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RFC 7215
Internet Engineering Task Force (IETF) L. Jakab
Request for Comments: 7215 Cisco Systems
Category: Experimental A. Cabellos-Aparicio
ISSN: 2070-1721 F. Coras
J. Domingo-Pascual
Technical University of Catalonia
D. Lewis
Cisco Systems
April 2014
Locator/Identifier Separation Protocol (LISP)
Network Element Deployment Considerations
Abstract
This document is a snapshot of different Locator/Identifier
Separation Protocol (LISP) deployment scenarios. It discusses the
placement of new network elements introduced by the protocol,
representing the thinking of the LISP working group as of Summer
2013. LISP deployment scenarios may have evolved since then. This
memo represents one stable point in that evolution of understanding.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7215.
Jakab, et al. Experimental [Page 1]
RFC 7215 LISP Deployment April 2014
Copyright Notice
Copyright (c) 2014 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
(http://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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................3
2. Tunnel Routers ..................................................5
2.1. Deployment Scenarios .......................................5
2.1.1. Customer Edge (CE) ..................................5
2.1.2. Provider Edge (PE) ..................................6
2.1.3. Tunnel Routers behind NAT ...........................8
2.1.3.1. ITR ........................................8
2.1.3.2. ETR ........................................9
2.1.3.3. Additional Notes ...........................9
2.2. Functional Models with Tunnel Routers ......................9
2.2.1. Split ITR/ETR .......................................9
2.2.2. Inter-Service-Provider Traffic Engineering .........11
2.3. Summary and Feature Matrix ................................13
3. Map-Servers and Map-Resolvers ..................................14
3.1. Map-Servers ...............................................14
3.2. Map-Resolvers .............................................16
4. Proxy Tunnel Routers ...........................................17
4.1. PITRs .....................................................17
4.2. PETRs .....................................................18
5. Migration to LISP ..............................................19
5.1. LISP+BGP ..................................................19
5.2. Mapping Service Provider (MSP) PITR Service ...............20
5.3. Proxy-ITR Route Distribution (PITR-RD) ....................20
5.4. Migration Summary .........................................23
6. Security Considerations ........................................24
7. Acknowledgements ...............................................24
8. References .....................................................24
8.1. Normative References ......................................24
8.2. Informative References ....................................24
Appendix A. Step-by-Step Example BGP-to-LISP Migration Procedure ..26
A.1. Customer Pre-Install and Pre-Turn-Up Checklist .............26
A.2. Customer Activating LISP Service ...........................28
A.3. Cut-Over Provider Preparation and Changes ..................29
1. Introduction
The Locator/Identifier Separation Protocol (LISP) is designed to
address the scaling issues of the global Internet routing system
identified in [RFC4984] by separating the current addressing scheme
into Endpoint IDentifiers (EIDs) and Routing LOCators (RLOCs). The
main protocol specification [RFC6830] describes how the separation is
achieved and which new network elements are introduced, and it
details the packet formats for the data and control planes.
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LISP assumes that such separation is between the edge and core and
uses mapping and encapsulation for forwarding. While the boundary
between both is not strictly defined, one widely accepted definition
places it at the border routers of stub autonomous systems, which may
carry a partial or complete default-free zone (DFZ) routing table.
The initial design of LISP took this location as a baseline for
protocol development. However, the applications of LISP go beyond
just decreasing the size of the DFZ routing table and include
improved multihoming and ingress traffic engineering (TE) support for
edge networks, and even individual hosts. Throughout this document,
we will use the term "LISP site" to refer to these networks/hosts
behind a LISP Tunnel Router. We formally define the following
two terms:
Network element: Facility or equipment used in the provision of a
communications service over the Internet [TELCO96].
LISP site: A single host or a set of network elements in an edge
network under the administrative control of a single organization,
delimited from other networks by LISP Tunnel Router(s).
Since LISP is a protocol that can be used for different purposes, it
is important to identify possible deployment scenarios and the
additional requirements they may impose on the protocol specification
and other protocols. Additionally, this document is intended as a
guide for the operational community for LISP deployments in their
networks. It is expected to evolve as LISP deployment progresses,
and the described scenarios are better understood or new scenarios
are discovered.
Each subsection considers an element type and discusses the impact of
deployment scenarios on the protocol specification. For definitions
of terms, please refer to the appropriate documents (as cited in the
respective sections).
This experimental document describes deployment considerations.
These considerations and the LISP specifications have areas that
require additional experience and measurement. LISP is not
recommended for deployment beyond experimental situations. Results
of experimentation may lead to modifications and enhancements of LISP
mechanisms. Additionally, at the time of this writing there is no
standardized security to implement. Beware that there are no
countermeasures for any of the threats identified in [LISP-THREATS].
See Section 15 of [RFC6830] for specific known issues that are in
need of further work during development, implementation, and
experimentation, and see [LISP-THREATS] for recommendations to
ameliorate the above-mentioned security threats.
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2. Tunnel Routers
The device that is the gateway between the edge and the core is
called a Tunnel Router (xTR); it performs one or both of two separate
functions:
1. Encapsulating packets originating from an end host to be
transported over intermediary (transit) networks towards the
other endpoint of the communication.
2. Decapsulating packets entering from intermediary (transit)
networks, originated at a remote end host.
The first function is performed by an Ingress Tunnel Router (ITR) and
the second by an Egress Tunnel Router (ETR).
Section 8 of the main LISP specification [RFC6830] has a short
discussion of where Tunnel Routers can be deployed and some of the
associated advantages and disadvantages. This section adds more
detail to the scenarios presented there and provides additional
scenarios as well. Furthermore, this section discusses functional
models, that is, network functions that can be achieved by deploying
Tunnel Routers in specific ways.
2.1. Deployment Scenarios
2.1.1. Customer Edge (CE)
The first scenario we discuss is the customer edge, when xTR
functionality is placed on the router(s) that connects the LISP site
to its upstream(s) but is under its control. As such, this is the
most common expected scenario for xTRs, and this document considers
it the reference location, comparing the other scenarios to this one.
