.
(1) Initialize the algorithm's data structures. Clear the list
of candidate vertices. Initialize the shortest-path tree to
only the root (which is the router doing the calculation).
Set Area A's TransitCapability to FALSE.
(2) Call the vertex just added to the tree vertex V. Examine
the link state advertisement associated with vertex V. This
is a lookup in the Area A's link state database based on the
Vertex ID. If this is a router links advertisement, and bit
V of the router links advertisement (see Section A.4.2) is
set, set Area A's TransitCapability to TRUE. In any case,
each link described by the advertisement gives the cost to
an adjacent vertex. For each described link, (say it joins
vertex V to vertex W):
(a) If this is a link to a stub network, examine the next
link in V's advertisement. Links to stub networks will
be considered in the second stage of the shortest path
calculation.
(b) Otherwise, W is a transit vertex (router or transit
network). Look up the vertex W's link state
advertisement (router links or network links) in Area
A's link state database. If the advertisement does not
exist, or its LS age is equal to MaxAge, or it does not
have a link back to vertex V, examine the next link in
V's advertisement.[22]
(c) If vertex W is already on the shortest-path tree,
examine the next link in the advertisement.
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(d) Calculate the link state cost D of the resulting path
from the root to vertex W. D is equal to the sum of the
link state cost of the (already calculated) shortest
path to vertex V and the advertised cost of the link
between vertices V and W. If D is:
o Greater than the value that already appears for
vertex W on the candidate list, then examine the
next link.
o Equal to the value that appears for vertex W on the
candidate list, calculate the set of next hops that
result from using the advertised link. Input to
this calculation is the destination (W), and its
parent (V). This calculation is shown in Section
16.1.1. This set of hops should be added to the
next hop values that appear for W on the candidate
list.
o Less than the value that appears for vertex W on the
candidate list, or if W does not yet appear on the
candidate list, then set the entry for W on the
candidate list to indicate a distance of D from the
root. Also calculate the list of next hops that
result from using the advertised link, setting the
next hop values for W accordingly. The next hop
calculation is described in Section 16.1.1; it takes
as input the destination (W) and its parent (V).
(3) If at this step the candidate list is empty, the shortest-
path tree (of transit vertices) has been completely built
and this stage of the procedure terminates. Otherwise,
choose the vertex belonging to the candidate list that is
closest to the root, and add it to the shortest-path tree
(removing it from the candidate list in the process). Note
that when there is a choice of vertices closest to the root,
network vertices must be chosen before router vertices in
order to necessarily find all equal-cost paths. This is
consistent with the tie-breakers that were introduced in the
modified Dijkstra algorithm used by OSPF's Multicast routing
extensions (MOSPF).
(4) Possibly modify the routing table. For those routing table
entries modified, the associated area will be set to Area A,
the path type will be set to intra-area, and the cost will
be set to the newly discovered shortest path's calculated
distance.
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If the newly added vertex is an area border router (call it
ABR), a routing table entry is added whose destination type
is "area border router". The Options field found in the
associated router links advertisement is copied into the
routing table entry's Optional capabilities field. If in
addition ABR is the endpoint of one of the calculating
router's configured virtual links that uses Area A as its
Transit area: the virtual link is declared up, the IP
address of the virtual interface is set to the IP address of
the outgoing interface calculated above for ABR, and the
virtual neighbor's IP address is set to the ABR interface
address (contained in ABR's router links advertisement) that
points back to the root of the shortest-path tree;
equivalently, this is the interface that points back to
ABR's parent vertex on the shortest-path tree (similar to
the calculation in Section 16.1.1).
If the newly added vertex is an AS boundary router, the
routing table entry of type "AS boundary router" for the
destination is located. Since routers can belong to more
than one area, it is possible that several sets of intra-
area paths exist to the AS boundary router, each set using a
different area. However, the AS boundary router's routing
table entry must indicate a set of paths which utilize a
single area. The area leading to the routing table entry is
selected as follows: The area providing the shortest path is
always chosen; if more than one area provides paths with the
same minimum cost, the area with the largest OSPF Area ID
(when considered as an unsigned 32-bit integer) is chosen.
Note that whenever an AS boundary router's routing table
entry is added/modified, the Options found in the associated
router links advertisement is copied into the routing table
entry's Optional capabilities field.
If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
Destination ID is the IP network number, which can be
obtained by masking the Vertex ID (Link State ID) with its
associated subnet mask (found in the body of the associated
network links advertisement). If the routing table entry
already exists (i.e., there is already an intra-area route
to the destination installed in the routing table), multiple
vertices have mapped to the same IP network. For example,
this can occur when a new Designated Router is being
established. In this case, the current routing table entry
should be overwritten if and only if the newly found path is
just as short and the current routing table entry's Link
State Origin has a smaller Link State ID than the newly
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added vertex' link state advertisement.
If there is no routing table entry for the network (the
usual case), a routing table entry for the IP network should
be added. The routing table entry's Link State Origin
should be set to the newly added vertex' link state
advertisement.
(5) Iterate the algorithm by returning to Step 2.
The stub networks are added to the tree in the procedure's
second stage. In this stage, all router vertices are again
examined. Those that have been determined to be unreachable in
the above first phase are discarded. For each reachable router
vertex (call it V), the associated router links advertisement is
found in the link state database. Each stub network link
appearing in the advertisement is then examined, and the
following steps are executed:
(1) Calculate the distance D of stub network from the root. D
is equal to the distance from the root to the router vertex
(calculated in stage 1), plus the stub network link's
advertised cost. Compare this distance to the current best
cost to the stub network. This is done by looking up the
stub network's current routing table entry. If the
calculated distance D is larger, go on to examine the next
stub network link in the advertisement.
(2) If this step is reached, the stub network's routing table
entry must be updated. Calculate the set of next hops that
would result from using the stub network link. This
calculation is shown in Section 16.1.1; input to this
calculation is the destination (the stub network) and the
parent vertex (the router vertex). If the distance D is the
same as the current routing table cost, simply add this set
of next hops to the routing table entry's list of next hops.
In this case, the routing table already has a Link State
Origin. If this Link State Origin is a router links
advertisement whose Link State ID is smaller than V's Router
ID, reset the Link State Origin to V's router links
advertisement.
Otherwise D is smaller than the routing table cost.
Overwrite the current routing table entry by setting the
routing table entry's cost to D, and by setting the entry's
list of next hops to the newly calculated set. Set the
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routing table entry's Link State Origin to V's router links
advertisement. Then go on to examine the next stub network
link.
For all routing table entries added/modified in the second
stage, the associated area will be set to Area A and the path
type will be set to intra-area. When the list of reachable
router links is exhausted, the second stage is completed. At
this time, all intra-area routes associated with Area A have
been determined.
The specification does not require that the above two stage
method be used to calculate the shortest path tree. However, if
another algorithm is used, an identical tree must be produced.
For this reason, it is important to note that links between
transit vertices must be bidirectional in ordered to be included
in the above tree. It should also be mentioned that more
efficient algorithms exist for calculating the tree; for
example, the incremental SPF algorithm described in [BBN].
16.1.1. The next hop calculation
This section explains how to calculate the current set of
next hops to use for a destination. Each next hop consists
of the outgoing interface to use in forwarding packets to
the destination together with the next hop router (if any).
The next hop calculation is invoked each time a shorter path
to the destination is discovered. This can happen in either
stage of the shortest-path tree calculation (see Section
16.1). In stage 1 of the shortest-path tree calculation a
shorter path is found as the destination is added to the
candidate list, or when the destination's entry on the
candidate list is modified (Step 2d of Stage 1). In stage 2
a shorter path is discovered each time the destination's
routing table entry is modified (Step 2 of Stage 2).
The set of next hops to use for the destination may be
recalculated several times during the shortest-path tree
calculation, as shorter and shorter paths are discovered.
In the end, the destination's routing table entry will
always reflect the next hops resulting from the absolute
shortest path(s).
Input to the next hop calculation is a) the destination and
b) its parent in the current shortest path between the root
(the calculating router) and the destination. The parent is
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always a transit vertex (i.e., always a router or a transit
network).
If there is at least one intervening router in the current
shortest path between the destination and the root, the
destination simply inherits the set of next hops from the
parent. Otherwise, there are two cases. In the first case,
the parent vertex is the root (the calculating router
itself). This means that the destination is either a
directly connected network or directly connected router.
The next hop in this case is simply the OSPF interface
connecting to the network/router; no next hop router is
required. If the connecting OSPF interface in this case is a
virtual link, the setting of the next hop should be deferred
until the calculation in Section 16.3.
In the second case, the parent vertex is a network that
directly connects the calculating router to the destination
router. The list of next hops is then determined by
examining the destination's router links advertisement. For
each link in the advertisement that points back to the
parent network, the link's Link Data field provides the IP
address of a next hop router. The outgoing interface to use
can then be derived from the next hop IP address (or it can
be inherited from the parent network).
16.2. Calculating the inter-area routes
The inter-area routes are calculated by examining summary link
advertisements. If the router has active attachments to
multiple areas, only backbone summary link advertisements are
examined. Routers attached to a single area examine that area's
summary links. In either case, the summary links examined below
are all part of a single area's link state database (call it
Area A).
Summary link advertisements are originated by the area border
routers. Each summary link advertisement in Area A is
considered in turn. Remember that the destination described by
a summary link advertisement is either a network (Type 3 summary
link advertisements) or an AS boundary router (Type 4 summary
link advertisements). For each summary link advertisement:
(1) If the cost specified by the advertisement is LSInfinity, or
if the advertisement's LS age is equal to MaxAge, then
examine the the next advertisement.
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(2) If the advertisement was originated by the calculating
router itself, examine the next advertisement.
(3) If the collection of destinations described by the summary
link advertisement falls into one of the router's configured
area address ranges (see Section 3.5) and the particular
area address range is active, the summary link advertisement
should be ignored. Active means that there are one or more
reachable (by intra-area paths) networks contained in the
area range. In this case, all addresses in the area range
are assumed to be either reachable via intra-area paths, or
else to be unreachable by any other means.
(4) Else, call the destination described by the advertisement N
(for Type 3 summary links, N's address is obtained by
masking the advertisement's Link State ID with the
network/subnet mask contained in the body of the
advertisement), and the area border originating the
advertisement BR. Look up the routing table entry for BR
having Area A as its associated area. If no such entry
exists for router BR (i.e., BR is unreachable in Area A), do
nothing with this advertisement and consider the next in the
list. Else, this advertisement describes an inter-area path
to destination N, whose cost is the distance to BR plus the
cost specified in the advertisement. Call the cost of this
inter-area path IAC.
