<- RFC Index (9301..9400)
RFC 9350
Internet Engineering Task Force (IETF) P. Psenak, Ed.
Request for Comments: 9350 Cisco Systems, Inc.
Category: Standards Track S. Hegde
ISSN: 2070-1721 Juniper Networks, Inc.
C. Filsfils
Cisco Systems, Inc.
K. Talaulikar
Cisco Systems, Inc
A. Gulko
Edward Jones
February 2023
IGP Flexible Algorithm
Abstract
IGP protocols historically compute the best paths over the network
based on the IGP metric assigned to the links. Many network
deployments use RSVP-TE or Segment Routing - Traffic Engineering (SR-
TE) to steer traffic over a path that is computed using different
metrics or constraints than the shortest IGP path. This document
specifies a solution that allows IGPs themselves to compute
constraint-based paths over the network. This document also
specifies a way of using Segment Routing (SR) Prefix-SIDs and SRv6
locators to steer packets along the constraint-based paths.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9350.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Requirements Language
3. Terminology
4. Flexible Algorithm
5. Flexible Algorithm Definition Advertisement
5.1. IS-IS Flexible Algorithm Definition Sub-TLV
5.2. OSPF Flexible Algorithm Definition TLV
5.3. Common Handling of the Flexible Algorithm Definition TLV
6. Sub-TLVs of IS-IS FAD Sub-TLV
6.1. IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV
6.2. IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV
6.3. IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV
6.4. IS-IS Flexible Algorithm Definition Flags Sub-TLV
6.5. IS-IS Flexible Algorithm Exclude SRLG Sub-TLV
7. Sub-TLVs of the OSPF FAD TLV
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV
8. IS-IS Flexible Algorithm Prefix Metric Sub-TLV
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV
10. OSPF Flexible Algorithm ASBR Reachability Advertisement
10.1. OSPFv2 Extended Inter-Area ASBR LSA
10.1.1. OSPFv2 Extended Inter-Area ASBR TLV
10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV
11. Advertisement of Node Participation in a Flex-Algorithm
11.1. Advertisement of Node Participation for Segment Routing
11.2. Advertisement of Node Participation for Other Data Planes
12. Advertisement of Link Attributes for Flex-Algorithm
13. Calculation of Flexible Algorithm Paths
13.1. Multi-area and Multi-domain Considerations
14. Flex-Algorithm and Forwarding Plane
14.1. Segment Routing MPLS Forwarding for Flex-Algorithm
14.2. SRv6 Forwarding for Flex-Algorithm
14.3. Other Data Planes' Forwarding for Flex-Algorithm
15. Operational Considerations
15.1. Inter-area Considerations
15.2. Usage of the SRLG Exclude Rule with Flex-Algorithm
15.3. Max-Metric Consideration
15.4. Flexible Algorithm Definition and Changes
15.5. Number of Flex-Algorithms
16. Backward Compatibility
17. Security Considerations
18. IANA Considerations
18.1. IGP IANA Considerations
18.1.1. IGP Algorithm Types Registry
18.1.2. IGP Metric-Type Registry
18.2. IGP Flexible Algorithm Definition Flags Registry
18.3. IS-IS IANA Considerations
18.3.1. IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV
Registry
18.3.2. IS-IS Sub-TLVs for TLVs Advertising Prefix
Reachability Registry
18.3.3. IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition
Sub-TLV Registry
18.4. OSPF IANA Considerations
18.4.1. OSPF Router Information (RI) TLVs Registry
18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs Registry
18.4.3. OSPFv3 Extended-LSA Sub-TLVs Registry
18.4.4. OSPF Flex-Algorithm Prefix Metric Bits Registry
18.4.5. Opaque Link-State Advertisements (LSA) Option Types
Registry
18.4.6. OSPFv2 Extended Inter-Area ASBR TLVs Registry
18.4.7. OSPFv2 Extended Inter-Area ASBR Sub-TLVs Registry
18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLVs
Registry
18.4.9. Link Attribute Application Identifiers Registry
19. References
19.1. Normative References
19.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
An IGP-computed path based on the shortest IGP metric is often
replaced by a traffic-engineered path due to requirements that are
not reflected by the IGP metric. Some networks engineer the IGP
metric assignments in a way that the IGP metric reflects the link
bandwidth or delay. If, for example, the IGP metric reflects the
bandwidth on the link and user traffic is delay sensitive, the best
IGP path may not reflect the best path from such a user's
perspective.
To overcome this limitation, various sorts of Traffic Engineering
have been deployed, including RSVP-TE and SR-TE, in which case the TE
component is responsible for computing paths based on additional
metrics and/or constraints. Such paths need to be installed in the
forwarding tables in addition to, or as a replacement for, the
original paths computed by IGPs. Tunnels are often used to represent
the engineered paths and mechanisms, like the one described in
[RFC3906], and are used to replace the original IGP paths with such
tunnel paths.
This document specifies a set of extensions to IS-IS, OSPFv2, and
OSPFv3 that enable a router to advertise TLVs that (a) identify a
calculation-type, (b) specify a metric-type, and (c) describe a set
of constraints on the topology that are to be used to compute the
best paths along the constrained topology. A given combination of
calculation-type, metric-type, and constraints is known as a
"Flexible Algorithm Definition". A router that sends such a set of
TLVs also assigns a Flex-Algorithm value to the specified combination
of calculation-type, metric-type, and constraints.
This document also specifies a way for a router to use IGPs to
associate one or more Segment Routing with the MPLS Data Plane (SR-
MPLS) Prefix-SIDs [RFC8660] or Segment Routing over IPv6 (SRv6)
locators [RFC8986] with a particular Flex-Algorithm. Each such
Prefix-SID or SRv6 locator then represents a path that is computed
according to the identified Flex-Algorithm. In SRv6, it is the
locator, not the Segment Identifier (SID), that holds the binding to
the algorithm.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
This section defines terms that are often used in this document.
Flexible Algorithm Definition (FAD): the set consisting of (a) a
calculation-type, (b) a metric-type, and (c) a set of constraints.
Flex-Algorithm: a numeric identifier in the range 128-255 that is
associated via configuration with the Flexible Algorithm
Definition.
Flexible Algorithm Participation: per the data plane configuration
state that expresses whether the node is participating in a
particular Flexible Algorithm. Not all routers in a given network
need to participate in a given Flexible Algorithm. The Flexible
Algorithm(s) that a given router participates in is determined by
configuration.
IGP Algorithm: value from the IANA "IGP Algorithm Types" registry
defined under the "Interior Gateway Protocol (IGP) Parameters"
registry group. IGP Algorithms represent the triplet
(calculation-type, metric-type, and constraints), where the second
and third elements of the triplet MAY be unspecified.
ABR: Area Border Router. In IS-IS terminology, it is also known as
the Level 1 (L1) / Level 2 (L2) router.
ASBR: Autonomous System Border Router.
4. Flexible Algorithm
Many possible constraints may be used to compute a path over a
network. Some networks are deployed as multiple planes. A simple
form of constraint may be to use a particular plane. A more
sophisticated form of constraint can include some extended metric, as
described in [RFC8570]. Constraints that restrict paths to links
with specific affinities or avoid links with specific affinities are
also possible. Combinations of these are also possible.
To provide maximum flexibility, a mechanism is provided that allows a
router to identify a particular calculation-type and metric-type,
describe a particular set of constraints, and assign a numeric
identifier, referred to as Flex-Algorithm, to the combination of that
calculation-type, metric-type, and those constraints. The mapping
between the Flex-Algorithm and its meaning is flexible and defined by
the user. As long as all routers in the domain have a common
understanding as to what a particular Flex-Algorithm represents, the
resulting routing computation is consistent and traffic is not
subject to any looping.
The set consisting of (a) a calculation-type, (b) a metric-type, and
(c) a set of constraints is referred to as a Flexible Algorithm
Definition.
The Flex-Algorithm is a numeric identifier in the range 128-255 that
is associated via configuration with the Flexible Algorithm
Definition.
The IANA "IGP Algorithm Types" registry defines the set of values for
IGP Algorithms. The following values are allocated by IANA from this
registry for Flex-Algorithms:
128-255 - Flex-Algorithms
5. Flexible Algorithm Definition Advertisement
To guarantee loop-free forwarding for paths computed for a particular
Flex-Algorithm, all routers that (a) are configured to participate in
a particular Flex-Algorithm and (b) are in the same Flex-Algorithm
Definition advertisement scope MUST agree on the definition of the
Flex-Algorithm. The following procedures ensure this condition is
fulfilled.
5.1. IS-IS Flexible Algorithm Definition Sub-TLV
The IS-IS Flexible Algorithm Definition (FAD) sub-TLV is used to
advertise the definition of the Flex-Algorithm.
The IS-IS FAD sub-TLV is advertised as a sub-TLV of the IS-IS Router
CAPABILITY TLV-242, which is defined in [RFC7981].
The IS-IS FAD sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Flex-Algorithm | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 26
Length: variable number of octets, dependent on the included sub-
TLVs.
Flex-Algorithm: Flexible Algorithm number. Single octet value
between 128 and 255 inclusive.