ISP1 ISP2
| |
| |
+----+ +----+
+--|xTR1|--|xTR2|--+
| +----+ +----+ |
| |
| LISP site |
+------------------+
Figure 1: xTRs at the Customer Edge
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From the LISP site's perspective, the main advantage of this type of
deployment (compared to the one described in the next section) is
having direct control over its ingress traffic engineering. This
makes it easy to set up and maintain active/active, active/backup, or
more complex TE policies, adding ISPs and additional xTRs at will,
without involving third parties.
Being under the same administrative control, reachability information
of all ETRs is easier to synchronize, because the necessary control
traffic can be allowed between the locators of the ETRs. A correct
synchronous global view of the reachability status is thus available,
and the Locator-Status-Bits can be set correctly in the LISP data
header of outgoing packets.
By placing the Tunnel Router at the edge of the site, existing
internal network configuration does not need to be modified.
Firewall rules, router configurations, and address assignments inside
the LISP site remain unchanged. This helps with incremental
deployment and allows a quick upgrade path to LISP. For larger sites
distributed in geographically diverse points of presence (PoPs) and
having many external connections and complex internal topology, it
may, however, make more sense to both encapsulate and decapsulate as
soon as possible, to benefit from the information in the IGP to
choose the best path. See Section 2.2.1 for a discussion of this
scenario.
Another thing to consider when placing Tunnel Routers is MTU issues.
Encapsulation increases the amount of overhead associated with each
packet. This added overhead decreases the effective end-to-end path
MTU (unless fragmentation and reassembly are used). Some transit
networks are known to provide larger MTU values than the typical
value of 1500 bytes for popular access technologies used at end hosts
(e.g., IEEE 802.3 and 802.11). However, placing the LISP router
connecting to such a network at the customer edge could possibly
bring up MTU issues, depending on the link type to the provider as
opposed to the following scenario. See [RFC4459] for MTU
considerations of tunneling protocols and how to mitigate potential
issues. Still, even with these mitigations, path MTU issues are
still possible.
2.1.2. Provider Edge (PE)
The other location at the core-edge boundary for deploying LISP
routers is at the Internet service provider edge. The main incentive
for this case is that the customer does not have to upgrade the CE
router(s) or change the configuration of any equipment.
Encapsulation/decapsulation happens in the provider's network, which
may be able to serve several customers with a single device. For
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large ISPs with many residential/business customers asking for LISP,
this can lead to important savings, since there is no need to upgrade
the software (or hardware, if that's the case) at each client's
location. Instead, they can upgrade the software (or hardware) on a
few PE routers serving the customers. This scenario is depicted in
Figure 2.
+----------+ +------------------+
| ISP1 | | ISP2 |
| | | |
| +----+ | | +----+ +----+ |
+--|xTR1|--+ +--|xTR2|--|xTR3|--+
+----+ +----+ +----+
| | |
| | |
+--<[LISP site]>---+-------+
Figure 2: xTRs at the Provider Edge
While this approach can make transition easy for customers and may be
cheaper for providers, the LISP site loses one of the main benefits
of LISP: ingress traffic engineering. Since the provider controls
the ETRs, additional complexity would be needed to allow customers to
modify their mapping entries.
The problem is aggravated when the LISP site is multihomed. Consider
the scenario in Figure 2: whenever a change to TE policies is
required, the customer contacts both ISP1 and ISP2 to make the
necessary changes on the routers (if they provide this possibility).
It is, however, unlikely that both ISPs will apply changes
simultaneously, which may lead to inconsistent state for the mappings
of the LISP site. Since the different upstream ISPs are usually
competing business entities, the ETRs may even be configured to
compete, to either attract all the traffic or get no traffic. The
former will happen if the customer pays per volume, the latter if the
connectivity has a fixed price. A solution could be to configure the
Map-Server(s) to do proxy-replying and have the Mapping Service
Provider (MSP) apply policies.
Additionally, since xTR1, xTR2, and xTR3 are in different
administrative domains, locator reachability information is unlikely
to be exchanged among them, making it difficult to set the
Locator-Status-Bits (LSBs) correctly on encapsulated packets.
Because of this, and due to the security concerns about LSBs as
described in [LISP-THREATS], their use is discouraged (set the L-bit
to 0). Map-Versioning is another alternative [RFC6834].
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Compared to the customer edge scenario, deploying LISP at the
provider edge might have the advantage of diminishing potential MTU
issues, because the Tunnel Router is closer to the core, where links
typically have higher MTUs than edge network links.
2.1.3. Tunnel Routers behind NAT
"NAT" in this section refers to IPv4 network address and port
translation.
2.1.3.1. ITR
_.--. _.--.
,-'' `--. +-------+ ,-'' `--.
' EID ` (Private) | NAT | (Public) ,' RLOC `.
( )---[ITR]---| |---------( )
. space ,' (Address) | Box |(Address) . space ,'
`--. _.-' +-------+ `--. _.-'
`--'' `--''
Figure 3: ITR behind NAT
Packets encapsulated by an ITR are just UDP packets from a NAT
device's point of view, and they are handled like any UDP packet;
there are no additional requirements for LISP data packets.
Map-Requests sent by an ITR, which create the state in the NAT table,
have a different 5-tuple in the IP header than the Map-Reply
generated by the authoritative ETR. Since the source address of this
packet is different from the destination address of the request
packet, no state will be matched in the NAT table and the packet will
be dropped. To avoid this, the NAT device has to do the following:
o Send all UDP packets with source port 4342, regardless of the
destination port, to the RLOC of the ITR. The simplest way to
achieve this is configuring 1:1 NAT mode from the external RLOC of
the NAT device to the ITR's RLOC (called "DMZ" mode in consumer
broadband routers).
o Rewrite the ITR-AFI and "Originating ITR RLOC Address" fields in
the payload.
This setup supports only a single ITR behind the NAT device.
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2.1.3.2. ETR
An ETR placed behind NAT is reachable from the outside by the
Internet-facing locator of the NAT device. It needs to know this
locator (and configure a loopback interface with it), so that it can
use it in Map-Reply and Map-Register messages. Thus, support for
dynamic locators for the mapping database is needed in LISP
equipment.
Again, only one ETR behind the NAT device is supported.
_.--. _.--.
,-'' `--. +-------+ ,-'' `--.