(5) Next, look up the routing table entry for the destination N.
(The entry's Destination Type is either Network or AS
boundary router.) If no entry exists for N or if the
entry's path type is "type 1 external" or "type 2 external",
then install the inter-area path to N, with associated area
Area A, cost IAC, next hop equal to the list of next hops to
router BR, and Advertising router equal to BR.
(6) Else, if the paths present in the table are intra-area
paths, do nothing with the advertisement (intra-area paths
are always preferred).
(7) Else, the paths present in the routing table are also
inter-area paths. Install the new path through BR if it is
cheaper, overriding the paths in the routing table.
Otherwise, if the new path is the same cost, add it to the
list of paths that appear in the routing table entry.
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16.3. Examining transit areas' summary links
This step is only performed by area border routers attached to
one or more transit areas. Transit areas are those areas
supporting one or more virtual links; their TransitCapability
parameter has been set to TRUE in Step 2 of the Dijkstra
algorithm (see Section 16.1). They are the only non-backbone
areas that can carry data traffic that neither originates nor
terminates in the area itself.
The purpose of the calculation below is to examine the transit
areas to see whether they provide any better (shorter) paths
than the paths previously calculated in Sections 16.1 and 16.2.
Any paths found that are better than or equal to previously
discovered paths are installed in the routing table.
The calculation proceeds as follows. All the transit areas'
summary link advertisements are examined in turn. Each such
summary link advertisement describes a route through a transit
area Area A to a Network N (N's address is obtained by masking
the advertisement's Link State ID with the network/subnet mask
contained in the body of the advertisement) or in the case of a
Type 4 summary link advertisement, to an AS boundary router N.
Suppose also that the summary link advertisement was originated
by an area border router BR.
(1) If the cost advertised by the summary link advertisement is
LSInfinity, or if the advertisement's LS age is equal to
MaxAge, then examine the next advertisement.
(2) If the summary link advertisement was originated by the
calculating router itself, examine the next advertisement.
(3) Look up the routing table entry for N. If it does not exist,
or if the route type is other than intra-area or inter-area,
or if the area associated with the routing table entry is
not the backbone area, then examine the next advertisement.
In other words, this calculation only updates backbone
intra-area routes found in Section 16.1 and inter-area
routes found in Section 16.2.
(4) Look up the routing table entry for the advertising router
BR associated with the Area A. If it is unreachable, examine
the next advertisement. Otherwise, the cost to destination N
is the sum of the cost in BR's Area A routing table entry
and the cost advertised in the advertisement. Call this cost
IAC.
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(5) If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC
is the same as N's current cost, add BR's list of next hops
to N's list of next hops. In any case, the area associated
with N's routing table entry must remain the backbone area,
and the path type (either intra-area or inter-area) must
also remain the same.
It is important to note that the above calculation never makes
unreachable destinations reachable, but instead just potentially
finds better paths to already reachable destinations. Also,
unlike Section 16.3 of [RFC 1247], the above calculation
installs any better cost found into the routing table entry,
from which it may be readvertised in summary link advertisements
to other areas.
As an example of the calculation, consider the Autonomous System
pictured in Figure 17. There is a single non-backbone area
(Area 1) that physically divides the backbone into two separate
pieces. To maintain connectivity of the backbone, a virtual link
has been configured between routers RT1 and RT4. On the right
side of the figure, Network N1 belongs to the backbone. The
dotted lines indicate that there is a much shorter intra-area
........................
. Area 1 (transit) . +
. . |
. +---+1 1+---+100 |
. |RT2|----------|RT4|=========|
. 1/+---+********* +---+ |
. /******* . |
. 1/*Virtual . |
1+---+/* Link . Net|work
=======|RT1|* . | N1
+---+\ . |
. \ . |
. \ . |
. 1\+---+1 1+---+20 |
. |RT3|----------|RT5|=========|
. +---+ +---+ |
. . |
........................ +
Figure 17: Routing through transit areas
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backbone path between router RT5 and Network N1 (cost 20) than
there is between Router RT4 and Network N1 (cost 100). Both
Router RT4 and Router RT5 will inject summary link
advertisements for Network N1 into Area 1.
After the shortest-path tree has been calculated for the
backbone in Section 16.1, Router RT1 (left end of the virtual
link) will have calculated a path through Router RT4 for all
data traffic destined for Network N1. However, since Router RT5
is so much closer to Network N1, all routers internal to Area 1
(e.g., Routers RT2 and RT3) will forward their Network N1
traffic towards Router RT5, instead of RT4. And indeed, after
examining Area 1's summary link advertisements by the above
calculation, Router RT1 will also forward Network N1 traffic
towards RT5. Note that in this example the virtual link enables
Network N1 traffic to be forwarded through the transit area Area
1, but the actual path the data traffic takes does not follow
the virtual link. In other words, virtual links allow transit
traffic to be forwarded through an area, but do not dictate the
precise path that the traffic will take.
16.4. Calculating AS external routes
AS external routes are calculated by examining AS external link
advertisements. Each of the AS external link advertisements is
considered in turn. Most AS external link advertisements
describe routes to specific IP destinations. An AS external
link advertisement can also describe a default route for the
Autonomous System (Destination ID = DefaultDestination,
network/subnet mask = 0x00000000). For each AS external link
advertisement:
(1) If the cost specified by the advertisement is LSInfinity, or
if the advertisement's LS age is equal to MaxAge, then
examine the next advertisement.
(2) If the advertisement was originated by the calculating
router itself, examine the next advertisement.
(3) Call the destination described by the advertisement N. N's
address is obtained by masking the advertisement's Link
State ID with the network/subnet mask contained in the body
of the advertisement. Look up the routing table entry for
the AS boundary router (ASBR) that originated the
advertisement. If no entry exists for router ASBR (i.e.,
ASBR is unreachable), do nothing with this advertisement and
consider the next in the list.
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Else, this advertisement describes an AS external path to
destination N. Examine the forwarding address specified in
the AS external link advertisement. This indicates the IP
address to which packets for the destination should be
forwarded. If the forwarding address is set to 0.0.0.0,
packets should be sent to the ASBR itself. Otherwise, look
up the forwarding address in the routing table.[23] An
intra-area or inter-area path must exist to the forwarding
address. If no such path exists, do nothing with the
advertisement and consider the next in the list.
Call the routing table distance to the forwarding address X
(when the forwarding address is set to 0.0.0.0, this is the
distance to the ASBR itself), and the cost specified in the
advertisement Y. X is in terms of the link state metric,
and Y is a type 1 or 2 external metric.
(4) Next, look up the routing table entry for the destination N.
If no entry exists for N, install the AS external path to N,
with next hop equal to the list of next hops to the
forwarding address, and advertising router equal to ASBR.
If the external metric type is 1, then the path-type is set
to type 1 external and the cost is equal to X+Y. If the
external metric type is 2, the path-type is set to type 2
external, the link state component of the route's cost is X,
and the type 2 cost is Y.
(5) Else, if the paths present in the table are not type 1 or
type 2 external paths, do nothing (AS external paths have
the lowest priority).
(6) Otherwise, compare the cost of this new AS external path to
the ones present in the table. Type 1 external paths are
always shorter than type 2 external paths. Type 1 external
paths are compared by looking at the sum of the distance to
the forwarding address and the advertised type 1 metric
(X+Y). Type 2 external paths are compared by looking at the
advertised type 2 metrics, and then if necessary, the
distance to the forwarding addresses.
If the new path is shorter, it replaces the present paths in
the routing table entry. If the new path is the same cost,
it is added to the routing table entry's list of paths.
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16.5. Incremental updates -- summary link advertisements
When a new summary link advertisement is received, it is not
necessary to recalculate the entire routing table. Call the
destination described by the summary link advertisement N (N's
address is obtained by masking the advertisement's Link State ID
with the network/subnet mask contained in the body of the
advertisement), and let Area A be the area to which the
advertisement belongs. There are then two separate cases:
Case 1: Area A is the backbone and/or the router is not an area
border router.
In this case, the following calculations must be performed.
First, if there is presently an inter-area route to the
destination N, N's routing table entry is invalidated,
saving the entry's values for later comparisons. Then the
calculation in Section 16.2 is run again for the single
destination N. In this calculation, all of Area A's summary
link advertisements that describe a route to N are examined.
In addition, if the router is an area border router attached
to one or more transit areas, the calculation in Section
16.3 must be run again for the single destination. If the
results of these calculations have changed the cost/path to
an AS boundary router (as would be the case for a Type 4
summary link advertisement) or to any forwarding addresses,
all AS external link advertisements will have to be
reexamined by rerunning the calculation in Section 16.4.
Otherwise, if N is now newly unreachable, the calculation in
Section 16.4 must be rerun for the single destination N, in
case an alternate external route to N exists.
Case 2: Area A is a transit area and the router is an area
border router.
In this case, the following calculations must be performed.
First, if N's routing table entry presently contains one or
more inter-area paths that utilize the transit area Area A,
these paths should be removed. If this removes all paths
from the routing table entry, the entry should be
invalidated. The entry's old values should be saved for
later comparisons. Next the calculation in Section 16.3 must
be run again for the single destination N. If the results of
this calculation have caused the cost to N to increase, the
complete routing table calculation must be rerun starting
with the Dijkstra algorithm specified in Section 16.1.
Otherwise, if the cost/path to an AS boundary router (as
would be the case for a Type 4 summary link advertisement)
or to any forwarding addresses has changed, all AS external
link advertisements will have to be reexamined by rerunning
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the calculation in Section 16.4. Otherwise, if N is now
newly unreachable, the calculation in Section 16.4 must be
rerun for the single destination N, in case an alternate
external route to N exists.
16.6. Incremental updates -- AS external link advertisements
When a new AS external link advertisement is received, it is not
necessary to recalculate the entire routing table. Call the
destination described by the AS external link advertisement N.
N's address is obtained by masking the advertisement's Link
State ID with the network/subnet mask contained in the body of
the advertisement. If there is already an intra-area or inter-
area route to the destination, no recalculation is necessary
(internal routes take precedence).
Otherwise, the procedure in Section 16.4 will have to be
performed, but only for those AS external link advertisements
whose destination is N. Before this procedure is performed, the
present routing table entry for N should be invalidated.