Metric-Type: type of metric from the IANA "IGP Metric-Type"
registry (Section 18.1.2) to be used during the calculation.
The following values are defined:
0: IGP Metric
1: Min Unidirectional Link Delay, as defined in Section 4.2 of
[RFC8570], encoded as an application-specific link
attribute, as specified in [RFC8919] and Section 12 of this
document.
2: Traffic Engineering Default Metric, as defined in
Section 3.7 of [RFC5305], encoded as an application-specific
link attribute, as specified in [RFC8919] and Section 12 of
this document.
Calc-Type: calculation-type. Value from 0-127 inclusive from the
IANA "IGP Algorithm Types" registry defined under the "Interior
Gateway Protocol (IGP) Parameters" registry. IGP Algorithms in
the range of 0-127 have a defined triplet (calculation-type,
metric-type, constraints). When used to specify the
calculation-type in the FAD sub-TLV, only the calculation-type
defined for the specified IGP Algorithm is used. The Metric/
Constraints MUST NOT be inherited. If the required
calculation-type is Shortest Path First, the value 0 MUST
appear in this field.
Priority: value between 0 and 255 inclusive that specifies the
priority of the advertisement. Numerically greater values are
preferred. Usage of the priority is described in Section 5.3.
Sub-TLVs: optional sub-TLVs.
The IS-IS FAD sub-TLV MAY be advertised in a Label Switched Path
(LSP) of any number. The IS-IS router MAY advertise more than one
IS-IS FAD sub-TLV for a given Flexible Algorithm (see Section 6).
The IS-IS FAD sub-TLV has an area/level scope. The Router Capability
TLV in which the FAD sub-TLV is present MUST have the S bit clear.
An IS-IS L1/L2 router MAY be configured to regenerate the winning FAD
from level 2, without any modification to it, to the level 1 area.
The regeneration of the FAD sub-TLV from level 2 to level 1 is
determined by the L1/L2 router, not by the originator of the FAD
advertisement in level 2. In such a case, the regenerated FAD sub-
TLV will be advertised in the level 1 Router Capability TLV
originated by the L1/L2 router.
An L1/L2 router MUST NOT regenerate any FAD sub-TLV from level 1 to
level 2.
5.2. OSPF Flexible Algorithm Definition TLV
The OSPF FAD TLV is advertised as a top-level TLV of the Router
Information (RI) Link State Advertisement (LSA), which is defined in
[RFC7770].
The OSPF FAD TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Metric-Type | Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 16
Length: variable number of octets, dependent on the included sub-
TLVs.
Flex-Algorithm: Flexible Algorithm number. Single octet value
between 128 and 255 inclusive.
Metric-Type: type of metric from the IANA "IGP Metric-Type"
registry (Section 18.1.2) to be used during the calculation.
The following values are defined:
0: IGP Metric
1: Min Unidirectional Link Delay, as defined in Section 4.2 of
[RFC7471], encoded as an application-specific link
attribute, as specified in [RFC8920] and Section 12 of this
document.
2: Traffic Engineering Metric, as defined in Section 2.5.5 of
[RFC3630], encoded as an application-specific link
attribute, as specified in [RFC8920] and Section 12 of this
document.
Calc-Type: as described in Section 5.1.
Priority: as described in Section 5.1.
Sub-TLVs: optional sub-TLVs.
When multiple OSPF FAD TLVs, for the same Flexible Algorithm, are
received from a given router, the receiver MUST use the first
occurrence of the TLV in the RI LSA. If the OSPF FAD TLV, for the
same Flex-Algorithm, appears in multiple RI LSAs that have different
flooding scopes, the OSPF FAD TLV in the RI LSA with the area-scoped
flooding scope MUST be used. If the OSPF FAD TLV, for the same
algorithm, appears in multiple RI LSAs that have the same flooding
scope, the OSPF FAD TLV in the RI LSA with the numerically smallest
Instance ID MUST be used and subsequent instances of the OSPF FAD TLV
MUST be ignored.
The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED. The AS
flooding scope SHOULD NOT be used unless local configuration policy
on the originating router indicates domain-wide flooding.
5.3. Common Handling of the Flexible Algorithm Definition TLV
This section describes the protocol-independent handling of the FAD
TLV (OSPF) or FAD sub-TLV (IS-IS). We will refer to it as FAD TLV in
this section, even though, in the case of IS-IS, it is a sub-TLV.
The value of the Flex-Algorithm MUST be between 128 and 255
inclusive. If it is not, the FAD TLV MUST be ignored.
Only a subset of the routers participating in the particular Flex-
Algorithm need to advertise the definition of the Flex-Algorithm.
Every router that is configured to participate in a particular Flex-
Algorithm MUST select the Flex-Algorithm Definition based on the
following ordered rules. This allows for the consistent Flex-
Algorithm Definition selection in cases where different routers
advertise different definitions for a given Flex-Algorithm:
1. From the advertisements of the FAD in the area (including both
locally generated advertisements and received advertisements),
select the one(s) with the numerically greatest priority value.
2. If there are multiple advertisements of the FAD with the same
numerically greatest priority, select the one that is originated
from the router with the numerically greatest System-ID, in the
case of IS-IS, or Router ID, in the case of OSPFv2 and OSPFv3.
For IS-IS, the System-ID is described in [ISO10589]. For OSPFv2
and OSPFv3, the standard Router ID is described in [RFC2328] and
[RFC5340], respectively.
The FAD selected according to these rules is also known as the
"winning FAD".
A router that is not configured to participate in a particular Flex-
Algorithm MUST ignore FAD sub-TLV advertisements for such Flex-
Algorithm.
A router that is not participating in a particular Flex-Algorithm MAY
advertise the FAD for such Flex-Algorithm. Receiving routers MUST
consider a received FAD advertisement regardless of the Flex-
Algorithm participation of that FAD advertisement's originator.
Any change in the Flex-Algorithm Definition may result in a temporary
disruption of traffic that is forwarded based on such Flex-Algorithm
paths. The impact is similar to any other event that requires
network-wide convergence.
If a node is configured to participate in a particular Flexible
Algorithm, but there is no valid Flex-Algorithm Definition available
for it or the selected Flex-Algorithm Definition includes
calculation-type, metric-type, constraint, flag, or sub-TLV that is
not supported by the node, it MUST stop participating in such
Flexible Algorithm. That implies that it MUST NOT announce
participation for such Flexible Algorithm, as specified in
Section 11, and it MUST remove any forwarding state associated with
it.
The Flex-Algorithm Definition is topology independent. It applies to
all topologies that a router participates in.
6. Sub-TLVs of IS-IS FAD Sub-TLV
One of the limitations of IS-IS [ISO10589] is that the length of a
TLV/sub-TLV is limited to a maximum of 255 octets. For the FAD sub-
TLV, there are a number of sub-sub-TLVs (defined below) that are
supported. For a given Flex-Algorithm, it is possible that the total
number of octets required to completely define a FAD exceeds the
maximum length supported by a single FAD sub-TLV. In such cases, the
FAD MAY be split into multiple such sub-TLVs, and the content of the
multiple FAD sub-TLVs are combined to provide a complete FAD for the
Flex-Algorithm. In such a case, the fixed portion of the FAD (see
Section 5.1) MUST be identical in all FAD sub-TLVs for a given Flex-
Algorithm from a given IS. In case the fixed portion of such FAD
sub-TLVs differ, the values in the fixed portion in the FAD sub-TLV
in the first occurrence in the lowest-numbered LSP from a given IS
MUST be used.
Any specification that introduces a new IS-IS FAD sub-sub-TLV MUST
specify whether the FAD sub-TLV may appear multiple times in the set
of FAD sub-TLVs for a given Flex-Algorithm from a given IS and how to
handle them if multiple are allowed.
6.1. IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV
The Flexible Algorithm Definition can specify "colors" that are used
by the operator to exclude links during the Flex-Algorithm path
computation.
The IS-IS Flexible Algorithm Exclude Admin Group (FAEAG) sub-TLV is
used to advertise the exclude rule that is used during the Flex-
Algorithm path calculation, as specified in Section 13.
The IS-IS FAEAG sub-TLV is a sub-TLV of the IS-IS FAD sub-TLV. It
has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group, as
defined in [RFC7308].
The IS-IS FAEAG sub-TLV MUST NOT appear more than once in a single
IS-IS FAD sub-TLV. If it appears more than once, the IS-IS FAD sub-
TLV MUST be ignored by the receiver.
The IS-IS FAEAG sub-TLV MUST NOT appear more than once in the set of
FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it
appears more than once in such a set, the IS-IS FAEAG sub-TLV in the
first occurrence in the lowest-numbered LSP from a given IS MUST be
used, and any other occurrences MUST be ignored.
6.2. IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV
The Flexible Algorithm Definition can specify "colors" that are used
by the operator to include links during the Flex-Algorithm path
computation.
The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV is used
to advertise the include-any rule that is used during the Flex-
Algorithm path calculation, as specified in Section 13.
The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV is a
sub-TLV of the IS-IS FAD sub-TLV. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 2
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group, as
defined in [RFC7308].
The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV MUST NOT
appear more than once in a single IS-IS FAD sub-TLV. If it appears
more than once, the IS-IS FAD sub-TLV MUST be ignored by the
receiver.
The IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV MUST NOT
appear more than once in the set of FAD sub-TLVs for a given Flex-
Algorithm from a given IS. If it appears more than once in such a
set, the IS-IS Flexible Algorithm Include-Any Admin Group sub-TLV in
the first occurrence in the lowest-numbered LSP from a given IS MUST
be used, and any other occurrences MUST be ignored.
6.3. IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV
The Flexible Algorithm Definition can specify "colors" that are used
by the operator to include links during the Flex-Algorithm path
computation.
The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV is used
to advertise the include-all rule that is used during the Flex-
Algorithm path calculation, as specified in Section 13.
The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV is a
sub-TLV of the IS-IS FAD sub-TLV. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group, as
defined in [RFC7308].
The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV MUST NOT
appear more than once in a single IS-IS FAD sub-TLV. If it appears
more than once, the IS-IS FAD sub-TLV MUST be ignored by the
receiver.
The IS-IS Flexible Algorithm Include-All Admin Group sub-TLV MUST NOT
appear more than once in the set of FAD sub-TLVs for a given Flex-
Algorithm from a given IS. If it appears more than once in such a
set, the IS-IS Flexible Algorithm Include-All Admin Group sub-TLV in
the first occurrence in the lowest-numbered LSP from a given IS MUST
be used, and any other occurrences MUST be ignored.
6.4. IS-IS Flexible Algorithm Definition Flags Sub-TLV
The IS-IS Flexible Algorithm Definition Flags (FADF) sub-TLV is a
sub-TLV of the IS-IS FAD sub-TLV. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 4
Length: variable, number of octets of the Flags field.
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
M-flag: when set, the Flex-Algorithm-specific prefix metric
MUST be used for inter-area and external prefix calculation.
This flag is not applicable to prefixes advertised as SRv6
locators.
A new IANA "IGP Flexible Algorithm Definition Flags" registry is
defined for allocation of bits in the Flags field -- see
Section 18.2.
Bits are defined/sent starting with bit 0 defined above. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order to minimize the number of bits that will need
to be transmitted.
Undefined bits MUST be transmitted as 0.
Bits that are not transmitted MUST be treated as if they are set to 0
on receipt.
The IS-IS FADF sub-TLV MUST NOT appear more than once in a single IS-
IS FAD sub-TLV. If it appears more than once, the IS-IS FAD sub-TLV
MUST be ignored by the receiver.
The IS-IS FADF sub-TLV MUST NOT appear more than once in the set of
FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it
appears more than once in such a set, the IS-IS FADF sub-TLV in the
first occurrence in the lowest-numbered LSP from a given IS MUST be
used, and any other occurrences MUST be ignored.
If the IS-IS FADF sub-TLV is not present inside the IS-IS FAD sub-
TLV, all the bits are assumed to be set to 0.
If a node is configured to participate in a particular Flexible
Algorithm, but the selected Flex-Algorithm Definition includes a bit
in the IS-IS FADF sub-TLV that is not supported by the node, it MUST
stop participating in such Flexible Algorithm.
New flag bits may be defined in the future. Implementations MUST
check all advertised flag bits in the received IS-IS FADF sub-TLV --
not just the subset currently defined.
The M-flag MUST not be used when calculating prefix reachability for
the SRv6 Locator prefix.
6.5. IS-IS Flexible Algorithm Exclude SRLG Sub-TLV
The Flexible Algorithm Definition can specify Shared Risk Link Groups
(SRLGs) that the operator wants to exclude during the Flex-Algorithm
path computation.
The IS-IS Flexible Algorithm Exclude SRLG (FAESRLG) sub-TLV is used
to advertise the exclude rule that is used during the Flex-Algorithm
path calculation, as specified in Section 13.
The IS-IS FAESRLG sub-TLV is a sub-TLV of the IS-IS FAD sub-TLV. It
has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Value |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5
Length: variable, dependent on number of SRLG values. MUST be a
multiple of 4 octets.
Shared Risk Link Group Value: SRLG value, as defined in
[RFC5307].
The IS-IS FAESRLG sub-TLV MUST NOT appear more than once in a single
IS-IS FAD sub-TLV. If it appears more than once, the IS-IS FAD sub-
TLV MUST be ignored by the receiver.
The IS-IS FAESRLG sub-TLV MAY appear more than once in the set of FAD
sub-TLVs for a given Flex-Algorithm from a given IS. This may be
necessary in cases where the total number of SRLG values that are
specified cause the FAD sub-TLV to exceed the maximum length of a
single FAD sub-TLV. In such a case, the receiver MUST use the union
of all values across all IS-IS FAESRLG sub-TLVs from such set.
7. Sub-TLVs of the OSPF FAD TLV
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV
The OSPF Flexible Algorithm Exclude Admin Group (FAEAG) sub-TLV is a
sub-TLV of the OSPF FAD TLV. Its usage is described in Section 6.1.
It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group, as
defined in [RFC7308].
The OSPF FAEAG sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV
The OSPF Flexible Algorithm Include-Any Admin Group sub-TLV is a sub-
TLV of the OSPF FAD TLV. The usage of this sub-TLV is described in
Section 6.2. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 2
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group, as
defined in [RFC7308].
The OSPF Flexible Algorithm Include-Any Admin Group sub-TLV MUST NOT
appear more than once in an OSPF FAD TLV. If it appears more than
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV
The OSPF Flexible Algorithm Include-All Admin Group sub-TLV is a sub-
TLV of the OSPF FAD TLV. The usage of this sub-TLV is described in
Section 6.3. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group, as
defined in [RFC7308].
The OSPF Flexible Algorithm Include-All Admin Group sub-TLV MUST NOT
appear more than once in an OSPF FAD TLV. If it appears more than
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV
The OSPF Flexible Algorithm Definition Flags (FADF) sub-TLV is a sub-
TLV of the OSPF FAD TLV. It has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 4
Length: variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
M-flag: when set, the Flex-Algorithm-specific prefix and ASBR
metric MUST be used for inter-area and external prefix
calculation. This flag is not applicable to prefixes
advertised as SRv6 locators.
A new IANA "IGP Flexible Algorithm Definition Flags" registry is
defined for allocation of bits in the Flags field -- see
Section 18.2.
Bits are defined/sent starting with bit 0 defined above. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order to minimize the number of bits that will need
to be transmitted.
Undefined bits MUST be transmitted as 0.
Bits that are not transmitted MUST be treated as if they are set to 0
on receipt.
The OSPF FADF sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
If the OSPF FADF sub-TLV is not present inside the OSPF FAD TLV, all
the bits are assumed to be set to 0.
If a node is configured to participate in a particular Flexible
Algorithm, but the selected Flex-Algorithm Definition includes a bit
in the OSPF FADF sub-TLV that is not supported by the node, it MUST
stop participating in such Flexible Algorithm.
New flag bits may be defined in the future. Implementations MUST
check all advertised flag bits in the received OSPF FADF sub-TLV --
not just the subset currently defined.
The M-flag MUST not be used when calculating prefix reachability for
the SRv6 Locator prefix.
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV
The OSPF Flexible Algorithm Exclude SRLG (FAESRLG) sub-TLV is a sub-
TLV of the OSPF FAD TLV. Its usage is described in Section 6.5. It
has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Value |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5
Length: variable, dependent on the number of SRLGs. MUST be a
multiple of 4 octets.
Shared Risk Link Group Value: SRLG value, as defined in
[RFC4203].
The OSPF FAESRLG sub-TLV MUST NOT appear more than once in an OSPF
FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be
ignored by the receiver.
8. IS-IS Flexible Algorithm Prefix Metric Sub-TLV
The IS-IS Flexible Algorithm Prefix Metric (FAPM) sub-TLV supports
the advertisement of a Flex-Algorithm-specific prefix metric
associated with a given prefix advertisement.
The IS-IS FAPM sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237
and has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Flex-Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 6
Length: 5 octets
Flex-Algorithm: single octet value between 128 and 255 inclusive.
Metric: 4 octets of metric information.
The IS-IS FAPM sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
If a prefix is advertised with a Flex-Algorithm prefix metric larger
than MAX_PATH_METRIC, as defined in [RFC5305], this prefix MUST NOT
be considered during the Flexible Algorithm computation.
The usage of the Flex-Algorithm prefix metric is described in
Section 13.
The IS-IS FAPM sub-TLV MUST NOT be advertised as a sub-TLV of the IS-
IS SRv6 Locator TLV [RFC9352]. The IS-IS SRv6 Locator TLV includes
the Algorithm and Metric fields, which MUST be used instead. If the
FAPM sub-TLV is present as a sub-TLV of the IS-IS SRv6 Locator TLV in
the received LSP, such FAPM sub-TLV MUST be ignored.
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV
The OSPF Flexible Algorithm Prefix Metric (FAPM) sub-TLV supports the
advertisement of a Flex-Algorithm-specific prefix metric associated
with a given prefix advertisement.
The OSPF FAPM sub-TLV is a sub-TLV of the:
* OSPFv2 Extended Prefix TLV [RFC7684] and
* following OSPFv3 TLVs, as defined in [RFC8362]:
- Inter-Area Prefix TLV
- External-Prefix TLV
The OSPF FAPM sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3 for OSPFv2, and 26 for OSPFv3
Length: 8 octets
Flex-Algorithm: single octet value between 128 and 255 inclusive.