' EID ` (Private) | NAT | (Public) ,' RLOC `.
( )---[ETR]---| |---------( )
. space ,' (Address) | Box |(Address) . space ,'
`--. _.-' +-------+ `--. _.-'
`--'' `--''
Figure 4: ETR behind NAT
2.1.3.3. Additional Notes
An implication of the issues described above is that LISP sites with
xTRs cannot be behind carrier-based NATs, since two different sites
would collide on the same forwarded UDP port. An alternative to
static hole-punching to explore is the use of the Port Control
Protocol (PCP) [RFC6887].
We only include this scenario due to completeness, to show that a
LISP site can be deployed behind NAT should it become necessary.
However, LISP deployments behind NAT should be avoided, if possible.
2.2. Functional Models with Tunnel Routers
This section describes how certain LISP deployments can provide
network functions.
2.2.1. Split ITR/ETR
In a simple LISP deployment, xTRs are located at the border of the
LISP site (see Section 2.1.1). In this scenario, packets are routed
inside the domain according to the EID. However, more complex
networks may want to route packets according to the destination RLOC.
This would enable them to choose the best egress point.
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The LISP specification separates the ITR and ETR functionality and
allows both entities to be deployed in separated network equipment.
ITRs can be deployed closer to the host (i.e., access routers). This
way, packets are encapsulated as soon as possible, and egress point
selection is driven by operational policy. In turn, ETRs can be
deployed at the border routers of the network, and packets are
decapsulated as soon as possible. Once decapsulated, packets are
routed based on the destination EID according to internal routing
policy.
We can see an example in Figure 5. The Source (S) transmits packets
using its EID, and in this particular case packets are encapsulated
at ITR_1. The encapsulated packets are routed inside the domain
according to the destination RLOC and can egress the network through
the best point (i.e., closer to the RLOC's Autonomous System (AS)).
On the other hand, inbound packets are received by ETR_1, which
decapsulates them. Then, packets are routed towards S according to
the EID, again following the best path.
+---------------------------------------+
| |
| +-------+ +-------+ +-------+
| | ITR_1 |---------+ | ETR_1 |-RLOC_A--| ISP_A |
| +-------+ | +-------+ +-------+
| +-+ | | |
| |S| | IGP | |
| +-+ | | |
| +-------+ | +-------+ +-------+
| | ITR_2 |---------+ | ETR_2 |-RLOC_B--| ISP_B |
| +-------+ +-------+ +-------+
| |
+---------------------------------------+
Figure 5: Split ITR/ETR Scenario
This scenario has a set of implications:
o The site must carry more-specific routes in order to choose the
best egress point, and typically BGP is used for this, increasing
the complexity of the network. However, this is usually already
the case for LISP sites that would benefit from this scenario.
o If the site is multihomed to different ISPs and any of the
upstream ISPs are doing unicast reverse path forwarding (uRPF)
filtering, this scenario may become impractical. To set the
correct source RLOC in the encapsulation header, ITRs need to
first determine which ETR will be used by the outgoing packet.
This adds complexity and reliability concerns.
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o In LISP, ITRs set the reachability bits when encapsulating data
packets. Hence, ITRs need a mechanism to be aware of the liveness
of all ETRs serving their site.
o The MTU within the site network must be large enough to
accommodate encapsulated packets.
o In this scenario, each ITR is serving fewer hosts than in the case
when it is deployed at the border of the network. It has been
shown that the cache hit rate grows logarithmically with the
amount of users [CACHE]. Taking this into account, when ITRs are
deployed closer to the host the effectiveness of the mapping cache
may be lower (i.e., the miss rate is higher). Another consequence
of this is that the site may transmit a higher amount of
Map-Requests, increasing the load on the distributed mapping
database.
o By placing the ITRs inside the site, they will still need global
RLOCs. This may add complexity to intra-site routing
configurations and more intra-site issues when there is a change
of providers.
2.2.2. Inter-Service-Provider Traffic Engineering
At the time of this writing, if two ISPs want to control their
ingress TE policies for transit traffic between them, they need to
rely on existing BGP mechanisms. This typically means deaggregating
prefixes to choose on which upstream link packets should enter. This
either is not feasible (if fine-grained per-customer control is
required, the very-specific prefixes may not be propagated) or
increases DFZ table size.
Typically, LISP is seen as applicable only to stub networks; however,
LISP can also be applied in a recursive manner, providing service
provider ingress/egress TE capabilities without impacting the DFZ
table size.
In order to implement this functionality with LISP, consider the
scenario depicted in Figure 6. The two ISPs willing to achieve
ingress/egress TE are labeled as ISP_A and ISP_B. They are servicing
Stub1 and Stub2, respectively. Both are required to be LISP sites
with their own xTRs. In this scenario, we assume that Stub1 and
Stub2 are communicating with each other; thus, ISP_A and ISP_B offer
transit for such communications. ISP_A has RLOC_A1 and RLOC_A2 as
upstream IP addresses, while ISP_B has RLOC_B1 and RLOC_B2. The
shared goal among ISP_A and ISP_B is to control the transit traffic
flow between RLOC_A1/A2 and RLOC_B1/B2.
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_.--.
Stub1 ... +-------+ ,-'' `--. +-------+ ... Stub2
\ | R_A1|----,' `. ---|R_B1 | /
--| | ( Transit ) | |--
... .../ | R_A2|-----. ,' ---|R_B2 | \... ...
+-------+ `--. _.-' +-------+
... ... ISP_A `--'' ISP_B ... ...
Figure 6: Inter-Service-Provider TE Scenario
Both ISPs deploy xTRs on RLOC_A1/A2 and RLOC_B1/B2, respectively and
reach a bilateral agreement to deploy their own private mapping
system. This mapping system contains bindings between the RLOCs of
Stub1 and Stub2 (owned by ISP_A and ISP_B, respectively) and RLOC_A1/
A2 and RLOC_B1/B2. Such bindings are in fact the TE policies between
both ISPs, and the convergence time is expected to be fast, since
ISPs only have to update/query a mapping to/from the database.