16.7. Events generated as a result of routing table changes
Changes to routing table entries sometimes cause the OSPF area
border routers to take additional actions. These routers need
to act on the following routing table changes:
o The cost or path type of a routing table entry has changed.
If the destination described by this entry is a Network or
AS boundary router, and this is not simply a change of AS
external routes, new summary link advertisements may have to
be generated (potentially one for each attached area,
including the backbone). See Section 12.4.3 for more
information. If a previously advertised entry has been
deleted, or is no longer advertisable to a particular area,
the advertisement must be flushed from the routing domain by
setting its LS age to MaxAge and reflooding (see Section
14.1).
o A routing table entry associated with a configured virtual
link has changed. The destination of such a routing table
entry is an area border router. The change indicates a
modification to the virtual link's cost or viability.
If the entry indicates that the area border router is newly
reachable (via TOS 0), the corresponding virtual link is now
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operational. An InterfaceUp event should be generated for
the virtual link, which will cause a virtual adjacency to
begin to form (see Section 10.3). At this time the virtual
link's IP interface address and the virtual neighbor's
Neighbor IP address are also calculated.
If the entry indicates that the area border router is no
longer reachable (via TOS 0), the virtual link and its
associated adjacency should be destroyed. This means an
InterfaceDown event should be generated for the associated
virtual link.
If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router links
advertisement for the backbone must be originated. This in
turn may cause further routing table changes.
16.8. Equal-cost multipath
The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to
calculate the routing table, and in the definition of the
routing table structure.
Each one of the multiple routes will be of the same type
(intra-area, inter-area, type 1 external or type 2 external),
cost, and will have the same associated area. However, each
route specifies a separate next hop and Advertising router.
There is no requirement that a router running OSPF keep track of
all possible equal-cost routes to a destination. An
implementation may choose to keep only a fixed number of routes
to any given destination. This does not affect any of the
algorithms presented in this specification.
16.9. Building the non-zero-TOS portion of the routing table
The OSPF protocol can calculate a different set of routes for
each IP TOS (see Section 2.4). Support for TOS-based routing is
optional. TOS-capable and non-TOS-capable routers can be mixed
in an OSPF routing domain. Routers not supporting TOS calculate
only the TOS 0 route to each destination. These routes are then
used to forward all data traffic, regardless of the TOS
indications in the data packet's IP header. A router that does
not support TOS indicates this fact to the other OSPF routers by
clearing the T-bit in the Options field of its router links
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advertisement.
The above sections detailing the routing table calculations
handle the TOS 0 case only. In general, for routers supporting
TOS-based routing, each piece of the routing table calculation
must be rerun separately for the non-zero TOS values. When
calculating routes for TOS X, only TOS X metrics can be used.
Any link state advertisement may specify a separate cost for
each TOS (a cost for TOS 0 must always be specified). The
encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
An advertisement can specify that it is restricted to TOS 0
(i.e., non-zero TOS is not handled) by clearing the T-bit in the
link state advertisement's Option field. Such advertisements
are not used when calculating routes for non-zero TOS. For this
reason, it is possible that a destination is unreachable for
some non-zero TOS. In this case, the TOS 0 path is used when
forwarding packets (see Section 11.1).
The following lists the modifications needed when running the
routing table calculation for a non-zero TOS value (called TOS
X). In general, routers and advertisements that do not support
TOS are omitted from the calculation.
Calculating the shortest-path tree (Section 16.1).
Routers that do not support TOS-based routing should be
omitted from the shortest-path tree calculation. These
routers are identified as those having the T-bit reset in
the Options field of their router links advertisements.
Such routers should never be added to the Dijktra
algorithm's candidate list, nor should their router links
advertisements be examined when adding the stub networks to
the tree. In particular, if the T-bit is reset in the
calculating router's own router links advertisement, it does
not run the shortest-path tree calculation for non-zero TOS
values.
Calculating the inter-area routes (Section 16.2).
Inter-area paths are the concatenation of a path to an area
border router with a summary link. When calculating TOS X
routes, both path components must also specify TOS X. In
other words, only TOS X paths to the area border router are
examined, and the area border router must be advertising a
TOS X route to the destination. Note that this means that
summary link advertisements having the T-bit reset in their
Options field are not considered.
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Examining transit areas' summary links (Section 16.3).
This calculation again considers the concatenation of a path
to an area border router with a summary link. As with
inter-area routes, only TOS X paths to the area border
router are examined, and the area border router must be
advertising a TOS X route to the destination.
Calculating AS external routes (Section 16.4).
This calculation considers the concatenation of a path to a
forwarding address with an AS external link. Only TOS X
paths to the forwarding address are examined, and the AS
boundary router must be advertising a TOS X route to the
destination. Note that this means that AS external link
advertisements having the T-bit reset in their Options field
are not considered.
In addition, the advertising AS boundary router must also be
reachable for its advertisements to be considered (see
Section 16.4). However, if the advertising router and the
forwarding address are not one in the same, the advertising
router need only be reachable via TOS 0.
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Footnotes
[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.
[2]It is possible for all of a router's interfaces to be unnumbered
point-to-point links. In this case, an IP address must be assigned
to the router. This address will then be advertised in the router's
router links advertisement as a host route.
[3]Note that in these cases both interfaces, the non-virtual and the
virtual, would have the same IP address.
[4]Note that no host route is generated for, and no IP packets can
be addressed to, interfaces to unnumbered point-to-point networks.
This is regardless of such an interface's state.
[5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
RT1, and the Backup Designated Router RT2. If Router RT1 crashes
(or maybe its interface to the network dies), the other routers on
the network will detect RT1's absence within RouterDeadInterval
seconds. All routers may not detect this at precisely the same
time; the routers that detect RT1's absence before RT2 does will,
for a time, select RT2 to be both Designated Router and Backup
Designated Router. When RT2 detects that RT1 is gone it will move
itself to Designated Router. At this time, the remaining router
having highest Router Priority will be selected as Backup Designated
Router.
[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
neighbor on virtual links is indicated by the routing table
calculation. However, in both these cases, the Hello Protocol is
still used. This ensures that communication between the neighbors
is bidirectional, and that each of the neighbors has a functioning
routing protocol layer.
[7]When the identity of the Designated Router is changing, it may be
quite common for a neighbor in this state to send the router a
Database Description packet; this means that there is some momentary
disagreement on the Designated Router's identity.
[8]Note that it is possible for a router to resynchronize any of its
fully established adjacencies by setting the adjacency's state back
to ExStart. This will cause the other end of the adjacency to
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process a SeqNumberMismatch event, and therefore to also go back to
ExStart state.
[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number) to some
router's Router ID.
[10]It is assumed that, for two different address ranges matching
the destination, one range is more specific than the other. Non-
contiguous subnet masks can be configured to violate this
assumption. Such subnet mask configurations cannot be handled by the
OSPF protocol.
[11]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
link state instance as it is flooded throughout the routing domain.
If two advertisements differ by more than this, they are assumed to
be different instances of the same advertisement. This can occur
when a router restarts and loses track of the advertisement's
previous LS sequence number. See Section 13.4 for more details.
[12]When two advertisements have different LS checksums, they are
assumed to be separate instances. This can occur when a router
restarts, and loses track of the advertisement's previous LS
sequence number. In the case where the two advertisements have the
same LS sequence number, it is not possible to determine which link
state is actually newer. If the wrong advertisement is accepted as
newer, the originating router will originate another instance. See
Section 13.4 for further details.
[13]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network links advertisement must be found based solely on its
Link State ID. The lookup in this case is still well defined, since
no two network links advertisements can have the same Link State ID.
[14]This clause covers the case: Inter-area routes are not
summarized to the backbone. This is because inter-area routes are
always associated with the backbone area.
[15]This clause is only invoked when Area A is a Transit area
supporting one or more virtual links. For example, in the area
configuration of Figure 6, Router RT11 need only originate a single
summary link having the (collapsed) destination N9-N11,H1 into its
connected Transit area Area 2, since all of its other eligible
routes have next hops belonging to Area 2 (and as such only need be
advertised by other area border routers; in this case, Routers RT10
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and RT7).
[16]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree only for
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of a single area's
shortest path tree [BBN]. However, these algorithms are beyond the
scope of this specification.
[17]This is how the Link state request list is emptied, which
eventually causes the neighbor state to transition to Full. See
Section 10.9 for more details.
[18]It should be a relatively rare occurrence for an advertisement's
LS age to reach MaxAge in this fashion. Usually, the advertisement
will be replaced by a more recent instance before it ages out.
[19]Only the TOS 0 routes are important here because all OSPF
protocol packets are sent with TOS = 0. See Appendix A.
[20]It may be the case that paths to certain destinations do not
vary based on TOS. For these destinations, the routing calculation
need not be repeated for each TOS value. In addition, there need
only be a single routing table entry for these destinations (instead
of a separate entry for each TOS value).
[21]Strictly speaking, because of equal-cost multipath, the
algorithm does not create a tree. We continue to use the "tree"
terminology because that is what occurs most often in the existing
literature.
[22]Note that the presence of any link back to V is sufficient; it
need not be the matching half of the link under consideration from V
to W. This is enough to ensure that, before data traffic flows
between a pair of neighboring routers, their link state databases
will be synchronized.
[23]When the forwarding address is non-zero, it should point to a
router belonging to another Autonomous System. See Section 12.4.5
for more details.
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References
[BBN] McQuillan, J., I. Richer and E. Rosen, "ARPANET
Routing Algorithm Improvements", BBN Technical
Report 3803, April 1978.
[DEC] Digital Equipment Corporation, "Information
processing systems -- Data communications --
Intermediate System to Intermediate System Intra-
Domain Routing Protocol", October 1987.
[McQuillan] McQuillan, J. et.al., "The New Routing Algorithm for
the Arpanet", IEEE Transactions on Communications,
May 1980.
[Perlman] Perlman, R., "Fault-Tolerant Broadcast of Routing
Information", Computer Networks, December 1983.
[RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791,
USC/Information Sciences Institute, September 1981.
[RFC 905] McKenzie, A., "ISO Transport Protocol specification
ISO DP 8073", RFC 905, ISO, April 1984.
[RFC 1112] Deering, S., "Host extensions for IP multicasting",
STD 5, RFC 1112, Stanford University, May 1988.
[RFC 1213] McCloghrie, K., and M. Rose, "Management Information
Base for network management of TCP/IP-based
internets: MIB-II", STD 17, RFC 1213, Hughes LAN
Systems, Performance Systems International, March
1991.