Flags: 1-octet value
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E| |
+-+-+-+-+-+-+-+-+
E bit: position 0: The type of external metric. If the bit is
set, the metric specified is a Type 2 external metric. This
bit is applicable only to OSPF external and Not-So-Stubby
Area (NSSA) external prefixes. This is semantically the
same as the E bit in Appendix A.4.5 of [RFC2328] and
Appendix A.4.7 of [RFC5340] for OSPFv2 and OSPFv3,
respectively.
Bits 1 through 7: MUST be cleared by the originator and
ignored by the receiver.
Reserved: MUST be set to 0 and ignored at reception.
Metric: 4 octets of metric information.
The OSPF FAPM sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
The usage of the Flex-Algorithm prefix metric is described in
Section 13.
10. OSPF Flexible Algorithm ASBR Reachability Advertisement
An OSPF ABR advertises the reachability of ASBRs in its attached
areas to enable routers within those areas to perform route
calculations for external prefixes advertised by the ASBRs. OSPF
extensions for advertisement of Flex-Algorithm-specific reachability
and the metric for ASBRs is similarly required for Flex-Algorithm
external prefix computations, as described further in Section 13.1.
10.1. OSPFv2 Extended Inter-Area ASBR LSA
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA is an OSPF Opaque
LSA [RFC5250] that is used to advertise additional attributes related
to the reachability of the OSPFv2 ASBR that is external to the area
yet internal to the OSPF domain. Semantically, the OSPFv2 EIA-ASBR
LSA is equivalent to the fixed format Type 4 summary-LSA [RFC2328].
Unlike the Type 4 summary-LSA, the Link State ID (LSID) of the EIA-
ASBR LSA does not carry the ASBR Router ID -- the ASBR Router ID is
carried in the body of the LSA. The OSPFv2 EIA-ASBR LSA is
advertised by an OSPFv2 ABR, and its flooding is defined to be area-
scoped only.
An OSPFv2 ABR generates the EIA-ASBR LSA for an ASBR when it is
advertising the Type 4 summary-LSA for it and has the need for
advertising additional attributes for that ASBR beyond what is
conveyed in the fixed-format Type 4 summary-LSA. An OSPFv2 ABR MUST
NOT advertise the EIA-ASBR LSA for an ASBR for which it is not
advertising the Type 4 summary-LSA. This ensures that the ABR does
not generate the EIA-ASBR LSA for an ASBR to which it does not have
reachability in the base OSPFv2 topology calculation. The OSPFv2 ABR
SHOULD NOT advertise the EIA-ASBR LSA for an ASBR when it does not
have additional attributes to advertise for that ASBR.
The OSPFv2 EIA-ASBR LSA has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
The LS age and Options fields are as defined in Appendix A.4.1 of
[RFC2328].
The LS Type MUST be 10, indicating that the Opaque LSA flooding scope
is area-local [RFC5250].
The Opaque Type used by the OSPFv2 EIA-ASBR LSA is 11. The Opaque
Type is used to differentiate the various types of OSPFv2 Opaque LSAs
and is described in Section 3 of [RFC5250].
The Opaque ID field is an arbitrary value used to maintain multiple
OSPFv2 EIA-ASBR LSAs. For OSPFv2 EIA-ASBR LSAs, the Opaque ID has no
semantic significance other than to differentiate OSPFv2 EIA-ASBR
LSAs originated by the same OSPFv2 ABR. If multiple OSPFv2 EIA-ASBR
LSAs specify the same ASBR, the attributes from the Opaque LSA with
the lowest Opaque ID SHOULD be used.
The Advertising Router, LS sequence number, and LS checksum fields
are as defined in Appendix A.4.1 of [RFC2328].
The Length field is as defined in Appendix A.4.1 of [RFC2328]. It
represents the total length (in octets) of the Opaque LSA, including
the LSA header and all TLVs (including padding).
The format of the TLVs within the body of the OSPFv2 EIA-ASBR LSA is
the same as the format used by the Traffic Engineering Extensions to
OSPFv2 [RFC3630]. The variable TLV section consists of one or more
nested TLV tuples. Nested TLVs are also referred to as sub-TLVs.
The TLV Length field defines the length of the value portion in
octets (thus, a TLV with no value portion would have a length of 0).
The TLV is padded to 4-octet alignment; padding is not included in
the Length field (so a 3-octet value would have a length of 3, but
the total size of the TLV would be 8 octets). Nested TLVs are also
32-bit aligned. For example, a 1-octet value would have the Length
field set to 1, and 3 octets of padding would be added to the end of
the value portion of the TLV. The padding is composed of zeros.
10.1.1. OSPFv2 Extended Inter-Area ASBR TLV
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) TLV is a top-level TLV
of the OSPFv2 EIA-ASBR LSA and is used to advertise additional
attributes associated with the reachability of an ASBR.
The OSPFv2 EIA-ASBR TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASBR Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable number of octets.
ASBR Router ID: 4 octets carrying the OSPF Router ID of the ASBR
whose information is being carried.
Sub-TLVs: variable
Only a single OSPFv2 EIA-ASBR TLV MUST be advertised in each OSPFv2
EIA-ASBR LSA, and the receiver MUST ignore all instances of this TLV
other than the first one in an LSA.
The OSPFv2 EIA-ASBR TLV MUST be present inside an OSPFv2 EIA-ASBR LSA
and MUST include at least a single sub-TLV; otherwise, the OSPFv2
EIA-ASBR LSA MUST be ignored by the receiver.
10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV
The OSPF Flexible Algorithm ASBR Metric (FAAM) sub-TLV supports the
advertisement of a Flex-Algorithm-specific metric associated with a
given ASBR reachability advertisement by an ABR.
The OSPF FAAM sub-TLV is a sub-TLV of the:
* OSPFv2 Extended Inter-Area ASBR TLV, as defined in Section 10.1.1,
and
* OSPFv3 Inter-Area-Router TLV, as defined in [RFC8362].
The OSPF FAAM sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1 for OSPFv2, and 33 for OSPFv3
Length: 8 octets
Flex-Algorithm: single octet value between 128 and 255 inclusive.
Reserved: 3 octets. MUST be set to 0 and ignored at reception.
Metric: 4 octets of metric information.
The OSPF FAAM sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
The advertisement of the ASBR reachability using the OSPF FAAM sub-
TLV inside the OSPFv2 EIA-ASBR LSA follows Section 12.4.3 of
[RFC2328] and inside the OSPFv3 E-Inter-Area-Router-LSA follows
Section 4.8.5 of [RFC5340]. The reachability of the ASBR is
evaluated in the context of the specific Flex-Algorithm.
The FAAM computed by the ABR will be equal to the metric to reach the
ASBR for a given Flex-Algorithm in a source area or the cumulative
metric via an ABR(s) when the ASBR is in a remote area. This is
similar in nature to how the metric is set when the ASBR reachability
metric is computed in the default algorithm for the metric in the
OSPFv2 Type 4 ASBR summary-LSA and the OSPFv3 Inter-Area-Router-LSA.
An OSPF ABR MUST NOT include the OSPF FAAM sub-TLV with a specific
Flex-Algorithm in its reachability advertisement for an ASBR between
areas unless that ASBR is reachable for it in the context of that
specific Flex-Algorithm.
An OSPF ABR MUST include the OSPF FAAM sub-TLVs as part of the ASBR
reachability advertisement between areas for any Flex-Algorithm for
which the winning FAD includes the M-flag and the ASBR is reachable
in the context of that specific Flex-Algorithm.
OSPF routers MUST use the OSPF FAAM sub-TLV to calculate the
reachability of the ASBRs if the winning FAD for the specific Flex-
Algorithm includes the M-flag. OSPF routers MUST NOT use the OSPF
FAAM sub-TLV to calculate the reachability of the ASBRs for the
specific Flex-Algorithm if the winning FAD for such Flex-Algorithm
does not include the M-flag. Instead, the OSPFv2 Type 4 summary-LSAs
or the OSPFv3 Inter-Area-Router-LSAs MUST be used, as specified in
Section 16.2 of [RFC2328] and Section 4.8.5 of [RFC5340] for OSPFv2
and OSPFv3, respectively.
The processing of a new or changed OSPF FAAM sub-TLV triggers the
processing of external routes similar to what is described in
Section 16.5 of [RFC2328] for OSPFv2 and Section 4.8.5 of [RFC5340]
for OSPFv3 for the specific Flex-Algorithm. The OSPF external and
NSSA external route calculation should be limited to a Flex-
Algorithm(s) for which the winning FAD(s) includes the M-flag.
Processing of the OSPF FAAM sub-TLV does not require the existence of
the equivalent OSPFv2 Type 4 summary-LSA or the OSPFv3 Inter-Area-
Router-LSA that is advertised by the same ABR inside the area. The
presence of the base LSA is not mandatory for the usage of the
extended LSA with the OSPF FAAM sub-TLV.