The packet flow is as follows. First, a packet originated at Stub1
towards Stub2 is LISP encapsulated by Stub1's xTR. The xTR of ISP_A
recursively encapsulates it, and according to the TE policies stored
in the private mapping system the ISP_A xTR chooses RLOC_B1 or
RLOC_B2 as the outer encapsulation destination. Note that the packet
transits between ISP_A and ISP_B double-encapsulated. Upon reception
at the xTR of ISP_B, the packet is decapsulated and sent towards
Stub2, which performs the last decapsulation.
This deployment scenario, which uses recursive LISP, includes three
important caveats. First, it is intended to be deployed between only
two ISPs. If more than two ISPs use this approach, then either the
xTRs deployed at the participating ISPs must query multiple mapping
systems, or the ISPs must agree on a common shared mapping system.
Furthermore, keeping this deployment scenario restricted to only two
ISPs maintains a scalable solution, given that only two entities need
to agree on using recursive LISP and only one private mapping system
is involved.
Second, the scenario is only recommended for ISPs providing
connectivity to LISP sites, such that source RLOCs of packets to be
recursively encapsulated belong to said ISP. Otherwise, the
participating ISPs must register prefixes they do not own in the
above-mentioned private mapping system. This results in either
requiring complex authentication mechanisms or enabling simple
traffic redirection attacks. Failure to follow these recommendations
may lead to operational security issues when deploying this scenario.
And third, recursive encapsulation models are typically complex to
troubleshoot and debug.
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Besides these recommendations, the main disadvantages of this
deployment case are:
o An extra LISP header is needed. This increases the packet size
and requires that the MTU between both ISPs accommodate double-
encapsulated packets.
o The ISP ITR must encapsulate packets and therefore must know the
RLOC-to-RLOC bindings. These bindings are stored in a mapping
database and may be cached in the ITR's mapping cache. Cache
misses lead to an additional lookup latency, unless a push-based
mapping system is used for the private mapping system.
o Maintaining the shared mapping database involves operational
overhead.
2.3. Summary and Feature Matrix
When looking at the deployment scenarios and functional models above,
there are several things to consider when choosing an appropriate
model, depending on the type of the organization doing the
deployment.
For home users and small sites that wish to multihome and have
control over their ISP options, the "CE" scenario offers the most
advantages: it's simple to deploy, and in some cases it only requires
a software upgrade of the Customer Premises Equipment (CPE), getting
mapping service, and configuring the router. It retains control of
TE and choosing upstreams by the user. It doesn't provide too many
advantages to ISPs, due to the lessened dependence on their services
in cases of multihomed clients. It is also unlikely that ISPs
wishing to offer LISP to their customers will choose the "CE" model,
as they would need to send a technician to each customer and,
potentially, a new CPE device. Even if they have remote control over
the router and a software upgrade could add LISP support, the
operation is too risky.
For a network operator, a good option to deploy is the "PE" scenario,
unless a hardware upgrade is required for its edge routers to support
LISP (in which case upgrading CPEs may be simpler). It retains
control of TE as well as the choice of Proxy Egress Tunnel Router
(PETR) and Map-Server/Map-Resolver. It also lowers potential MTU
issues, as discussed above. Network operators should also explore
the "inter-service-provider TE" (recursive) functional model for
their TE needs.
To optimize their traffic flow, large organizations can benefit the
most from the "split ITR/ETR" functional model.
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The following table gives a quick overview of the features supported
by each of the deployment scenarios discussed above (marked with an
"x" in the appropriate column): "CE" for customer edge, "PE" for
provider edge, "Split" for split ITR/ETR, and "Recursive" for
inter-service-provider traffic engineering. The discussed features
include:
Control of ingress TE: This scenario allows the LISP site to easily
control LISP ingress traffic engineering policies.
No modifications to existing int. network infrastructure: This
scenario doesn't require the LISP site to modify internal network
configurations.
Locator-Status-Bits sync: This scenario allows easy synchronization
of the Locator Status Bits.
MTU/PMTUD issues minimized: The scenario minimizes potential MTU and
Path MTU Discovery (PMTUD) issues.
Feature CE PE Split Recursive NAT
--------------------------------------------------------------------
Control of ingress TE x - x x x
No modifications to existing
int. network infrastructure x x - - x
Locator-Status-Bits sync x - x x -
MTU/PMTUD issues minimized - x - - -
3. Map-Servers and Map-Resolvers
Map-Servers and Map-Resolvers make up the LISP mapping system and
provide a means to find authoritative EID-to-RLOC mapping
information, conforming to [RFC6833]. They are meant to be deployed
in RLOC space, and their operation behind NAT is not supported.
3.1. Map-Servers
The Map-Server learns EID-to-RLOC mapping entries from an
authoritative source and publishes them in the distributed mapping
database. These entries are learned through authenticated
Map-Register messages sent by authoritative ETRs. Also, upon
reception of a Map-Request, the Map-Server verifies that the
destination EID matches an EID-Prefix for which it is authoritative
and then re-encapsulates and forwards it to a matching ETR.
Map-Server functionality is described in detail in [RFC6833].
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The Map-Server is provided by a Mapping Service Provider (MSP). The
MSP participates in the global distributed mapping database
infrastructure by setting up connections to other participants
according to the specific mapping system that is employed (e.g.,
Alternative Logical Topology (ALT) [RFC6836], Delegated Database Tree
(DDT) [LISP-DDT]). Participation in the mapping database and the
storing of EID-to-RLOC mapping data are subject to the policies of
the "root" operators, who should check ownership rights for the
EID-Prefixes stored in the database by participants. These policies
are out of scope for this document.
The LISP DDT protocol is used by LISP MSPs to provide reachability
between those providers' Map-Resolvers and Map-Servers. The DDT root
is currently operated by a collection of organizations on an open
basis. See [DDT-ROOT] for more details. Similarly to the DNS root,
it has several different server instances using names of the letters
of the Greek alphabet (alpha, delta, etc.), operated by independent
organizations. When this document was published, there were 6 such
instances, with one of them being anycasted. [DDT-ROOT] provides the
list of server instances on its web site and configuration files for
several Map-Server implementations. The DDT root and LISP Mapping
Providers both rely on and abide by existing allocation policies as
defined by Regional Internet Registries (RIRs) to determine prefix
ownership for use as EIDs.