[RFC 1247] Moy, J., "OSPF Version 2", RFC 1247, Proteon, Inc.,
July 1991.
[RFC 1519] Fuller, V., T. Li, J. Yu, and K. Varadhan,
"Classless Inter-Domain Routing (CIDR): an Address
Assignment and Aggregation Strategy", RFC1519,
BARRNet, cisco, MERIT, OARnet, September 1993.
[RFC 1340] Reynolds, J., and J. Postel, "Assigned Numbers", STD
2, RFC 1340, USC/Information Sciences Institute,
July 1992.
[RFC 1349] Almquist, P., "Type of Service in the Internet
Protocol Suite", RFC 1349, July 1992.
Moy [Page 164]
RFC 1583 OSPF Version 2 March 1994
[RS-85-153] Leiner, B., et.al., "The DARPA Internet Protocol
Suite", DDN Protocol Handbook, April 1985.
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RFC 1583 OSPF Version 2 March 1994
A. OSPF data formats
This appendix describes the format of OSPF protocol packets and OSPF
link state advertisements. The OSPF protocol runs directly over the
IP network layer. Before any data formats are described, the
details of the OSPF encapsulation are explained.
Next the OSPF Options field is described. This field describes
various capabilities that may or may not be supported by pieces of
the OSPF routing domain. The OSPF Options field is contained in OSPF
Hello packets, Database Description packets and in OSPF link state
advertisements.
OSPF packet formats are detailed in Section A.3. A description of
OSPF link state advertisements appears in Section A.4.
A.1 Encapsulation of OSPF packets
OSPF runs directly over the Internet Protocol's network layer. OSPF
packets are therefore encapsulated solely by IP and local data-link
headers.
OSPF does not define a way to fragment its protocol packets, and
depends on IP fragmentation when transmitting packets larger than
the network MTU. The OSPF packet types that are likely to be large
(Database Description Packets, Link State Request, Link State
Update, and Link State Acknowledgment packets) can usually be split
into several separate protocol packets, without loss of
functionality. This is recommended; IP fragmentation should be
avoided whenever possible. Using this reasoning, an attempt should
be made to limit the sizes of packets sent over virtual links to 576
bytes. However, if necessary, the length of OSPF packets can be up
to 65,535 bytes (including the IP header).
The other important features of OSPF's IP encapsulation are:
o Use of IP multicast. Some OSPF messages are multicast, when
sent over multi-access networks. Two distinct IP multicast
addresses are used. Packets sent to these multicast addresses
should never be forwarded; they are meant to travel a single hop
only. To ensure that these packets will not travel multiple
hops, their IP TTL must be set to 1.
AllSPFRouters
This multicast address has been assigned the value
224.0.0.5. All routers running OSPF should be prepared to
receive packets sent to this address. Hello packets are
always sent to this destination. Also, certain OSPF
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protocol packets are sent to this address during the
flooding procedure.
AllDRouters
This multicast address has been assigned the value
224.0.0.6. Both the Designated Router and Backup Designated
Router must be prepared to receive packets destined to this
address. Certain OSPF protocol packets are sent to this
address during the flooding procedure.
o OSPF is IP protocol number 89. This number has been registered
with the Network Information Center. IP protocol number
assignments are documented in [RFC 1340].
o Routing protocol packets are sent with IP TOS of 0. The OSPF
protocol supports TOS-based routing. Routes to any particular
destination may vary based on TOS. However, all OSPF routing
protocol packets are sent using the normal service TOS value of
binary 0000 defined in [RFC 1349].
o Routing protocol packets are sent with IP precedence set to
Internetwork Control. OSPF protocol packets should be given
precedence over regular IP data traffic, in both sending and
receiving. Setting the IP precedence field in the IP header to
Internetwork Control [RFC 791] may help implement this
objective.
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A.2 The Options field
The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all link state advertisements. The Options
field enables OSPF routers to support (or not support) optional
capabilities, and to communicate their capability level to other
OSPF routers. Through this mechanism routers of differing
capabilities can be mixed within an OSPF routing domain.
When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain link state advertisements
to a neighbor because of its reduced functionality. Lastly, listing
capabilities in link state advertisements allows routers to route
traffic around reduced functionality routers, by excluding them from
parts of the routing table calculation.
Two capabilities are currently defined. For each capability, the
effect of the capability's appearance (or lack of appearance) in
Hello packets, Database Description packets and link state
advertisements is specified below. For example, the
ExternalRoutingCapability (below called the E-bit) has meaning only
in OSPF Hello Packets. Routers should reset (i.e. clear) the
unassigned part of the capability field when sending Hello packets
or Database Description packets and when originating link state
advertisements.
Additional capabilities may be assigned in the future. Routers
encountering unrecognized capabilities in received Hello Packets,
Database Description packets or link state advertisements should
ignore the capability and process the packet/advertisement normally.
+-+-+-+-+-+-+-+-+
| | | | | | |E|T|
+-+-+-+-+-+-+-+-+
The Options field
T-bit
This describes the router's TOS capability. If the T-bit is
reset, then the router supports only a single TOS (TOS 0). Such
a router is also said to be incapable of TOS-routing, and
elsewhere in this document referred to as a TOS-0-only router.
The absence of the T-bit in a router links advertisement causes
the router to be skipped when building a non-zero TOS shortest-
path tree (see Section 16.9). In other words, routers incapable
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of TOS routing will be avoided as much as possible when
forwarding data traffic requesting a non-zero TOS. The absence
of the T-bit in a summary link advertisement or an AS external
link advertisement indicates that the advertisement is
describing a TOS 0 route only (and not routes for non-zero TOS).
E-bit
This bit reflects the associated area's
ExternalRoutingCapability. AS external link advertisements are
not flooded into/through OSPF stub areas (see Section 3.6). The
E-bit ensures that all members of a stub area agree on that
area's configuration. The E-bit is meaningful only in OSPF
Hello packets. When the E-bit is reset in the Hello packet sent
out a particular interface, it means that the router will
neither send nor receive AS external link state advertisements
on that interface (in other words, the interface connects to a
stub area). Two routers will not become neighbors unless they
agree on the state of the E-bit.
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A.3 OSPF Packet Formats
There are five distinct OSPF packet types. All OSPF packet types
begin with a standard 24 byte header. This header is described
first. Each packet type is then described in a succeeding section.
In these sections each packet's division into fields is displayed,
and then the field definitions are enumerated.
All OSPF packet types (other than the OSPF Hello packets) deal with
lists of link state advertisements. For example, Link State Update
packets implement the flooding of advertisements throughout the OSPF
routing domain. Because of this, OSPF protocol packets cannot be
parsed unless the format of link state advertisements is also
understood. The format of Link state advertisements is described in
Section A.4.
The receive processing of OSPF packets is detailed in Section 8.2.
The sending of OSPF packets is explained in Section 8.1.
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A.3.1 The OSPF packet header
Every OSPF packet starts with a common 24 byte header. This header
contains all the necessary information to determine whether the
packet should be accepted for further processing. This
determination is described in Section 8.2 of the specification.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | Type | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version #
The OSPF version number. This specification documents version 2
of the protocol.
Type
The OSPF packet types are as follows. The format of each of
these packet types is described in a succeeding section.
Type Description
________________________________
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
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Packet length
The length of the protocol packet in bytes. This length
includes the standard OSPF header.
Router ID
The Router ID of the packet's source. In OSPF, the source and
destination of a routing protocol packet are the two ends of an
(potential) adjacency.
Area ID
A 32 bit number identifying the area that this packet belongs
to. All OSPF packets are associated with a single area. Most
travel a single hop only. Packets travelling over a virtual
link are labelled with the backbone Area ID of 0.0.0.0.
Checksum
The standard IP checksum of the entire contents of the packet,
starting with the OSPF packet header but excluding the 64-bit
authentication field. This checksum is calculated as the 16-bit
one's complement of the one's complement sum of all the 16-bit
words in the packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit words, the
packet is padded with a byte of zero before checksumming.
AuType
Identifies the authentication scheme to be used for the packet.
Authentication is discussed in Appendix D of the specification.
Consult Appendix D for a list of the currently defined
authentication types.
Authentication
A 64-bit field for use by the authentication scheme.
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A.3.2 The Hello packet
Hello packets are OSPF packet type 1. These packets are sent
periodically on all interfaces (including virtual links) in order to
establish and maintain neighbor relationships. In addition, Hello
Packets are multicast on those physical networks having a multicast
or broadcast capability, enabling dynamic discovery of neighboring
routers.
All routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
These parameters are included in Hello packets, so that differences
can inhibit the forming of neighbor relationships. A detailed
explanation of the receive processing for Hello packets is presented
in Section 10.5. The sending of Hello packets is covered in Section
9.5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 1 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HelloInterval | Options | Rtr Pri |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RouterDeadInterval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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Network mask
The network mask associated with this interface. For example,
if the interface is to a class B network whose third byte is
used for subnetting, the network mask is 0xffffff00.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
HelloInterval
The number of seconds between this router's Hello packets.
Rtr Pri
This router's Router Priority. Used in (Backup) Designated
Router election. If set to 0, the router will be ineligible to
become (Backup) Designated Router.
RouterDeadInterval
The number of seconds before declaring a silent router down.
Designated Router
The identity of the Designated Router for this network, in the
view of the advertising router. The Designated Router is
identified here by its IP interface address on the network. Set
to 0.0.0.0 if there is no Designated Router.
Backup Designated Router
The identity of the Backup Designated Router for this network,
in the view of the advertising router. The Backup Designated
Router is identified here by its IP interface address on the
network. Set to 0.0.0.0 if there is no Backup Designated
Router.
Neighbor
The Router IDs of each router from whom valid Hello packets have
been seen recently on the network. Recently means in the last
RouterDeadInterval seconds.
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A.3.3 The Database Description packet
Database Description packets are OSPF packet type 2. These packets
are exchanged when an adjacency is being initialized. They describe
the contents of the topological database. Multiple packets may be
used to describe the database. For this purpose a poll-response
procedure is used. One of the routers is designated to be master,
the other a slave. The master sends Database Description packets
(polls) which are acknowledged by Database Description packets sent
by the slave (responses). The responses are linked to the polls via
the packets' DD sequence numbers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 2 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 0 | Options |0|0|0|0|0|I|M|MS
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DD sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| A |
+- Link State Advertisement -+
| Header |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
The format of the Database Description packet is very similar to
both the Link State Request and Link State Acknowledgment packets.