11. Advertisement of Node Participation in a Flex-Algorithm
When a router is configured to participate in a particular Flex-
Algorithm and is advertising such participation, it is participating
in that Flex-Algorithm.
Paths for various data planes MAY be computed for a specific Flex-
Algorithm. Each data plane uses its own specific forwarding over
such Flex-Algorithm paths. To guarantee the presence of the data-
plane-specific forwarding, associated with a particular Flex-
Algorithm, a router MUST advertise its participation for a particular
Flex-Algorithm for each data plane. Some data planes may share a
common participation advertisement (e.g., SR-MPLS and SRv6).
Advertisement of the participation for any particular Flex-Algorithm
in any data plane is subject to the condition specified in
Section 5.3.
11.1. Advertisement of Node Participation for Segment Routing
[RFC8665], [RFC8666], and [RFC8667] (IGP Segment Routing extensions)
describe how the SR-Algorithm is used to compute the IGP best path.
Routers advertise support for the SR-Algorithm as a node capability,
as described in the above-mentioned IGP Segment Routing extensions.
To advertise participation for a particular Flex-Algorithm for
Segment Routing, including both SR-MPLS and SRv6, the Flex-Algorithm
value MUST be advertised in the SR-Algorithm TLV (OSPF) or sub-TLV
(IS-IS).
Segment Routing Flex-Algorithm participation advertisement is
topology independent. When a router advertises participation in an
SR-Algorithm, the participation applies to all topologies in which
the advertising node participates.
11.2. Advertisement of Node Participation for Other Data Planes
This section describes considerations related to how other data
planes can advertise their participation in a specific Flex-
Algorithm.
Data-plane-specific Flex-Algorithm participation advertisements MAY
be topology specific or MAY be topology independent, depending on the
data plane itself.
Data-plane-specific advertisement for Flex-Algorithm participation
MUST be defined for each data plane and is outside the scope of this
document.
12. Advertisement of Link Attributes for Flex-Algorithm
Various link attributes may be used during the Flex-Algorithm path
calculation. For example, include or exclude rules based on link
affinities can be part of the Flex-Algorithm Definition, as defined
in Sections 6 and 7.
Application-specific link attributes, as specified in [RFC8919] or
[RFC8920], that are to be used during Flex-Algorithm calculation MUST
use the Application-Specific Link Attribute (ASLA) advertisements
defined in [RFC8919] or [RFC8920] unless, in the case of IS-IS, the
L-flag is set in the ASLA advertisement. When the L-flag is set,
then legacy advertisements MUST be used, subject to the procedures
and constraints defined in Section 4.2 of [RFC8919] and Section 6.
The mandatory use of ASLA advertisements applies to link attributes
specifically mentioned in this document (Min Unidirectional Link
Delay, TE Default Metric, Administrative Group, Extended
Administrative Group, and Shared Risk Link Group) and any other link
attributes that may be used in support of Flex-Algorithm in the
future.
A new Application Identifier Bit is defined to indicate that the ASLA
advertisement is associated with the Flex-Algorithm application.
This bit is set in the Standard Application Bit Mask (SABM) defined
in [RFC8919] or [RFC8920]:
Bit 3: Flexible Algorithm (X-bit)
ASLA Admin Group Advertisements to be used by the Flexible Algorithm
application MAY use either the Administrative Group or Extended
Administrative Group encodings.
A receiver supporting this specification MUST accept both ASLA
Administrative Group and Extended Administrative Group TLVs, as
defined in [RFC8919] or [RFC8920]. In the case of IS-IS, if the
L-flag is set in the ASLA advertisement, as defined in Section 4.2 of
[RFC8919], then the receiver MUST be able to accept both the
Administrative Group TLV, as defined in [RFC5305], and the Extended
Administrative Group TLV, as defined in [RFC7308].
13. Calculation of Flexible Algorithm Paths
A router MUST be configured to participate in a given Flex-Algorithm
K and MUST select the FAD based on the rules defined in Section 5.3
before it can compute any path for that Flex-Algorithm.
No specific two-way connectivity check is performed during the Flex-
Algorithm path computation. The result of the existing Flex-
Algorithm-agnostic, two-way connectivity check is used during the
Flex-Algorithm path computation.
As described in Section 11, participation for any particular Flex-
Algorithm MUST be advertised on a per data plane basis. Calculation
of the paths for any particular Flex-Algorithm is data plane
specific.
Multiple data planes MAY use the same Flex-Algorithm value at the
same time and, as such, share the FAD for it. Traffic for each data
plane will be forwarded based on the data-plane-specific forwarding
entries.
The Flex-Algorithm Definition is data plane independent and is used
by all Flex-Algorithm data planes.
The way various data planes handle nodes that do not participate in
Flexible Algorithm is data plane specific. If the data plane only
wants to consider participating nodes during the Flex-Algorithm
calculation, then when computing paths for a given Flex-Algorithm,
all nodes that do not advertise participation for that Flex-Algorithm
in their data-plane-specific advertisements MUST be pruned from the
topology. Segment Routing, including both SR-MPLS and SRv6, are data
planes that MUST use such pruning when computing Flex-Algorithm
paths.
When computing the path for a given Flex-Algorithm, the metric-type
that is part of the Flex-Algorithm Definition (Section 5) MUST be
used.
When computing the path for a given Flex-Algorithm, the calculation-
type that is part of the Flex-Algorithm Definition (Section 5) MUST
be used.
Various links that include or exclude rules can be part of the Flex-
Algorithm Definition. To refer to a particular bit within an Admin
Group or Extended Admin Group, we use the term "color".
Rules, in the order as specified below, MUST be used to prune links
from the topology during the Flex-Algorithm computation.
For all links in the topology:
1. Check if any exclude Administrative Group rule is part of the
Flex-Algorithm Definition. If such exclude rule exists, check if
any color that is part of the exclude rule is also set on the
link. If such a color is set, the link MUST be pruned from the
computation.
2. Check if any exclude SRLG rule is part of the Flex-Algorithm
Definition. If such exclude rule exists, check if the link is
part of any SRLG that is also part of the SRLG exclude rule. If
the link is part of such SRLG, the link MUST be pruned from the
computation.
3. Check if any include-any Administrative Group rule is part of the
Flex-Algorithm Definition. If such include-any rule exists,
check if any color that is part of the include-any rule is also
set on the link. If no such color is set, the link MUST be
pruned from the computation.
4. Check if any include-all Administrative Group rule is part of the
Flex-Algorithm Definition. If such include-all rule exists,
check if all colors that are part of the include-all rule are
also set on the link. If all such colors are not set on the
link, the link MUST be pruned from the computation.
5. If the Flex-Algorithm Definition uses something other than the
IGP metric (Section 5), and such metric is not advertised for the
particular link in a topology for which the computation is done,
such link MUST be pruned from the computation. A metric of value
0 MUST NOT be assumed in such a case.
13.1. Multi-area and Multi-domain Considerations
Any IGP Shortest Path Tree calculation is limited to a single area.
This applies to Flex-Algorithm calculations as well. Given that the
computing router does not have visibility of the topology of the next
areas or domain, the Flex-Algorithm-specific path to an inter-area or
inter-domain prefix will be computed for the local area only. The
egress L1/L2 router (ABR in OSPF), or ASBR for an inter-domain case,
will be selected based on the best path for the given Flex-Algorithm
in the local area, and such egress ABR or ASBR router will be
responsible to compute the best Flex-Algorithm-specific path over the
next area or domain. This may produce an end-to-end path, which is
suboptimal based on Flex-Algorithm constraints. In cases where the
ABR or ASBR has no reachability to a prefix for a given Flex-
Algorithm in the next area or domain, the traffic could be dropped by
the ABR/ASBR.
To allow the optimal end-to-end path for an inter-area or inter-
domain prefix for any Flex-Algorithm to be computed, the FAPM has
been defined in Sections 8 and 9. For external route calculation for
prefixes originated by ASBRs in remote areas in OSPF, the FAAM has
been defined in Section 10.2 for the ABR to indicate its ASBR
reachability along with the metric for the specific Flex-Algorithm.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, an ABR or an ASBR MUST include the FAPM (see
Sections 8 and 9) when advertising the prefix that is reachable in a
given Flex-Algorithm between areas or domains. Such metric will be
equal to the metric to reach the prefix for that Flex-Algorithm in
its source area or domain. This is similar in nature to how the
metric is set when prefixes are advertised between areas or domains
for the default algorithm. When a prefix is unreachable in its
source area or domain in a specific Flex-Algorithm, then an ABR or
ASBR MUST NOT include the FAPM for that Flex-Algorithm when
advertising the prefix between areas or domains.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, the FAPM MUST be used during the calculation of
prefix reachability for the inter-area and external prefixes. If the
FAPM for the Flex-Algorithm is not advertised with the inter-area or
external prefix reachability advertisement, the prefix MUST be
considered as unreachable for that Flex-Algorithm. Similarly, in the
case of OSPF, for ASBRs in remote areas, if the FAAM is not
advertised by the local ABR(s), the ASBR MUST be considered as
unreachable for that Flex-Algorithm, and the external prefix
advertisements from such an ASBR are not considered for that Flex-
Algorithm.