It is expected that the DDT root organizations will continue to
evolve in response to experimentation with LISP deployments for
Internet edge multihoming and VPN use cases.
In all cases, the MSP configures its Map-Server(s) to publish the
prefixes of its clients in the distributed mapping database and start
encapsulating and forwarding Map-Requests to the ETRs of the AS.
These ETRs register their prefix(es) with the Map-Server(s) through
periodic authenticated Map-Register messages. In this context, for
some LISP sites, there is a need for mechanisms to:
o Automatically distribute EID-Prefix(es) shared keys between the
ETRs and the EID-registrar Map-Server.
o Dynamically obtain the address of the Map-Server in the ETR of
the AS.
The Map-Server plays a key role in the reachability of the
EID-Prefixes it is serving. On one hand, it is publishing these
prefixes into the distributed mapping database, and on the other
hand, it is encapsulating and forwarding Map-Requests to the
authoritative ETRs of these prefixes. ITRs encapsulating towards
EIDs for which a failed Map-Server is responsible will be unable to
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look up any of their covering prefixes. The only exceptions are the
ITRs that already contain the mappings in their local caches. In
this case, ITRs can reach ETRs until the entry expires (typically
24 hours). For this reason, redundant Map-Server deployments are
desirable. A set of Map-Servers providing high-availability service
to the same set of prefixes is called a redundancy group. ETRs are
configured to send Map-Register messages to all Map-Servers in the
redundancy group. The configuration for fail-over (or
load-balancing, if desired) among the members of the group depends on
the technology behind the mapping system being deployed. Since ALT
is based on BGP and DDT takes its inspiration from the Domain Name
System (DNS), deployments can leverage current industry best
practices for redundancy in BGP and DNS. These best practices are
out of scope for this document.
Additionally, if a Map-Server has no reachability for any ETR serving
a given EID block, it should not originate that block into the
mapping system.
3.2. Map-Resolvers
A Map-Resolver is a network infrastructure component that accepts
LISP-encapsulated Map-Requests, typically from an ITR, and finds the
appropriate EID-to-RLOC mapping by consulting the distributed mapping
database. Map-Resolver functionality is described in detail in
[RFC6833].
Anyone with access to the distributed mapping database can set up a
Map-Resolver and provide EID-to-RLOC mapping lookup service.
Database access setup is mapping system specific.
For performance reasons, it is recommended that LISP sites use
Map-Resolvers that are topologically close to their ITRs. ISPs
supporting LISP will provide this service to their customers,
possibly restricting access to their user base. LISP sites not in
this position can use open access Map-Resolvers, if available.
However, regardless of the availability of open access resolvers, the
MSP providing the Map-Server(s) for a LISP site should also make
available Map-Resolver(s) for the use of that site.
In medium- to large-size ASes, ITRs must be configured with the RLOC
of a Map-Resolver; this type of operation can be done manually.
However, in Small Office/Home Office (SOHO) scenarios, a mechanism
for autoconfiguration should be provided.
One solution to avoid manual configuration in LISP sites of any size
is the use of anycast [RFC4786] RLOCs for Map-Resolvers, similar to
the DNS root server infrastructure. Since LISP uses UDP
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encapsulation, the use of anycast would not affect reliability. LISP
routers are then shipped with a preconfigured list of well-known
Map-Resolver RLOCs, which can be edited by the network administrator,
if needed.
The use of anycast also helps improve mapping lookup performance.
Large MSPs can increase the number and geographical diversity of
their Map-Resolver infrastructure, using a single anycasted RLOC.
Once LISP deployment is advanced enough, very large content providers
may also be interested in running this kind of setup, to ensure
minimal connection setup latency for those connecting to their
network from LISP sites.
While Map-Servers and Map-Resolvers implement different
functionalities within the LISP mapping system, they can coexist on
the same device. For example, MSPs offering both services can deploy
a single Map-Resolver/Map-Server in each PoP where they have a
presence.
4. Proxy Tunnel Routers
4.1. PITRs
Proxy Ingress Tunnel Routers (PITRs) are part of the non-LISP/LISP
transition mechanism, allowing non-LISP sites to reach LISP sites.
They announce via BGP certain EID-Prefixes (aggregated, whenever
possible) to attract traffic from non-LISP sites towards EIDs in the
covered range. They do the mapping system lookup and encapsulate
received packets towards the appropriate ETR. Note that for the
reverse path, LISP sites can reach non-LISP sites by simply not
encapsulating traffic. See [RFC6832] for a detailed description of
PITR functionality.
The success of new protocols depends greatly on their ability to
maintain backwards compatibility and interoperate with the
protocol(s) they intend to enhance or replace, and on the incentives
to deploy the necessary new software or equipment. A LISP site needs
an interworking mechanism to be reachable from non-LISP sites. A
PITR can fulfill this role, enabling early adopters to see the
benefits of LISP, similar to tunnel brokers helping the transition
from IPv4 to IPv6. A site benefits from new LISP functionality
(proportionally with existing global LISP deployment) when migrating
to LISP, so it has the incentives to deploy the necessary Tunnel
Routers. In order to be reachable from non-LISP sites, it has two
options: keep announcing its prefix(es) with BGP, or have a PITR
announce prefix(es) covering them.
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If the goal of reducing the DFZ routing table size is to be reached,
the second option is preferred. Moreover, the second option allows
LISP-based ingress traffic engineering from all sites. However, the
placement of PITRs significantly influences performance and
deployment incentives. Section 5 is dedicated to the migration to a
LISP-enabled Internet and includes deployment scenarios for PITRs.
4.2. PETRs
In contrast to PITRs, PETRs are not required for the correct
functioning of all LISP sites. There are two cases where they can be
of great help:
o LISP sites with unicast reverse path forwarding (uRPF)
restrictions, and
o Communication between sites using different address family RLOCs.
In the first case, uRPF filtering is applied at the LISP site's
upstream provider's PE router. When forwarding traffic to non-LISP
sites, an ITR does not encapsulate packets, leaving the original IP
headers intact. As a result, packets will have EIDs in their source
address. Since we are discussing the transition period, we can
assume that a prefix covering the EIDs belonging to the LISP site is
advertised to the global routing tables by a PITR, and the PE router
has a route towards it. However, the next hop will not be on the
interface towards the CE router, so non-encapsulated packets will
fail uRPF checks.