The main part of all three is a list of items, each item describing
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a piece of the topological database. The sending of Database
Description Packets is documented in Section 10.8. The reception of
Database Description packets is documented in Section 10.6.
0 These fields are reserved. They must be 0.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
I-bit
The Init bit. When set to 1, this packet is the first in the
sequence of Database Description Packets.
M-bit
The More bit. When set to 1, it indicates that more Database
Description Packets are to follow.
MS-bit
The Master/Slave bit. When set to 1, it indicates that the
router is the master during the Database Exchange process.
Otherwise, the router is the slave.
DD sequence number
Used to sequence the collection of Database Description Packets.
The initial value (indicated by the Init bit being set) should
be unique. The DD sequence number then increments until the
complete database description has been sent.
The rest of the packet consists of a (possibly partial) list of the
topological database's pieces. Each link state advertisement in the
database is described by its link state advertisement header. The
link state advertisement header is documented in Section A.4.1. It
contains all the information required to uniquely identify both the
advertisement and the advertisement's current instance.
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A.3.4 The Link State Request packet
Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring router, a router may
find that parts of its topological database are out of date. The
Link State Request packet is used to request the pieces of the
neighbor's database that are more up to date. Multiple Link State
Request packets may need to be used. The sending of Link State
Request packets is the last step in bringing up an adjacency.
A router that sends a Link State Request packet has in mind the
precise instance of the database pieces it is requesting, defined by
LS sequence number, LS checksum, and LS age, although these fields
are not specified in the Link State Request Packet itself. The
router may receive even more recent instances in response.
The sending of Link State Request packets is documented in Section
10.9. The reception of Link State Request packets is documented in
Section 10.7.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 3 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Each advertisement requested is specified by its LS type, Link State
ID, and Advertising Router. This uniquely identifies the
advertisement, but not its instance. Link State Request packets are
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understood to be requests for the most recent instance (whatever
that might be).
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A.3.5 The Link State Update packet
Link State Update packets are OSPF packet type 4. These packets
implement the flooding of link state advertisements. Each Link
State Update packet carries a collection of link state
advertisements one hop further from its origin. Several link state
advertisements may be included in a single packet.
Link State Update packets are multicast on those physical networks
that support multicast/broadcast. In order to make the flooding
procedure reliable, flooded advertisements are acknowledged in Link
State Acknowledgment packets. If retransmission of certain
advertisements is necessary, the retransmitted advertisements are
always carried by unicast Link State Update packets. For more
information on the reliable flooding of link state advertisements,
consult Section 13.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 4 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # advertisements |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- +-+
| Link state advertisements |
+- +-+
| ... |
# advertisements
The number of link state advertisements included in this update.
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The body of the Link State Update packet consists of a list of link
state advertisements. Each advertisement begins with a common 20
byte header, the link state advertisement header. This header is
described in Section A.4.1. Otherwise, the format of each of the
five types of link state advertisements is different. Their formats
are described in Section A.4.
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A.3.6 The Link State Acknowledgment packet
Link State Acknowledgment Packets are OSPF packet type 5. To make
the flooding of link state advertisements reliable, flooded
advertisements are explicitly acknowledged. This acknowledgment is
accomplished through the sending and receiving of Link State
Acknowledgment packets. Multiple link state advertisements can be
acknowledged in a single Link State Acknowledgment packet.
Depending on the state of the sending interface and the source of
the advertisements being acknowledged, a Link State Acknowledgment
packet is sent either to the multicast address AllSPFRouters, to the
multicast address AllDRouters, or as a unicast. The sending of Link
State Acknowledgement packets is documented in Section 13.5. The
reception of Link State Acknowledgement packets is documented in
Section 13.7.
The format of this packet is similar to that of the Data Description
packet. The body of both packets is simply a list of link state
advertisement headers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 5 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | AuType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| A |
+- Link State Advertisement -+
| Header |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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Each acknowledged link state advertisement is described by its link
state advertisement header. The link state advertisement header is
documented in Section A.4.1. It contains all the information
required to uniquely identify both the advertisement and the
advertisement's current instance.
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A.4 Link state advertisement formats
There are five distinct types of link state advertisements. Each
link state advertisement begins with a standard 20-byte link state
advertisement header. This header is explained in Section A.4.1.
Succeeding sections then diagram the separate link state
advertisement types.
Each link state advertisement describes a piece of the OSPF routing
domain. Every router originates a router links advertisement. In
addition, whenever the router is elected Designated Router, it
originates a network links advertisement. Other types of link state
advertisements may also be originated (see Section 12.4). All link
state advertisements are then flooded throughout the OSPF routing
domain. The flooding algorithm is reliable, ensuring that all
routers have the same collection of link state advertisements. (See
Section 13 for more information concerning the flooding algorithm).
This collection of advertisements is called the link state (or
topological) database.
From the link state database, each router constructs a shortest path
tree with itself as root. This yields a routing table (see Section
11). For the details of the routing table build process, see
Section 16.
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A.4.1 The Link State Advertisement header
All link state advertisements begin with a common 20 byte header.
This header contains enough information to uniquely identify the
advertisement (LS type, Link State ID, and Advertising Router).
Multiple instances of the link state advertisement may exist in the
routing domain at the same time. It is then necessary to determine
which instance is more recent. This is accomplished by examining
the LS age, LS sequence number and LS checksum fields that are also
contained in the link state advertisement header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age
The time in seconds since the link state advertisement was
originated.
Options
The optional capabilities supported by the described portion of
the routing domain. OSPF's optional capabilities are documented
in Section A.2.
LS type
The type of the link state advertisement. Each link state type
has a separate advertisement format. The link state types are
as follows (see Section 12.1.3 for further explanation):
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LS Type Description
___________________________________
1 Router links
2 Network links
3 Summary link (IP network)
4 Summary link (ASBR)
5 AS external link
Link State ID
This field identifies the portion of the internet environment
that is being described by the advertisement. The contents of
this field depend on the advertisement's LS type. For example,
in network links advertisements the Link State ID is set to the
IP interface address of the network's Designated Router (from
which the network's IP address can be derived). The Link State
ID is further discussed in Section 12.1.4.
Advertising Router
The Router ID of the router that originated the link state
advertisement. For example, in network links advertisements
this field is set to the Router ID of the network's Designated
Router.
LS sequence number
Detects old or duplicate link state advertisements. Successive
instances of a link state advertisement are given successive LS
sequence numbers. See Section 12.1.6 for more details.
LS checksum
The Fletcher checksum of the complete contents of the link state
advertisement, including the link state advertisement header but
excepting the LS age field. See Section 12.1.7 for more details.
length
The length in bytes of the link state advertisement. This
includes the 20 byte link state advertisement header.
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A.4.2 Router links advertisements
Router links advertisements are the Type 1 link state
advertisements. Each router in an area originates a router links
advertisement. The advertisement describes the state and cost of
the router's links (i.e., interfaces) to the area. All of the
router's links to the area must be described in a single router
links advertisement. For details concerning the construction of
router links advertisements, see Section 12.4.1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | TOS 0 metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
In router links advertisements, the Link State ID field is set to
the router's OSPF Router ID. The T-bit is set in the
advertisement's Option field if and only if the router is able to
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calculate a separate set of routes for each IP TOS. Router links
advertisements are flooded throughout a single area only.
bit V
When set, the router is an endpoint of an active virtual link
that is using the described area as a Transit area (V is for
virtual link endpoint).
bit E
When set, the router is an AS boundary router (E is for
external)
bit B
When set, the router is an area border router (B is for border)
# links
The number of router links described by this advertisement.
This must be the total collection of router links (i.e.,
interfaces) to the area.
The following fields are used to describe each router link (i.e.,
interface). Each router link is typed (see the below Type field).
The Type field indicates the kind of link being described. It may
be a link to a transit network, to another router or to a stub
network. The values of all the other fields describing a router
link depend on the link's Type. For example, each link has an
associated 32-bit data field. For links to stub networks this field
specifies the network's IP address mask. For other link types the
Link Data specifies the router's associated IP interface address.
Type
A quick description of the router link. One of the following.
Note that host routes are classified as links to stub networks
whose network mask is 0xffffffff.
Type Description
__________________________________________________
1 Point-to-point connection to another router
2 Connection to a transit network
3 Connection to a stub network
4 Virtual link
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Link ID
Identifies the object that this router link connects to. Value
depends on the link's Type. When connecting to an object that
also originates a link state advertisement (i.e., another router
or a transit network) the Link ID is equal to the neighboring
advertisement's Link State ID. This provides the key for
looking up said advertisement in the link state database. See
Section 12.2 for more details.
Type Link ID
______________________________________
1 Neighboring router's Router ID
2 IP address of Designated Router
3 IP network/subnet number
4 Neighboring router's Router ID
Link Data
Contents again depend on the link's Type field. For connections
to stub networks, it specifies the network's IP address mask.
For unnumbered point-to-point connections, it specifies the
interface's MIB-II [RFC 1213] ifIndex value. For the other link
types it specifies the router's associated IP interface address.
This latter piece of information is needed during the routing
table build process, when calculating the IP address of the next
hop. See Section 16.1.1 for more details.
# TOS
The number of different TOS metrics given for this link, not
counting the required metric for TOS 0. For example, if no
additional TOS metrics are given, this field should be set to 0.
TOS 0 metric
The cost of using this router link for TOS 0.
For each link, separate metrics may be specified for each Type of
Service (TOS). The metric for TOS 0 must always be included, and
was discussed above. Metrics for non-zero TOS are described below.
The encoding of TOS in OSPF link state advertisements is described
in Section 12.3. Note that the cost for non-zero TOS values that
are not specified defaults to the TOS 0 cost. Metrics must be
listed in order of increasing TOS encoding. For example, the metric
for TOS 16 must always follow the metric for TOS 8 when both are
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specified.
TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF link state advertisements is described in Section
12.3.
metric
The cost of using this outbound router link, for traffic of the
specified TOS.
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A.4.3 Network links advertisements
Network links advertisements are the Type 2 link state
advertisements. A network links advertisement is originated for
each transit network in the area. A transit network is a multi-
access network that has more than one attached router. The network
links advertisement is originated by the network's Designated
Router. The advertisement describes all routers attached to the
network, including the Designated Router itself. The
advertisement's Link State ID field lists the IP interface address
of the Designated Router.