The Flex-Algorithm prefix metrics and the OSPF Flex-Algorithm ASBR
metrics MUST NOT be used during the Flex-Algorithm computation unless
the FAD selected based on the rules defined in Section 5.3 includes
the M-flag, as described in Sections 6.4 or 7.4.
In the case of OSPF, when calculating external routes in a Flex-
Algorithm, if the winning FAD includes the M-flag, and the
advertising ASBR is in a remote area, the metric will be the sum of
the following:
* the FAPM for that Flex-Algorithm advertised with the external
route by the ASBR
* the metric to reach the ASBR for that Flex-Algorithm from the
local ABR, i.e., the FAAM for that Flex-Algorithm advertised by
the ABR in the local area for that ASBR
* the Flex-Algorithm-specific metric to reach the local ABR
This is similar in nature to how the metric is calculated for routes
learned from remote ASBRs in the default algorithm using the OSPFv2
Type 4 ASBR summary-LSA and the OSPFv3 Inter-Area-Router-LSA.
If the FAD selected based on the rules defined in Section 5.3 does
not include the M-flag, then the IGP metrics associated with the
prefix reachability advertisements used by the base IS-IS and OSPF
protocol MUST be used for the Flex-Algorithm route computation.
Similarly, in the case of external route calculations in OSPF, the
ASBR reachability is determined based on the base OSPFv2 Type 4
summary-LSA and the OSFPv3 Inter-Area-Router-LSA.
It is NOT RECOMMENDED to use the Flex-Algorithm for inter-area or
inter-domain prefix reachability without the M-flag set. The reason
is that, without the explicit Flex-Algorithm prefix metric
advertisement (and the Flex-Algorithm ASBR metric advertisement in
the case of OSPF external route calculation), it is not possible to
conclude whether the ABR or ASBR has reachability to the inter-area
or inter-domain prefix for a given Flex-Algorithm in the next area or
domain. Sending the Flex-Algorithm traffic for such a prefix towards
the ABR or ASBR may result in traffic looping or persistent traffic
drop.
During the route computation, it is possible for the Flex-Algorithm-
specific metric to exceed the maximum value that can be stored in an
unsigned 32-bit variable. In such scenarios, the value MUST be
considered to be of value 0xFFFFFFFF during the computation and
advertised as such.
The FAPM MUST NOT be advertised with IS-IS L1 or L2 intra-area,
OSPFv2 intra-area, or OSPFv3 intra-area routes. If the FAPM is
advertised for these route-types, it MUST be ignored during the
prefix reachability calculation.
The M-flag in the FAD is not applicable to prefixes advertised as
SRv6 locators. The IS-IS SRv6 Locator TLV [RFC9352] includes the
Algorithm and Metric fields. When the SRv6 Locator is advertised
between areas or domains, the Metric field in the Locator TLV of IS-
IS MUST be used irrespective of the M-flag in the FAD advertisement.
OSPF external and NSSA external prefix advertisements MAY include a
non-zero forwarding address in the prefix advertisements in the base
protocol. In such a scenario, the Flex-Algorithm-specific
reachability of the external prefix is determined by Flex-Algorithm-
specific reachability of the forwarding address.
In OSPF, the procedures for translation of NSSA external prefix
advertisements into external prefix advertisements performed by an
NSSA ABR [RFC3101] remain unchanged for Flex-Algorithm. An NSSA
translator MUST include the OSPF FAPM sub-TLVs for all Flex-
Algorithms that are in the original NSSA external prefix
advertisement from the NSSA ASBR in the translated external prefix
advertisement generated by it, regardless of its participation in
those Flex-Algorithms or its having reachability to the NSSA ASBR in
those Flex-Algorithms.
An area could become partitioned from the perspective of the Flex-
Algorithm due to the constraints and/or metric being used for it
while maintaining the continuity in the base algorithm. When that
happens, some destinations inside that area could become unreachable
in that Flex-Algorithm. These destinations will not be able to use
an inter-area path. This is the consequence of the fact that the
inter-area prefix reachability advertisement would not be available
for these intra-area destinations within the area. It is RECOMMENDED
to minimize the risk of such partitioning by providing enough
redundancy inside the area for each Flex-Algorithm being used.
14. Flex-Algorithm and Forwarding Plane
This section describes how Flex-Algorithm paths are used in
forwarding.
14.1. Segment Routing MPLS Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SR MPLS
forwarding.
Prefix-SID advertisements include an SR-Algorithm value and, as such,
are associated with the specified SR-Algorithm. Prefix-SIDs are also
associated with a specific topology that is inherited from the
associated prefix reachability advertisement. When the algorithm
value advertised is a Flex-Algorithm value, the Prefix-SID is
associated with paths calculated using that Flex-Algorithm in the
associated topology.
A Flex-Algorithm path MUST be installed in the MPLS forwarding plane
using the MPLS label that corresponds to the Prefix-SID that was
advertised for that Flex-algorithm. If the Prefix-SID for a given
Flex-Algorithm is not known, the Flex-Algorithm-specific path cannot
be installed in the MPLS forwarding plane.
Traffic that is supposed to be routed via Flex-Algorithm-specific
paths MUST be dropped when there are no such paths available.
Loop Free Alternate (LFA) paths ([RFC6571] or its variants) for a
given Flex-Algorithm MUST be computed using the same constraints as
the calculation of the primary paths for that Flex-Algorithm. LFA
paths MUST only use Prefix-SIDs advertised specifically for the given
algorithm. LFA paths MUST NOT use an Adjacency SID that belongs to a
link that has been pruned from the Flex-Algorithm computation.
If LFA protection is being used to protect a given Flex-Algorithm
path, all routers in the area participating in the given Flex-
Algorithm SHOULD advertise at least one Flex-Algorithm-specific Node-
SID. These Node-SIDs are used to steer traffic over the LFA-computed
backup path.
14.2. SRv6 Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SRv6
forwarding.
In SRv6, a node is provisioned with a (topology, algorithm) specific
locator for each of the topology/algorithm pairs supported by that
node. Each locator is an aggregate prefix for all SIDs provisioned
on that node that have the matching topology/algorithm.
The SRv6 locator advertisement in IS-IS [RFC9352] includes the Multi-
Topology Identifier (MTID) value that associates the locator with a
specific topology. SRv6 locator advertisements also include an
algorithm value that explicitly associates the locator with a
specific algorithm. When the algorithm value advertised with a
locator represents a Flex-Algorithm, the paths to the locator prefix
MUST be calculated using the specified Flex-Algorithm in the
associated topology.
Forwarding entries for the locator prefixes advertised in IS-IS MUST
be installed in the forwarding plane of the receiving SRv6-capable
routers when the associated topology/algorithm is participating in
them. Forwarding entries for locators associated with Flex-
Algorithms in which the node is not participating MUST NOT be
installed in the forwarding plane.
When the locator is associated with a Flex-Algorithm, LFA paths to
the locator prefix MUST be calculated using such Flex-Algorithm in
the associated topology to guarantee that they follow the same
constraints as the calculation of the primary paths. LFA paths MUST
only use SRv6 SIDs advertised specifically for the given Flex-
Algorithm.
If LFA protection is being used to protect locators associated with a
given Flex-Algorithm, all routers in the area participating in the
given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm-
specific locator and END SID per node and one END.X SID for every
link that has not been pruned from such Flex-Algorithm computation.
These locators and SIDs are used to steer traffic over the LFA-
computed backup path.
14.3. Other Data Planes' Forwarding for Flex-Algorithm
Any data plane that wants to use Flex-Algorithm-specific forwarding
needs to install some form of Flex-Algorithm-specific forwarding
entries.
Data-plane-specific forwarding for Flex-Algorithms MUST be defined
for each data plane and is outside the scope of this document.
15. Operational Considerations
15.1. Inter-area Considerations
The scope of the Flex-Algorithm computation and the scope of the FAD
is an area. In IS-IS, the Router Capability TLV in which the FAD
sub-TLV is advertised MUST have the S bit clear, which prevents it
from being flooded outside the level in which it was originated.
Even though in OSPF the FAD sub-TLV can be flooded in an RI LSA that
has an AS flooding scope, the FAD selection is performed for each
individual area in which it is being used.
There is no requirement for the FAD for a particular Flex-Algorithm
to be identical in all areas in the network. For example, traffic
for the same Flex-Algorithm may be optimized for minimal delay (e.g.,
using delay metric) in one area or level while being optimized for
available bandwidth (e.g., using IGP metric) in another area or
level.
As described in Section 5.1, IS-IS allows the regeneration of the
winning FAD from level 2, without any modification to it, into a
level 1 area. This allows the operator to configure the FAD in one
or multiple routers in level 2, without the need to repeat the same
task in each level 1 area, if the intent is to have the same FAD for
the particular Flex-Algorithm across all levels. This can similarly
be achieved in OSPF by using the AS flooding scope of the RI LSA in
which the FAD sub-TLV for the particular Flex-Algorithm is
advertised.
Regeneration of the FAD from a level 1 area to the level 2 area is
not supported in IS-IS, so if the intent is to regenerate the FAD
between IS-IS levels, the FAD MUST be defined on a router(s) that is
in level 2. In OSPF, the FAD definition can be done in any area and
propagated to all routers in the OSPF routing domain by using the AS
flooding scope of the RI LSA.