To avoid this filtering, the affected ITR encapsulates packets
towards the locator of the PETR for non-LISP destinations. Now the
source address of the packets, as seen by the PE router, is the ITR's
locator, which will not fail the uRPF check. The PETR then
decapsulates and forwards the packets.
The second use case is IPv4-to-IPv6 transition. Service providers
using older access network hardware that only supports IPv4 can still
offer IPv6 to their clients by providing a CPE device running LISP,
and PETR(s) for accessing IPv6-only non-LISP sites and LISP sites,
with IPv6-only locators. Packets originating from the client LISP
site for these destinations would be encapsulated towards the PETR's
IPv4 locator. The PETR is in a native IPv6 network, decapsulating
and forwarding packets. For non-LISP destinations, the packet
travels natively from the PETR. For LISP destinations with IPv6-only
locators, the packet will go through a PITR in order to reach its
destination.
For more details on PETRs, see [RFC6832].
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PETRs can be deployed by ISPs wishing to offer value-added services
to their customers. As is the case with PITRs, PETRs too may
introduce path stretch (the ratio between the cost of the selected
path and that of the optimal path). Because of this, the ISP needs
to consider the tradeoff of using several devices close to the
customers to minimize it, or fewer devices farther away from the
customers to minimize cost instead.
Since the deployment incentives for PITRs and PETRs are different, it
is likely that they will be deployed in separate devices, except for
the Content Delivery Network (CDN) case, which may deploy both in a
single device.
In all cases, the existence of a PETR involves another step in the
configuration of a LISP router. CPE routers, which are typically
configured by DHCP, stand to benefit most from PETRs.
Autoconfiguration of the PETR locator could be achieved by a DHCP
option or by adding a PETR field to either Map-Notify or Map-Reply
messages.
5. Migration to LISP
This section discusses a deployment architecture to support the
migration to a LISP-enabled Internet. The loosely defined terms
"early transition phase", "late transition phase", and "LISP Internet
phase" refer to time periods when LISP sites are a minority, a
majority, or represent all edge networks, respectively.
5.1. LISP+BGP
For sites wishing to migrate to LISP with their Provider-Independent
(PI) prefix, the least disruptive way is to upgrade their border
routers to support LISP and register the prefix into the LISP mapping
system, but to keep announcing it with BGP as well. This way, LISP
sites will reach them over LISP, while legacy sites will be
unaffected by the change. The main disadvantage of this approach is
that no decrease in the DFZ routing table size is achieved. Still,
just increasing the number of LISP sites is an important gain, as an
increasing LISP/non-LISP site ratio may decrease the need for
BGP-based traffic engineering that leads to prefix deaggregation.
That, in turn, may lead to a decrease in the DFZ size and churn in
the late transition phase.
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This scenario is not limited to sites that already have their
prefixes announced with BGP. Newly allocated EID blocks could follow
this strategy as well during the early LISP deployment phase,
depending on the cost/benefit analysis of the individual networks.
Since this leads to an increase in the DFZ size, the following
architecture should be preferred for new allocations.
5.2. Mapping Service Provider (MSP) PITR Service
In addition to publishing their clients' registered prefixes in the
mapping system, MSPs with enough transit capacity can offer PITR
service to them as a separate service. This service is especially
useful for new PI allocations to sites without existing BGP
infrastructure wishing to avoid BGP altogether. The MSP announces
the prefix into the DFZ, and the client benefits from ingress traffic
engineering without prefix deaggregation. The downside of this
scenario is added path stretch.
Routing all non-LISP ingress traffic through a third party that is
not one of its ISPs is only feasible for sites with modest amounts of
traffic (like those using the IPv6 tunnel broker services today),
especially in the first stage of the transition to LISP, with a
significant number of legacy sites. This is because the handling of
said traffic is likely to result in additional costs, which would be
passed down to the client. When the LISP/non-LISP site ratio becomes
high enough, this approach can prove increasingly attractive.
Compared to LISP+BGP, this approach avoids DFZ bloat caused by prefix
deaggregation for traffic engineering purposes, resulting in slower
routing table increase in the case of new allocations and potential
decrease for existing ones. Moreover, MSPs serving different clients
with adjacent aggregatable prefixes may lead to additional decrease,
but quantifying this decrease is subject to future research study.
5.3. Proxy-ITR Route Distribution (PITR-RD)
Instead of a LISP site or the MSP announcing its EIDs with BGP to the
DFZ, this function can be outsourced to a third party, a PITR Service
Provider (PSP). This will result in a decrease in operational
complexity at both the site and the MSP.
The PSP manages a set of distributed PITR(s) that will advertise the
corresponding EID-Prefixes through BGP to the DFZ. These PITRs will
then encapsulate the traffic they receive for those EIDs towards the
RLOCs of the LISP site, ensuring their reachability from non-LISP
sites.
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While it is possible for a PSP to manually configure each client's
EID-Routes to be announced, this approach offers little flexibility
and is not scalable. This section presents a scalable architecture
that offers automatic distribution of EID-Routes to LISP sites and
service providers.
The architecture requires no modification to existing LISP network
elements, but it introduces a new (conceptual) network element, the
EID-Route Server, which is defined as a router that either propagates
routes learned from other EID-Route Servers or originates EID-Routes.
The EID-Routes that it originates are those for which it is
authoritative. It propagates these routes to Proxy-ITRs within the
AS of the EID-Route Server. It is worth noting that a BGP-capable
router can also be considered an EID-Route Server.
Further, an EID-Route is defined as a prefix originated via the Route
Server of the MSP, which should be aggregated if the MSP has multiple
customers inside a single large continuous prefix. This prefix is
propagated to other PITRs both within the MSP and to other PITR
operators with which it peers. EID-Route Servers are operated by
either the LISP site, MSPs, or PSPs and may be collocated with a
Map-Server or PITR, but they are functionally discrete entities.
They distribute EID-Routes, using BGP, to other domains according to
policies set by participants.
MSP (AS64500)
RS ---> PITR
| /
| _.--./
,-'' /`--.