The distance from the network to all attached routers is zero, for
all Types of Service. This is why the TOS and metric fields need
not be specified in the network links advertisement. For details
concerning the construction of network links advertisements, see
Section 12.4.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network Mask
The IP address mask for the network. For example, a class A
network would have the mask 0xff000000.
Attached Router
The Router IDs of each of the routers attached to the network.
Actually, only those routers that are fully adjacent to the
Designated Router are listed. The Designated Router includes
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itself in this list. The number of routers included can be
deduced from the link state advertisement header's length field.
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A.4.4 Summary link advertisements
Summary link advertisements are the Type 3 and 4 link state
advertisements. These advertisements are originated by area border
routers. A separate summary link advertisement is made for each
destination (known to the router) which belongs to the AS, yet is
outside the area. For details concerning the construction of
summary link advertisements, see Section 12.4.3.
Type 3 link state advertisements are used when the destination is an
IP network. In this case the advertisement's Link State ID field is
an IP network number (if necessary, the Link State ID can also have
one or more of the network's "host" bits set; see Appendix F for
details). When the destination is an AS boundary router, a Type 4
advertisement is used, and the Link State ID field is the AS
boundary router's OSPF Router ID. (To see why it is necessary to
advertise the location of each ASBR, consult Section 16.4.) Other
than the difference in the Link State ID field, the format of Type 3
and 4 link state advertisements is identical.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
For stub areas, Type 3 summary link advertisements can also be used
to describe a (per-area) default route. Default summary routes are
used in stub areas instead of flooding a complete set of external
routes. When describing a default summary route, the
advertisement's Link State ID is always set to DefaultDestination
(0.0.0.0) and the Network Mask is set to 0.0.0.0.
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Separate costs may be advertised for each IP Type of Service. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3. Note that the cost for TOS 0 must be included, and is
always listed first. If the T-bit is reset in the advertisement's
Option field, only a route for TOS 0 is described by the
advertisement. Otherwise, routes for the other TOS values are also
described; if a cost for a certain TOS is not included, its cost
defaults to that specified for TOS 0.
Network Mask
For Type 3 link state advertisements, this indicates the
destination network's IP address mask. For example, when
advertising the location of a class A network the value
0xff000000 would be used. This field is not meaningful and must
be zero for Type 4 link state advertisements.
For each specified Type of Service, the following fields are
defined. The number of TOS routes included can be calculated from
the link state advertisement header's length field. Values for TOS
0 must be specified; they are listed first. Other values must be
listed in order of increasing TOS encoding. For example, the cost
for TOS 16 must always follow the cost for TOS 8 when both are
specified.
TOS The Type of Service that the following cost concerns. The
encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
metric
The cost of this route. Expressed in the same units as the
interface costs in the router links advertisements.
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A.4.5 AS external link advertisements
AS external link advertisements are the Type 5 link state
advertisements. These advertisements are originated by AS boundary
routers. A separate advertisement is made for each destination
(known to the router) which is external to the AS. For details
concerning the construction of AS external link advertisements, see
Section 12.4.3.
AS external link advertisements usually describe a particular
external destination. For these advertisements the Link State ID
field specifies an IP network number (if necessary, the Link State
ID can also have one or more of the network's "host" bits set; see
Appendix F for details). AS external link advertisements are also
used to describe a default route. Default routes are used when no
specific route exists to the destination. When describing a default
route, the Link State ID is always set to DefaultDestination
(0.0.0.0) and the Network Mask is set to 0.0.0.0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Separate costs may be advertised for each IP Type of Service. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3. Note that the cost for TOS 0 must be included, and is
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always listed first. If the T-bit is reset in the advertisement's
Option field, only a route for TOS 0 is described by the
advertisement. Otherwise, routes for the other TOS values are also
described; if a cost for a certain TOS is not included, its cost
defaults to that specified for TOS 0.
Network Mask
The IP address mask for the advertised destination. For
example, when advertising a class A network the mask 0xff000000
would be used.
For each specified Type of Service, the following fields are
defined. The number of TOS routes included can be calculated from
the link state advertisement header's length field. Values for TOS
0 must be specified; they are listed first. Other values must be
listed in order of increasing TOS encoding. For example, the cost
for TOS 16 must always follow the cost for TOS 8 when both are
specified.
bit E
The type of external metric. If bit E is set, the metric
specified is a Type 2 external metric. This means the metric is
considered larger than any link state path. If bit E is zero,
the specified metric is a Type 1 external metric. This means
that is is comparable directly (without translation) to the link
state metric.
Forwarding address
Data traffic for the advertised destination will be forwarded to
this address. If the Forwarding address is set to 0.0.0.0, data
traffic will be forwarded instead to the advertisement's
originator (i.e., the responsible AS boundary router).
TOS The Type of Service that the following cost concerns. The
encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
metric
The cost of this route. Interpretation depends on the external
type indication (bit E above).
External Route Tag
A 32-bit field attached to each external route. This is not
used by the OSPF protocol itself. It may be used to communicate
information between AS boundary routers; the precise nature of
such information is outside the scope of this specification.
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B. Architectural Constants
Several OSPF protocol parameters have fixed architectural values.
These parameters have been referred to in the text by names such as
LSRefreshTime. The same naming convention is used for the
configurable protocol parameters. They are defined in Appendix C.
The name of each architectural constant follows, together with its
value and a short description of its function.
LSRefreshTime
The maximum time between distinct originations of any particular
link state advertisement. When the LS age field of one of the
router's self-originated advertisements reaches the value
LSRefreshTime, a new instance of the link state advertisement is
originated, even though the contents of the advertisement (apart
from the link state header) will be the same. The value of
LSRefreshTime is set to 30 minutes.
MinLSInterval
The minimum time between distinct originations of any particular
link state advertisement. The value of MinLSInterval is set to
5 seconds.
MaxAge
The maximum age that a link state advertisement can attain. When
an advertisement's LS age field reaches MaxAge, it is reflooded
in an attempt to flush the advertisement from the routing domain
(See Section 14). Advertisements of age MaxAge are not used in
the routing table calculation. The value of MaxAge must be
greater than LSRefreshTime. The value of MaxAge is set to 1
hour.
CheckAge
When the age of a link state advertisement (that is contained in
the link state database) hits a multiple of CheckAge, the
advertisement's checksum is verified. An incorrect checksum at
this time indicates a serious error. The value of CheckAge is
set to 5 minutes.
MaxAgeDiff
The maximum time dispersion that can occur, as a link state
advertisement is flooded throughout the AS. Most of this time
is accounted for by the link state advertisements sitting on
router output queues (and therefore not aging) during the
flooding process. The value of MaxAgeDiff is set to 15 minutes.
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LSInfinity
The metric value indicating that the destination described by a
link state advertisement is unreachable. Used in summary link
advertisements and AS external link advertisements as an
alternative to premature aging (see Section 14.1). It is defined
to be the 24-bit binary value of all ones: 0xffffff.
DefaultDestination
The Destination ID that indicates the default route. This route
is used when no other matching routing table entry can be found.
The default destination can only be advertised in AS external
link advertisements and in stub areas' type 3 summary link
advertisements. Its value is the IP address 0.0.0.0.
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C. Configurable Constants
The OSPF protocol has quite a few configurable parameters. These
parameters are listed below. They are grouped into general
functional categories (area parameters, interface parameters, etc.).
Sample values are given for some of the parameters.
Some parameter settings need to be consistent among groups of
routers. For example, all routers in an area must agree on that
area's parameters, and all routers attached to a network must agree
on that network's IP network number and mask.
Some parameters may be determined by router algorithms outside of
this specification (e.g., the address of a host connected to the
router via a SLIP line). From OSPF's point of view, these items are
still configurable.
C.1 Global parameters
In general, a separate copy of the OSPF protocol is run for each
area. Because of this, most configuration parameters are
defined on a per-area basis. The few global configuration
parameters are listed below.
Router ID
This is a 32-bit number that uniquely identifies the router
in the Autonomous System. One algorithm for Router ID
assignment is to choose the largest or smallest IP address
assigned to the router. If a router's OSPF Router ID is
changed, the router's OSPF software should be restarted
before the new Router ID takes effect. Before restarting in
order to change its Router ID, the router should flush its
self-originated link state advertisements from the routing
domain (see Section 14.1), or they will persist for up to
MaxAge minutes.
TOS capability
This item indicates whether the router will calculate
separate routes based on TOS. For more information, see
Sections 4.5 and 16.9.
C.2 Area parameters
All routers belonging to an area must agree on that area's
configuration. Disagreements between two routers will lead to
an inability for adjacencies to form between them, with a
resulting hindrance to the flow of routing protocol and data
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traffic. The following items must be configured for an area:
Area ID
This is a 32-bit number that identifies the area. The Area
ID of 0.0.0.0 is reserved for the backbone. If the area
represents a subnetted network, the IP network number of the
subnetted network may be used for the Area ID.
List of address ranges
An OSPF area is defined as a list of address ranges. Each
address range consists of the following items:
[IP address, mask]
Describes the collection of IP addresses contained
in the address range. Networks and hosts are
assigned to an area depending on whether their
addresses fall into one of the area's defining
address ranges. Routers are viewed as belonging to
multiple areas, depending on their attached
networks' area membership.
Status Set to either Advertise or DoNotAdvertise. Routing
information is condensed at area boundaries.
External to the area, at most a single route is
advertised (via a summary link advertisement) for
each address range. The route is advertised if and
only if the address range's Status is set to
Advertise. Unadvertised ranges allow the existence
of certain networks to be intentionally hidden from
other areas. Status is set to Advertise by default.
As an example, suppose an IP subnetted network is to be its
own OSPF area. The area would be configured as a single
address range, whose IP address is the address of the
subnetted network, and whose mask is the natural class A, B,
or C address mask. A single route would be advertised
external to the area, describing the entire subnetted
network.
AuType
Each area can be configured for a separate type of
authentication. See Appendix D for a discussion of the
defined authentication types.
ExternalRoutingCapability
Whether AS external advertisements will be flooded
into/throughout the area. If AS external advertisements are
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excluded from the area, the area is called a "stub".
Internal to stub areas, routing to external destinations
will be based solely on a default summary route. The
backbone cannot be configured as a stub area. Also, virtual
links cannot be configured through stub areas. For more
information, see Section 3.6.
StubDefaultCost
If the area has been configured as a stub area, and the
router itself is an area border router, then the
StubDefaultCost indicates the cost of the default summary
link that the router should advertise into the area. There
can be a separate cost configured for each IP TOS. See
Section 12.4.3 for more information.