15.2. Usage of the SRLG Exclude Rule with Flex-Algorithm
There are two different ways in which SRLG information can be used
with Flex-Algorithms:
* In a context of a single Flex-Algorithm, it can be used for
computation of backup paths, as described in
[RTGWG-SEGMENT-ROUTING-TI-LFA]. This usage does not require
association of any specific SRLG constraint with the given Flex-
Algorithm Definition.
* In the context of multiple Flex-Algorithms, it can be used for
creating disjoint sets of paths by pruning the links belonging to
a specific SRLG from the topology on which a specific Flex-
Algorithm computes its paths. This usage:
- facilitates the usage of already deployed SRLG configurations
for the setup of disjoint paths between two or more Flex-
Algorithms and
- requires explicit association of a given Flex-Algorithm with a
specific set of SRLG constraints, as defined in Sections 6.5
and 7.5.
The two usages mentioned above are orthogonal.
15.3. Max-Metric Consideration
Both IS-IS and OSPF have a mechanism to set the IGP metric on a link
to a value that would make the link either unreachable or serve as
the link of last resort. Similar functionality would be needed for
the Min Unidirectional Link Delay and TE metric, as these can be used
to compute Flex-Algorithm paths.
The link can be made unreachable for all Flex-Algorithms that use the
Min Unidirectional Link Delay as a metric, as described in
Section 5.1, by removing the Flex-Algorithm ASLA Min Unidirectional
Link Delay advertisement for the link. The link can be made the link
of last resort by setting the delay value in the Flex-Algorithm ASLA
delay advertisement for the link to the value of 16,777,215 (2^24 -
1).
The link can be made unreachable for all Flex-Algorithms that use the
TE metric, as described in Section 5.1, by removing the Flex-
Algorithm ASLA TE metric advertisement for the link. The link can be
made the link of last resort by setting the TE metric value in the
Flex-Algorithm ASLA delay advertisement for the link to the value of
(2^24 - 1) in IS-IS and (2^32 - 1) in OSPF.
15.4. Flexible Algorithm Definition and Changes
When configuring a node to participate in a specific Flex-Algorithm,
the components of the FAD (calculation-type, metric-type, and
constraints) should be considered carefully. The configuration of
participation in a particular Flex-Algorithm doesn't guarantee that
the node will actively participate in it, because it may not support
the calculation-type, the metric-type, or some constraint advertised
by the winning FAD (see Section 5.3). Changes in the FAD
configuration should also be considered in light of the capabilities
of the participating routers in the scope of the FAD advertisement.
As Section 5.3 notes, a change in the Flex-Algorithm Definition may
require network-wide Shortest Path First (SPF) recomputation and
network reconvergence. This potential for disruption should be taken
into consideration when planning and making changes to the FAD.
15.5. Number of Flex-Algorithms
The maximum number of Flex-Algorithms is determined by the algorithm
range 128-255, as specified in Section 4. Although possible, it is
not expected that all of them will be used simultaneously.
Typically, only a limited subset of Flex-Algorithms is expected to be
deployed in the network.
16. Backward Compatibility
This extension brings no new backward-compatibility issues. IS-IS,
OSPFv2, and OSPFv3 all have well-defined handling of unrecognized
TLVs and sub-TLVs that allows the introduction of new extensions,
similar to those defined here, without introducing any
interoperability issues.
17. Security Considerations
This document adds two new ways to disrupt IGP networks:
* An attacker can hijack a particular Flex-Algorithm by advertising
a FAD with a priority of 255 (or any priority higher than that of
the legitimate nodes).
* An attacker could make it look like a router supports a particular
Flex-Algorithm when it actually doesn't, or vice versa.
Both of these attacks can be addressed by the existing security
extensions, as described in [RFC5304] and [RFC5310] for IS-IS, in
[RFC2328] and [RFC7474] for OSPFv2, and in [RFC4552] and [RFC5340]
for OSPFv3.
If the node that is authenticated is taken over by an attacker, such
rogue node can advertise the FAD for any Flex-Algorithm. Doing so
may result in traffic for such Flex-Algorithm to be misrouted, or not
delivered at all, for example, by using an unsupported metric-type,
calculation-type, or constraint. Such attack is not preventable
through authentication, and it is not different from advertising any
other incorrect information through IS-IS or OSPF.
18. IANA Considerations
18.1. IGP IANA Considerations
18.1.1. IGP Algorithm Types Registry
This document makes the following registration in the "IGP Algorithm
Types" registry:
+=========+=====================+=====================+
| Value | Description | Reference |
+=========+=====================+=====================+
| 128-255 | Flexible Algorithms | RFC 9350, Section 4 |
+---------+---------------------+---------------------+
Table 1: IGP Algorithm Types Registry
18.1.2. IGP Metric-Type Registry
IANA has created the "IGP Metric-Type" registry within the "Interior
Gateway Protocol (IGP) Parameters" registry group. The registration
policy is "Standards Action" [RFC8126] [RFC7120]. Values are
assigned from the range 0-255 and have been registered as follows.
+======+======================================+===========+
| Type | Description | Reference |
+======+======================================+===========+
| 0 | IGP Metric | RFC 9350, |
| | | Section |
| | | 5.1 |
+------+--------------------------------------+-----------+
| 1 | Min Unidirectional Link Delay as | RFC 9350, |
| | defined in [RFC8570], Section 4.2 | Section |
| | and [RFC7471], Section 4.2 | 5.1 |
+------+--------------------------------------+-----------+
| 2 | Traffic Engineering Default Metric | RFC 9350, |
| | as defined in [RFC5305], Section 3.7 | Section |
| | and Traffic Engineering Metric as | 5.1 |
| | defined in [RFC3630], Section 2.5.5 | |
+------+--------------------------------------+-----------+
Table 2: IGP Metric-Type Registry
18.2. IGP Flexible Algorithm Definition Flags Registry
IANA has created the "IGP Flexible Algorithm Definition Flags"
registry within the "Interior Gateway Protocol (IGP) Parameters"
registry group. The registration policy is "Standards Action". New
registrations should be assigned in ascending bit order (see
Section 6.4); the following single bit has been assigned as follows.
+=====+=============================+====================+
| Bit | Name | Reference |
+=====+=============================+====================+
| 0 | Prefix Metric Flag (M-flag) | RFC 9350, Sections |
| | | 6.4 and 7.4 |
+-----+-----------------------------+--------------------+
Table 3: IGP Flexible Algorithm Definition Flags Registry
18.3. IS-IS IANA Considerations
18.3.1. IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV Registry
This document makes the following registration in the "IS-IS Sub-TLVs
for IS-IS Router CAPABILITY TLV" registry.
+=======+=====================================+=============+
| Value | Description | Reference |
+=======+=====================================+=============+
| 26 | Flexible Algorithm Definition (FAD) | RFC 9350, |
| | | Section 5.1 |
+-------+-------------------------------------+-------------+
Table 4: IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV
Registry
18.3.2. IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability
Registry
This document makes the following registration in the "IS-IS Sub-TLVs
for TLVs Advertising Prefix Reachability" registry.
+======+==================+====+=====+=====+=====+=====+===========+
| Type | Description | 27 | 135 | 235 | 236 | 237 | Reference |
+======+==================+====+=====+=====+=====+=====+===========+
| 6 | Flexible | n | y | y | y | y | RFC 9350, |
| | Algorithm Prefix | | | | | | Section 8 |
| | Metric (FAPM) | | | | | | |
+------+------------------+----+-----+-----+-----+-----+-----------+
Table 5: IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability
Registry
18.3.3. IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Registry
IANA has created the "IS-IS Sub-Sub-TLVs for Flexible Algorithm
Definition Sub-TLV" registry within the "IS-IS TLV Codepoints"
registry group. The registration procedure is "Expert Review" (note
that the "IS-IS TLV Codepoints" registry group includes Expert Review
guidance that applies to all registries thereunder).
The sub-sub-TLVs defined in this document have been assigned as
follows.
+=======+========================================+=============+
| Type | Description | Reference |
+=======+========================================+=============+
| 0 | Reserved | RFC 9350 |
+-------+----------------------------------------+-------------+
| 1 | Flexible Algorithm Exclude Admin Group | RFC 9350, |
| | | Section 6.1 |
+-------+----------------------------------------+-------------+
| 2 | Flexible Algorithm Include-Any Admin | RFC 9350, |
| | Group | Section 6.2 |
+-------+----------------------------------------+-------------+
| 3 | Flexible Algorithm Include-All Admin | RFC 9350, |
| | Group | Section 6.3 |
+-------+----------------------------------------+-------------+
| 4 | Flexible Algorithm Definition Flags | RFC 9350, |
| | | Section 6.4 |
+-------+----------------------------------------+-------------+
| 5 | Flexible Algorithm Exclude SRLG | RFC 9350, |
| | | Section 6.5 |
+-------+----------------------------------------+-------------+
| 6-255 | Unassigned | |
+-------+----------------------------------------+-------------+
Table 6: IS-IS Sub-Sub-TLVs for Flexible Algorithm
Definition Sub-TLV Registry
18.4. OSPF IANA Considerations
18.4.1. OSPF Router Information (RI) TLVs Registry
This document makes the following registration in the "OSPF Router
Information (RI) TLVs" registry.