LISP site ---,' | v `.
( | DFZ )----- Mapping system
non-LISP site ----. | ^ ,'
`--. / _.-'
| `--''
v /
PITR
PSP (AS64501)
Figure 7: PITR-RD Architecture
The architecture described above decouples EID origination from route
propagation, with the following benefits:
o Can accurately represent business relationships between PITR
operators
o Is more mapping system agnostic
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o Makes minor changes to PITR implementation; no changes to other
components
In the example in Figure 7, we have a MSP providing services to the
LISP site. The LISP site does not run BGP and gets an EID allocation
directly from a RIR, or from the MSP, which may be a Local Internet
Registry (LIR). Existing PI allocations can be migrated as well.
The MSP ensures the presence of the prefix in the mapping system and
runs an EID-Route Server to distribute it to PSPs. Since the LISP
site does not run BGP, the prefix will be originated with the AS
number of the MSP.
In the simple case depicted in Figure 7, the EID-Route of a LISP site
will be originated by the Route Server and announced to the DFZ by
the PSP's PITRs with AS path 64501 64500. From that point on, the
usual BGP dynamics apply. This way, routes announced by the PITR are
still originated by the authoritative Route Server. Note that the
peering relationships between MSPs/PSPs and those in the underlying
forwarding plane may not be congruent, making the AS path to a PITR
shorter than it is in reality.
The non-LISP site will select the best path towards the EID-Prefix
according to its local BGP policies. Since AS-path length is usually
an important metric for selecting paths, careful placement of PITRs
could significantly reduce path stretch between LISP and non-LISP
sites.
The architecture allows for flexible policies between MSPs/PSPs.
Consider the EID-Route Server networks as control plane overlays,
facilitating the implementation of policies necessary to reflect the
business relationships between participants. The results are then
injected into the common underlying forwarding plane. For example,
some MSPs/PSPs may agree to exchange EID-Prefixes and only announce
them to each of their forwarding plane customers. Global
reachability of an EID-Prefix depends on the MSP from which the LISP
site buys service and is also subject to agreement between the above-
mentioned parties.
In terms of impact on the DFZ, this architecture results in a slower
routing table increase for new allocations, since traffic engineering
will be done at the LISP level. For existing allocations migrating
to LISP, the DFZ may decrease, since MSPs may be able to aggregate
the prefixes announced.
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Compared to LISP+BGP, this approach avoids DFZ bloat caused by prefix
deaggregation for traffic engineering purposes, resulting in slower
routing table increase in the case of new allocations and potential
decrease for existing ones. Moreover, MSPs serving different clients
with adjacent aggregatable prefixes may lead to additional decrease,
but quantifying this decrease is subject to future research study.
The flexibility and scalability of this architecture do not come
without a cost, however: A PSP operator has to establish either
transit or peering relationships to improve its connectivity.
5.4. Migration Summary
Registering a domain name typically entails an annual fee that should
cover the operating expenses for publishing the domain in the global
DNS. This situation is similar for several other registration
services. A LISP MSP client publishing an EID-Prefix in the LISP
mapping system has the option of signing up for PITR services as
well, for an extra fee. These services may be offered by the MSP
itself, but it is expected that specialized PSPs will do it. Clients
that do not sign up will be responsible for getting non-LISP traffic
to their EIDs (using the LISP+BGP scenario).
Additionally, Tier 1 ISPs have incentives to offer PITR services to
non-subscribers in strategic places just to attract more traffic from
competitors and thus more revenue.
The following table presents the expected effects that the transition
scenarios at various phases will have on the DFZ routing table size:
Phase | LISP+BGP | MSP PITR | PITR-RD
-----------------+--------------+-----------------+----------------
Early transition | no change | slower increase | slower increase
Late transition | may decrease | slower increase | slower increase
LISP Internet | considerable decrease
It is expected that PITR-RD will coexist with LISP+BGP during the
migration, with the latter being more popular in the early transition
phase. As the transition progresses and the MSP PITR and PITR-RD
ecosystem gets more ubiquitous, LISP+BGP should become less
attractive, slowing down the increase of the number of routes in
the DFZ.
Note that throughout Section 5 we focused on the effects of LISP
deployment on the DFZ routing table size. Other metrics may be
impacted as well but to the best of our knowledge have not been
measured as yet.
Jakab, et al. Experimental [Page 23]
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6. Security Considerations
All security implications of LISP deployments are to be discussed in
separate documents. [LISP-THREATS] gives an overview of LISP threat
models, including ETR operators attracting traffic by overclaiming an
EID-Prefix (Section 4.4.3 of [LISP-THREATS]). Securing mapping
lookups is discussed in [LISP-SEC].
7. Acknowledgements
Many thanks to Margaret Wasserman for her contribution to the IETF 76
presentation that kickstarted this work. The authors would also like
to thank Damien Saucez, Luigi Iannone, Joel Halpern, Vince Fuller,
Dino Farinacci, Terry Manderson, Noel Chiappa, Hannu Flinck, Paul
Vinciguerra, Fred Templin, Brian Haberman, and everyone else who
provided input.
8. References
8.1. Normative References
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830,
January 2013.
[RFC6832] Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
"Interworking between Locator/ID Separation Protocol
(LISP) and Non-LISP Sites", RFC 6832, January 2013.
[RFC6833] Fuller, V. and D. Farinacci, "Locator/ID Separation
Protocol (LISP) Map-Server Interface", RFC 6833,
January 2013.
8.2. Informative References
[CACHE] Jung, J., Sit, E., Balakrishnan, H., and R. Morris, "DNS
performance and the effectiveness of caching", IEEE/ACM
Transactions on Networking (TON), Volume 10, Issue 5,
pages 589-603, October 2002.
[DDT-ROOT] "Introduction to LISP DDT: DDT Root", March 2014,
<http://ddt-root.org/>.
[LISP-DDT] Fuller, V., Lewis, D., Ermagan, V., and A. Jain, "LISP
Delegated Database Tree", Work in Progress, March 2013.