C.3 Router interface parameters
Some of the configurable router interface parameters (such as IP
interface address and subnet mask) actually imply properties of
the attached networks, and therefore must be consistent across
all the routers attached to that network. The parameters that
must be configured for a router interface are:
IP interface address
The IP protocol address for this interface. This uniquely
identifies the router over the entire internet. An IP
address is not required on serial lines. Such a serial line
is called "unnumbered".
IP interface mask
Also referred to as the subnet mask, this indicates the
portion of the IP interface address that identifies the
attached network. Masking the IP interface address with the
IP interface mask yields the IP network number of the
attached network. On point-to-point networks and virtual
links, the IP interface mask is not defined. On these
networks, the link itself is not assigned an IP network
number, and so the addresses of each side of the link are
assigned independently, if they are assigned at all.
Interface output cost(s)
The cost of sending a packet on the interface, expressed in
the link state metric. This is advertised as the link cost
for this interface in the router's router links
advertisement. There may be a separate cost for each IP
Type of Service. The interface output cost(s) must always
be greater than 0.
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RxmtInterval
The number of seconds between link state advertisement
retransmissions, for adjacencies belonging to this
interface. Also used when retransmitting Database
Description and Link State Request Packets. This should be
well over the expected round-trip delay between any two
routers on the attached network. The setting of this value
should be conservative or needless retransmissions will
result. It will need to be larger on low speed serial lines
and virtual links. Sample value for a local area network: 5
seconds.
InfTransDelay
The estimated number of seconds it takes to transmit a Link
State Update Packet over this interface. Link state
advertisements contained in the update packet must have
their age incremented by this amount before transmission.
This value should take into account the transmission and
propagation delays of the interface. It must be greater
than 0. Sample value for a local area network: 1 second.
Router Priority
An 8-bit unsigned integer. When two routers attached to a
network both attempt to become Designated Router, the one
with the highest Router Priority takes precedence. If there
is still a tie, the router with the highest Router ID takes
precedence. A router whose Router Priority is set to 0 is
ineligible to become Designated Router on the attached
network. Router Priority is only configured for interfaces
to multi-access networks.
HelloInterval
The length of time, in seconds, between the Hello Packets
that the router sends on the interface. This value is
advertised in the router's Hello Packets. It must be the
same for all routers attached to a common network. The
smaller the HelloInterval, the faster topological changes
will be detected, but more OSPF routing protocol traffic
will ensue. Sample value for a X.25 PDN network: 30
seconds. Sample value for a local area network: 10 seconds.
RouterDeadInterval
After ceasing to hear a router's Hello Packets, the number
of seconds before its neighbors declare the router down.
This is also advertised in the router's Hello Packets in
their RouterDeadInterval field. This should be some
multiple of the HelloInterval (say 4). This value again
must be the same for all routers attached to a common
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network.
Authentication key
This configured data allows the authentication procedure to
generate and/or verify the authentication field in the OSPF
header. This value again must be the same for all routers
attached to a common network. For example, if the AuType
indicates simple password, the Authentication key would be a
64-bit password. This key would be inserted directly into
the OSPF header when originating routing protocol packets.
There could be a separate password for each network.
C.4 Virtual link parameters
Virtual links are used to restore/increase connectivity of the
backbone. Virtual links may be configured between any pair of
area border routers having interfaces to a common (non-backbone)
area. The virtual link appears as an unnumbered point-to-point
link in the graph for the backbone. The virtual link must be
configured in both of the area border routers.
A virtual link appears in router links advertisements (for the
backbone) as if it were a separate router interface to the
backbone. As such, it has all of the parameters associated with
a router interface (see Section C.3). Although a virtual link
acts like an unnumbered point-to-point link, it does have an
associated IP interface address. This address is used as the IP
source in OSPF protocol packets it sends along the virtual link,
and is set dynamically during the routing table build process.
Interface output cost is also set dynamically on virtual links
to be the cost of the intra-area path between the two routers.
The parameter RxmtInterval must be configured, and should be
well over the expected round-trip delay between the two routers.
This may be hard to estimate for a virtual link; it is better to
err on the side of making it too large. Router Priority is not
used on virtual links.
A virtual link is defined by the following two configurable
parameters: the Router ID of the virtual link's other endpoint,
and the (non-backbone) area through which the virtual link runs
(referred to as the virtual link's Transit area). Virtual links
cannot be configured through stub areas.
C.5 Non-broadcast, multi-access network parameters
OSPF treats a non-broadcast, multi-access network much like it
treats a broadcast network. Since there may be many routers
attached to the network, a Designated Router is selected for the
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network. This Designated Router then originates a networks
links advertisement, which lists all routers attached to the
non-broadcast network.
However, due to the lack of broadcast capabilities, it is
necessary to use configuration parameters in the Designated
Router selection. These parameters need only be configured in
those routers that are themselves eligible to become Designated
Router (i.e., those router's whose Router Priority for the
network is non-zero):
List of all other attached routers
The list of all other routers attached to the non-broadcast
network. Each router is listed by its IP interface address
on the network. Also, for each router listed, that router's
eligibility to become Designated Router must be defined.
When an interface to a non-broadcast network comes up, the
router sends Hello Packets only to those neighbors eligible
to become Designated Router, until the identity of the
Designated Router is discovered.
PollInterval
If a neighboring router has become inactive (Hello Packets
have not been seen for RouterDeadInterval seconds), it may
still be necessary to send Hello Packets to the dead
neighbor. These Hello Packets will be sent at the reduced
rate PollInterval, which should be much larger than
HelloInterval. Sample value for a PDN X.25 network: 2
minutes.
C.6 Host route parameters
Host routes are advertised in router links advertisements as
stub networks with mask 0xffffffff. They indicate either router
interfaces to point-to-point networks, looped router interfaces,
or IP hosts that are directly connected to the router (e.g., via
a SLIP line). For each host directly connected to the router,
the following items must be configured:
Host IP address
The IP address of the host.
Cost of link to host
The cost of sending a packet to the host, in terms of the
link state metric. There may be multiple costs configured,
one for each IP TOS. However, since the host probably has
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only a single connection to the internet, the actual
configured cost(s) in many cases is unimportant (i.e., will
have no effect on routing).
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D. Authentication
All OSPF protocol exchanges are authenticated. The OSPF packet
header (see Section A.3.1) includes an authentication type field,
and 64-bits of data for use by the appropriate authentication scheme
(determined by the type field).
The authentication type is configurable on a per-area basis.
Additional authentication data is configurable on a per-interface
basis. For example, if an area uses a simple password scheme for
authentication, a separate password may be configured for each
network contained in the area.
Authentication types 0 and 1 are defined by this specification. All
other authentication types are reserved for definition by the IANA
(iana@ISI.EDU). The current list of authentication types is
described below in Table 20.
AuType Description
___________________________________________
0 No authentication
1 Simple password
All others Reserved for assignment by the
IANA (iana@ISI.EDU)
Table 20: OSPF authentication types.
D.1 AuType 0 -- No authentication
Use of this authentication type means that routing exchanges in
the area are not authenticated. The 64-bit field in the OSPF
header can contain anything; it is not examined on packet
reception.
D.2 AuType 1 -- Simple password
Using this authentication type, a 64-bit field is configured on
a per-network basis. All packets sent on a particular network
must have this configured value in their OSPF header 64-bit
authentication field. This essentially serves as a "clear" 64-
bit password.
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This guards against routers inadvertently joining the area.
They must first be configured with their attached networks'
passwords before they can participate in the routing domain.
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E. Differences from RFC 1247
This section documents the differences between this memo and RFC
1247. These differences include a fix for a problem involving OSPF
virtual links, together with minor enhancements and clarifications
to the protocol. All differences are backward-compatible.
Implementations of this memo and of RFC 1247 will interoperate.
E.1 A fix for a problem with OSPF Virtual links
In RFC 1247, certain configurations of OSPF virtual links can
cause routing loops. The root of the problem is that while there
is an information mismatch at the boundary of any virtual link's
Transit area, a backbone path can still cross the boundary. RFC
1247 attempted to compensate for this information mismatch by
adjusting any backbone path as it enters the transit area (see
Section 16.3 in RFC 1247). However, this proved not to be
enough. This memo fixes the problem by having all area border
routers determine, by looking at summary links, whether better
backbone paths can be found through the transit areas.
This fix simplifies the OSPF virtual link logic, and consists of
the following components:
o A new bit has been defined in the router links
advertisement, called bit V. Bit V is set in a router's
router links advertisement for Area A if and only if the
router is an endpoint of an active virtual link that uses
Area A as its Transit area (see Sections 12.4.1 and A.4.2).
This enables the other routers attached to Area A to
discover whether the area supports any virtual links (i.e.,
is a transit area). This discovery is done during the
calculation of Area A's shortest-path tree (see Section
16.1).
o To aid in the description of the algorithm, a new parameter
has been added to the OSPF area structure:
TransitCapability. This parameter indicates whether the area
supports any active virtual links. Equivalently, it
indicates whether the area can carry traffic that neither
originates nor terminates in the area itself.
o The calculation in Section 16.3 of RFC 1247 has been
replaced. The new calculation, performed by area border
routers only, examines the summary links belonging to all
attached transit areas to see whether the transit areas can
provide better paths than those already found in Sections
16.1 and 16.2.
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o The incremental calculations in Section 16.5 have been
updated as a result of the new calculations in Section 16.3.
E.2 Supporting supernetting and subnet 0
In RFC 1247, an OSPF router cannot originate separate AS
external link advertisements (or separate summary link
advertisements) for two networks that have the same address but
different masks. This situation can arise when subnet 0 of a
network has been assigned (a practice that is generally
discouraged), or when using supernetting as described in [RFC
1519] (a practice that is generally encouraged to reduce the
size of routing tables), or even when in transition from one
mask to another on a subnet. Using supernetting as an example,
you might want to aggregate the four class C networks
192.9.4.0-192.9.7.0, advertising one route for the aggregation
and another for the single class C network 192.9.4.0.
The reason behind this limitation is that in RFC 1247, the Link
State ID of AS external link advertisements and summary link
advertisements is set equal to the described network's IP
address. In the above example, RFC 1247 would assign both
advertisements the Link State ID of 192.9.4.0, making them in
essence the same advertisement. This memo fixes the problem by
relaxing the setting of the Link State ID so that any of the
"host" bits of the network address can also be set. This allows
you to disambiguate advertisements for networks having the same
address but different masks. Given an AS external link
advertisement (or a summary link advertisement), the described
network's address can now be obtained by masking the Link State
ID with the network mask carried in the body of the
advertisement. Again using the above example, the aggregate can
now be advertised using a Link State ID of 192.9.4.0 and the
single class C network advertised simultaneously using the Link
State ID of 192.9.4.255.