+=======+=========================================+=============+
| Value | Description | Reference |
+=======+=========================================+=============+
| 16 | Flexible Algorithm Definition (FAD) TLV | RFC 9350, |
| | | Section 5.2 |
+-------+-----------------------------------------+-------------+
Table 7: OSPF Router Information (RI) TLVs Registry
18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs Registry
This document makes the following registration in the "OSPFv2
Extended Prefix TLV Sub-TLVs" registry.
+=======+=========================================+===========+
| Value | Description | Reference |
+=======+=========================================+===========+
| 3 | Flexible Algorithm Prefix Metric (FAPM) | RFC 9350, |
| | | Section 9 |
+-------+-----------------------------------------+-----------+
Table 8: OSPFv2 Extended Prefix TLV Sub-TLVs Registry
18.4.3. OSPFv3 Extended-LSA Sub-TLVs Registry
This document makes the following registrations in the "OSPFv3
Extended-LSA Sub-TLVs" registry.
+=======+=========================================+==============+
| Value | Description | Reference |
+=======+=========================================+==============+
| 26 | Flexible Algorithm Prefix Metric (FAPM) | RFC 9350, |
| | | Section 9 |
+-------+-----------------------------------------+--------------+
| 33 | OSPF Flexible Algorithm ASBR Metric | RFC 9350, |
| | | Section 10.2 |
+-------+-----------------------------------------+--------------+
Table 9: OSPFv3 Extended-LSA Sub-TLVs Registry
18.4.4. OSPF Flex-Algorithm Prefix Metric Bits Registry
IANA has created the "OSPF Flex-Algorithm Prefix Metric Bits"
registry under the "Open Shortest Path First (OSPF) Parameters"
registry. The registration procedure is "IETF Review". Bits 1-7 are
unassigned, and the initial value has been assigned as follows.
+============+=======================+=====================+
| Bit Number | Description | Reference |
+============+=======================+=====================+
| 0 | E bit - External Type | RFC 9350, Section 9 |
+------------+-----------------------+---------------------+
Table 10: OSPF Flex-Algorithm Prefix Metric Bits Registry
18.4.5. Opaque Link-State Advertisements (LSA) Option Types Registry
This document makes the following registration in the "Opaque Link-
State Advertisements (LSA) Option Types" registry within the "Open
Shortest Path First (OSPF) Opaque Link-State Advertisements (LSA)
Option Types" registry group.
+=======+==========================+==============+
| Value | Opaque Type | Reference |
+=======+==========================+==============+
| 11 | OSPFv2 Extended Inter- | RFC 9350, |
| | Area ASBR (EIA-ASBR) LSA | Section 10.1 |
+-------+--------------------------+--------------+
Table 11: Opaque Link-State Advertisements
(LSA) Option Types Registry
18.4.6. OSPFv2 Extended Inter-Area ASBR TLVs Registry
IANA has created the "OSPFv2 Extended Inter-Area ASBR TLVs" registry
within the "Open Shortest Path First v2 (OSPFv2) Parameters" registry
group. The registration procedure is "IETF Review" or "IESG
Approval". The initial value has been assigned as follows.
+=======+==========================+===========+
| Value | Description | Reference |
+=======+==========================+===========+
| 1 | Extended Inter-Area ASBR | RFC 9350 |
+-------+--------------------------+-----------+
Table 12: OSPFv2 Extended Inter-Area ASBR
TLVs Registry
The values 2-32767 are unassigned, the values 32768-33023 are
reserved for Experimental Use, and the values 0 and 33024-65535 are
reserved.
18.4.7. OSPFv2 Extended Inter-Area ASBR Sub-TLVs Registry
IANA has created the "OSPFv2 Extended Inter-Area ASBR Sub-TLVs"
registry under the "Open Shortest Path First v2 (OSPFv2) Parameters"
registry. The registration procedure is "IETF Review" or "IESG
Approval". The initial value has been assigned as follows.
+=======+=====================================+===========+
| Value | Description | Reference |
+=======+=====================================+===========+
| 1 | OSPF Flexible Algorithm ASBR Metric | RFC 9350 |
+-------+-------------------------------------+-----------+
Table 13: OSPFv2 Extended Inter-Area ASBR Sub-TLVs Registry
The values 2-32767 are unassigned, the values 32768-33023 are
reserved for Experimental Use, and the values 0 and 33024-65535 are
reserved.
18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLVs Registry
IANA has created the "OSPF Flexible Algorithm Definition TLV Sub-
TLVs" registry within the "Open Shortest Path First (OSPF)
Parameters" registry group. The registration procedure is "IETF
Review" or "IESG Approval".
The "OSPF Flexible Algorithm Definition TLV Sub-TLVs" registry will
define sub-TLVs at any level of nesting for the Flexible Algorithm
TLV, and new values can be allocated via the registration procedure.
This document registers the following sub-TLVs.
+============+========================================+=============+
| Bit Number | Description | Reference |
+============+========================================+=============+
| 0 | Reserved | RFC 9350 |
+------------+----------------------------------------+-------------+
| 1 | Flexible Algorithm | RFC 9350, |
| | Exclude Admin Group | Section 7.1 |
+------------+----------------------------------------+-------------+
| 2 | Flexible Algorithm | RFC 9350, |
| | Include-Any Admin Group | Section 7.2 |
+------------+----------------------------------------+-------------+
| 3 | Flexible Algorithm | RFC 9350, |
| | Include-All Admin Group | Section 7.3 |
+------------+----------------------------------------+-------------+
| 4 | Flexible Algorithm | RFC 9350, |
| | Definition Flags | Section 7.4 |
+------------+----------------------------------------+-------------+
| 5 | Flexible Algorithm | RFC 9350, |
| | Exclude SRLG | Section 7.5 |
+------------+----------------------------------------+-------------+
Table 14: OSPF Flexible Algorithm Definition TLV Sub-TLVs Registry
The values 6-32767 are unassigned, and values 32768-33023 are for
Experimental Use; these will not be registered with IANA.
Types in the range 33024-65535 are not to be assigned at this time.
Before any assignments can be made in the 33024-65535 range, there
MUST be an IETF specification that specifies IANA considerations that
cover the range being assigned.
18.4.9. Link Attribute Application Identifiers Registry
This document registers the following bit in the "Link Attribute
Application Identifiers" registry.
+=====+============================+======================+
| Bit | Description | Reference |
+=====+============================+======================+
| 3 | Flexible Algorithm (X-bit) | RFC 9350, Section 12 |
+-----+----------------------------+----------------------+
Table 15: Link Attribute Application Identifiers Registry
19. References
19.1. Normative References
[ISO10589] ISO, "Information technology - Telecommunications and
information exchange between systems - Intermediate System
to Intermediate System intra-domain routeing information
exchange protocol for use in conjunction with the protocol
for providing the connectionless-mode network service (ISO
8473)", Second Edition, ISO/IEC 10589:2002, November 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
Traffic Engineering (MPLS-TE)", RFC 7308,
DOI 10.17487/RFC7308, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
December 2019, <https://www.rfc-editor.org/info/rfc8666>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[RFC8919] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
RFC 8919, DOI 10.17487/RFC8919, October 2020,
<https://www.rfc-editor.org/info/rfc8919>.
[RFC8920] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020,
<https://www.rfc-editor.org/info/rfc8920>.
[RFC9352] Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
and Z. Hu, "IS-IS Extensions to Support Segment Routing
over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
February 2023, <https://www.rfc-editor.org/info/rfc9352>.
19.2. Informative References
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 3101, DOI 10.17487/RFC3101, January 2003,
<https://www.rfc-editor.org/info/rfc3101>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<https://www.rfc-editor.org/info/rfc6571>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[ROUTING-PLANES-USING-SR]
Hegde, S. and A. Gulko, "Separating Routing Planes using
Segment Routing", Work in Progress, Internet-Draft, draft-
gulkohegde-routing-planes-using-sr-00, 13 March 2017,
<https://datatracker.ietf.org/doc/html/draft-gulkohegde-
routing-planes-using-sr-00>.
[RTGWG-SEGMENT-ROUTING-TI-LFA]
Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
09, 23 December 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-09>.
Acknowledgements
This document, among other things, addresses the problem that
[ROUTING-PLANES-USING-SR] was trying to solve. All authors of that
document agreed to join this document.
Thanks to Eric Rosen, Tony Przygienda, William Britto A. J., Gunter
Van de Velde, Dirk Goethals, Manju Sivaji, and Baalajee S. for their
detailed review and excellent comments.
Thanks to Cengiz Halit for his review and feedback during the initial
phase of the solution definition.
Thanks to Kenji Kumaki for his comments.
Thanks to Acee Lindem for editorial comments.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
82109 Bratislava
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde
Juniper Networks, Inc.
Embassy Business Park
Bangalore 560093
KA
India
Email: shraddha@juniper.net
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Ketan Talaulikar
Cisco Systems, Inc
India
Email: ketant.ietf@gmail.com
Arkadiy Gulko
Edward Jones
Email: arkadiy.gulko@edwardjones.com