Jakab, et al. Experimental [Page 24]
RFC 7215 LISP Deployment April 2014
[LISP-SEC] Maino, F., Ermagan, V., Cabellos-Aparicio, A., Saucez, D.,
and O. Bonaventure, "LISP-Security (LISP-SEC)", Work in
Progress, October 2013.
[LISP-THREATS]
Saucez, D., Iannone, L., and O. Bonaventure, "LISP Threats
Analysis", Work in Progress, April 2014.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with
In-the-Network Tunneling", RFC 4459, April 2006.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006.
[RFC4984] Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
Workshop on Routing and Addressing", RFC 4984,
September 2007.
[RFC6834] Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
Separation Protocol (LISP) Map-Versioning", RFC 6834,
January 2013.
[RFC6835] Farinacci, D. and D. Meyer, "The Locator/ID Separation
Protocol Internet Groper (LIG)", RFC 6835, January 2013.
[RFC6836] Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol Alternative Logical
Topology (LISP+ALT)", RFC 6836, January 2013.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887,
April 2013.
[TELCO96] Federal Communications Commission, "Telecommunications Act
of 1996", 1996, <http://transition.fcc.gov/telecom.html>.
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Appendix A. Step-by-Step Example BGP-to-LISP Migration Procedure
To help the operational community deploy LISP, this informative
section offers a step-by-step guide for migrating a BGP-based
Internet presence to a LISP site. It includes a pre-install/
pre-turn-up checklist, and customer and provider activation
procedures.
A.1. Customer Pre-Install and Pre-Turn-Up Checklist
1. Determine how many current physical service provider connections
the customer has, and their existing bandwidth and traffic
engineering requirements.
This information will determine the number of routing locators,
and the priorities and weights that should be configured on
the xTRs.
2. Make sure the customer router has LISP capabilities.
* Check the OS version of the CE router. If LISP is an add-on,
check to see if it is installed.
This information can be used to determine if the platform is
appropriate to support LISP, in order to determine if a
software and/or hardware upgrade is required.
* Have the customer upgrade (if necessary, software and/or
hardware) to be LISP capable.
3. Obtain the current running configuration of the CE router. A
suggested LISP router configuration example can be customized to
the customer's existing environment.
4. Verify MTU handling.
* Request an increase in MTU to 1556 or more on service provider
connections. Prior to the MTU change, verify the transmission
of a 1500-byte packet from the PxTR to the RLOC with the Don't
Fragment (DF) bit set.
* Ensure that the customer is not filtering ICMP Unreachable or
Time Exceeded messages on their firewall or router.
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LISP, like any tunneling protocol, will increase the size of
packets when the LISP header is appended. If increasing the MTU
of the access links is not possible, care must be taken that ICMP
is not being filtered in order to allow Path MTU Discovery to
take place.
5. Validate member prefix allocation.
This step checks to see whether the prefix used by the customer
is a direct (Provider-Independent) prefix or a prefix assigned by
a physical service provider (Provider Aggregatable). If the
prefixes are assigned by other service providers, then a Letter
of Agreement is required to announce prefixes through the Proxy
Service Provider.
6. Verify the member RLOCs and their reachability.
This step ensures that the RLOCs configured on the CE router are
in fact reachable and working.
7. Prepare for cut-over.
* If possible, have a host outside of all security and filtering
policies connected to the console port of the edge router or
switch.
* Make sure the customer has access to the router in order to
configure it.
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A.2. Customer Activating LISP Service
1. The customer configures LISP on CE router(s) according to the
configuration recommended by the service provider.
The LISP configuration consists of the EID-Prefix, the locators,
and the weights and priorities of the mapping between the two
values. In addition, the xTR must be configured with
Map-Resolver(s), Map-Server(s), and the shared key for
registering to Map-Server(s). If required, Proxy-ETR(s) may be
configured as well.
In addition to the LISP configuration:
* Ensure that the default routes(s) to next-hop external
neighbors is included and RLOCs are present in the
configuration.
* If two or more routers are used, ensure that all RLOCs are
included in the LISP configuration on all routers.
* It will be necessary to redistribute the default route via IGP
between the external routers.
2. When transition is ready, perform a soft shutdown on existing
eBGP peer session(s).
* From the CE router, use the LISP Internet Groper (LIG)
[RFC6835] to ensure that registration is successful.
* To verify LISP connectivity, find and ping LISP connected
sites. If possible, find ping destinations that are not
covered by a prefix in the global BGP routing system, because
PITRs may deliver the packets even if LISP connectivity is not
working. Traceroutes may help determine if this is the case.
* To verify connectivity to non-LISP sites, try accessing a
landmark (e.g., a major Internet site) via a web browser.
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RFC 7215 LISP Deployment April 2014
A.3. Cut-Over Provider Preparation and Changes
1. Verify site configuration, and then verify active registration on
Map-Server(s).
* Authentication key.
* EID-Prefix.
2. Add EID space to map-cache on proxies.
3. Add networks to BGP advertisement on proxies.
* Modify route-maps/policies on PxTRs.
* Modify route policies on core routers (if non-connected
member).
* Modify ingress policies on core routers.
* Ensure route announcement in looking glass servers,
RouteViews.
4. Perform traffic verification test.
* Ensure that MTU handling is as expected (PMTUD working).
* Ensure Proxy-ITR map-cache population.
* Ensure access from traceroute/ping servers around Internet.
* Use a looking glass to check for external visibility of
registration via several Map-Resolvers.
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Authors' Addresses
Lorand Jakab
Cisco Systems
170 Tasman Drive
San Jose, CA 95134
USA
EMail: lojakab@cisco.com
Albert Cabellos-Aparicio
Technical University of Catalonia
C/Jordi Girona, s/n
BARCELONA 08034
Spain
EMail: acabello@ac.upc.edu
Florin Coras
Technical University of Catalonia
C/Jordi Girona, s/n
BARCELONA 08034
Spain
EMail: fcoras@ac.upc.edu
Jordi Domingo-Pascual
Technical University of Catalonia
C/Jordi Girona, s/n
BARCELONA 08034
Spain
EMail: jordi.domingo@ac.upc.edu
Darrel Lewis
Cisco Systems
170 Tasman Drive
San Jose, CA 95134
USA
EMail: darlewis@cisco.com
Jakab, et al. Experimental [Page 30]