Appendix F gives one possible algorithm for setting one or more
"host" bits in the Link State ID in order to disambiguate
advertisements. It should be noted that this is a local
decision. Each router in an OSPF system is free to use its own
algorithm, since only those advertisements originated by the
router itself are affected.
It is believed that this change will be more or less compatible
with implementations of RFC 1247. Implementations of RFC 1247
will probably either a) install routing table entries that won't
be used or b) do the correct processing as outlined in this memo
or c) mark the advertisement as unusable when presented with a
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Link State ID that has one or more of the host bits set.
However, in the interest of interoperability, implementations of
this memo should only set the host bits in Link State IDs when
absolutely necessary.
The change affects Sections 12.1.4, 12.4.3, 12.4.5, 16.2, 16.3,
16.4, 16.5, 16.6, A.4.4 and A.4.5.
E.3 Obsoleting LSInfinity in router links advertisements
The metric of LSInfinity can no longer be used in router links
advertisements to indicate unusable links. This is being done
for several reasons:
o It removes any possible confusion in an OSPF area as to just
which routers/networks are reachable in the area. For
example, the above virtual link fix relies on detecting the
existence of virtual links when running the Dijkstra.
However, when one-directional links (i.e., cost of
LSInfinity in one direction, but not the other) are
possible, some routers may detect the existence of virtual
links while others may not. This may defeat the fix for the
virtual link problem.
o It also helps OSPF's Multicast routing extensions (MOSPF),
because one-way reachability can lead to places that are
reachable via unicast but not multicast, or vice versa.
The two prior justifications for using LSInfinity in router
links advertisements were 1) it was a way to not support TOS
before TOS was optional and 2) it went along with strong TOS
interpretations. These justifications are no longer valid.
However, LSInfinity will continue to mean "unreachable" in
summary link advertisements and AS external link advertisements,
as some implementations use this as an alternative to the
premature aging procedure specified in Section 14.1.
This change has one other side effect. When two routers are
connected via a virtual link whose underlying path is non-TOS-
capable, they must now revert to being non-TOS-capable routers
themselves, instead of the previous behavior of advertising the
non-zero TOS costs of the virtual link as LSInfinity. See
Section 15 for details.
E.4 TOS encoding updated
The encoding of TOS in OSPF link state advertisements has been
updated to reflect the new TOS value (minimize monetary cost)
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defined by [RFC 1349]. The OSPF encoding is defined in Section
12.3, which is identical in content to Section A.5 of [RFC
1349].
E.5 Summarizing routes into transit areas
RFC 1247 mandated that routes associated with Area A are never
summarized back into Area A. However, this memo further reduces
the number of summary links originated by refusing to summarize
into Area A those routes having next hops belonging to Area A.
This is an optimization over RFC 1247 behavior when virtual
links are present. For example, in the area configuration of
Figure 6, Router RT11 need only originate a single summary link
having the (collapsed) destination N9-N11,H1 into its connected
transit area Area 2, since all of its other eligible routes have
next hops belonging to Area 2 (and as such only need be
advertised by other area border routers; in this case, Routers
RT10 and RT7). This is the logical equivalent of a Distance
Vector protocol's split horizon logic.
This change appears in Section 12.4.3.
E.6 Summarizing routes into stub areas
RFC 1247 mandated that area border routers attached to stub
areas must summarize all inter-area routes into the stub areas.
However, while area border routers connected to OSPF stub areas
must originate default summary links into the stub area, they
need not summarize other routes into the stub area. The amount
of summarization done into stub areas can instead be put under
configuration control. The network administrator can then make
the trade-off between optimal routing and database size.
This change appears in Sections 12.4.3 and 12.4.4.
E.7 Flushing anomalous network links advertisements
Text was added indicating that a network links advertisement
whose Link State ID is equal to one of the router's own IP
interface addresses should be considered to be self-originated,
regardless of the setting of the advertisement's Advertising
Router. If the Advertising Router of such an advertisement is
not equal to the router's own Router ID, the advertisement
should be flushed from the routing domain using the premature
aging procedure specified in Section 14.1. This case should be
rare, and it indicates that the router's Router ID has changed
since originating the advertisement.
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Failure to flush these anomalous advertisements could lead to
multiple network links advertisements having the same Link State
ID. This in turn could cause the Dijkstra calculation in Section
16.1 to fail, since it would be impossible to tell which network
links advertisement is valid (i.e., more recent).
This change appears in Sections 13.4 and 14.1.
E.8 Required Statistics appendix deleted
Appendix D of RFC 1247, which specified a list of required
statistics for an OSPF implementation, has been deleted. That
appendix has been superseded by the two documents: the OSPF
Version 2 Management Information Base and the OSPF Version 2
Traps.
E.9 Other changes
The following small changes were also made to RFC 1247:
o When representing unnumbered point-to-point networks in
router links advertisements, the corresponding Link Data
field should be set to the unnumbered interface's MIB-II
[RFC 1213] ifIndex value.
o A comment was added to Step 3 of the Dijkstra algorithm in
Section 16.1. When removing vertices from the candidate
list, and when there is a choice of vertices closest to the
root, network vertices must be chosen before router vertices
in order to necessarily find all equal-cost paths.
o A comment was added to Section 12.4.3 noting that a summary
link advertisement cannot express a reachable destination
whose path cost equals or exceeds LSInfinity.
o A comment was added to Section 15 noting that a virtual link
whose underlying path has cost greater than hexadecimal
0xffff (the maximum size of an interface cost in a router
links advertisement) should be considered inoperational.
o An option was added to the definition of area address
ranges, allowing the network administrator to specify that a
particular range should not be advertised to other OSPF
areas. This enables the existence of certain networks to be
hidden from other areas. This change appears in Sections
12.4.3 and C.2.
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o A note was added reminding implementors that bit E (the AS
boundary router indication) should never be set in a router
links advertisement for a stub area, since stub areas cannot
contain AS boundary routers. This change appears in Section
12.4.1.
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F. An algorithm for assigning Link State IDs
In RFC 1247, the Link State ID in AS external link advertisements
and summary link advertisements is set to the described network's IP
address. This memo relaxes that requirement, allowing one or more of
the network's host bits to be set in the Link State ID. This allows
the router to originate separate advertisements for networks having
the same addresses, yet different masks. Such networks can occur in
the presence of supernetting and subnet 0s (see Section E.2 for more
information).
This appendix gives one possible algorithm for setting the host bits
in Link State IDs. The choice of such an algorithm is a local
decision. Separate routers are free to use different algorithms,
since the only advertisements affected are the ones that the router
itself originates. The only requirement on the algorithms used is
that the network's IP address should be used as the Link State ID
(the RFC 1247 behavior) whenever possible.
The algorithm below is stated for AS external link advertisements.
This is only for clarity; the exact same algorithm can be used for
summary link advertisements. Suppose that the router wishes to
originate an AS external link advertisement for a network having
address NA and mask NM1. The following steps are then used to
determine the advertisement's Link State ID:
(1) Determine whether the router is already originating an AS
external link advertisement with Link State ID equal to NA (in
such an advertisement the router itself will be listed as the
advertisement's Advertising Router). If not, set the Link State
ID equal to NA (the RFC 1247 behavior) and the algorithm
terminates. Otherwise,
(2) Obtain the network mask from the body of the already existing AS
external link advertisement. Call this mask NM2. There are then
two cases:
o NM1 is longer (i.e., more specific) than NM2. In this case,
set the Link State ID in the new advertisement to be the
network [NA,NM1] with all the host bits set (i.e., equal to
NA or'ed together with all the bits that are not set in NM1,
which is network [NA,NM1]'s broadcast address).
o NM2 is longer than NM1. In this case, change the existing
advertisement (having Link State ID of NA) to reference the
new network [NA,NM1] by incrementing the sequence number,
changing the mask in the body to NM1 and using the cost for
the new network. Then originate a new advertisement for the
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old network [NA,NM2], with Link State ID equal to NA or'ed
together with the bits that are not set in NM2 (i.e.,
network [NA,NM2]'s broadcast address).
The above algorithm assumes that all masks are contiguous; this
ensures that when two networks have the same address, one mask is
more specific than the other. The algorithm also assumes that no
network exists having an address equal to another network's
broadcast address. Given these two assumptions, the above algorithm
always produces unique Link State IDs. The above algorithm can also
be reworded as follows: When originating an AS external link state
advertisement, try to use the network number as the Link State ID.
If that produces a conflict, examine the two networks in conflict.
One will be a subset of the other. For the less specific network,
use the network number as the Link State ID and for the more
specific use the network's broadcast address instead (i.e., flip all
the "host" bits to 1). If the most specific network was originated
first, this will cause you to originate two link state
advertisements at once.
As an example of the algorithm, consider its operation when the
following sequence of events occurs in a single router (Router A).
(1) Router A wants to originate an AS external link advertisement
for [10.0.0.0,255.255.255.0]:
(a) A Link State ID of 10.0.0.0 is used.
(2) Router A then wants to originate an AS external link
advertisement for [10.0.0.0,255.255.0.0]:
(a) The advertisement for [10.0.0,0,255.255.255.0] is
reoriginated using a new Link State ID of 10.0.0.255.
(b) A Link State ID of 10.0.0.0 is used for
[10.0.0.0,255.255.0.0].
(3) Router A then wants to originate an AS external link
advertisement for [10.0.0.0,255.0.0.0]:
(a) The advertisement for [10.0.0.0,255.255.0.0] is reoriginated
using a new Link State ID of 10.0.255.255.
(b) A Link State ID of 10.0.0.0 is used for
[10.0.0.0,255.0.0.0].
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(c) The network [10.0.0.0,255.255.255.0] keeps its Link State ID
of 10.0.0.255.
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Security Considerations
All OSPF protocol exchanges are authenticated. This is accomplished
through authentication fields contained in the OSPF packet header.
For more information, see Sections 8.1, 8.2, and Appendix D.
Author's Address
John Moy
Proteon, Inc.
9 Technology Drive
Westborough, MA 01581
Phone: 508-898-2800
Fax: 508-898-3176
Email: jmoy@proteon.com
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