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RFC 4456

BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)

Pages: 12
Draft Standard
Errata
Obsoletes:  27961966
Updated by:  7606

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Network Working Group                                           T. Bates
Request for Comments: 4456                                       E. Chen
Obsoletes: 2796, 1966                                      Cisco Systems
Category: Standards Track                                     R. Chandra
                                                           Sonoa Systems
                                                              April 2006


                         BGP Route Reflection:
            An Alternative to Full Mesh Internal BGP (IBGP)


Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

The Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol designed for TCP/IP internets. Typically, all BGP speakers within a single AS must be fully meshed so that any external routing information must be re-distributed to all other routers within that Autonomous System (AS). This represents a serious scaling problem that has been well documented with several alternatives proposed. This document describes the use and design of a method known as "route reflection" to alleviate the need for "full mesh" Internal BGP (IBGP). This document obsoletes RFC 2796 and RFC 1966.
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Table of Contents

1. Introduction ....................................................2 2. Specification of Requirements ...................................2 3. Design Criteria .................................................3 4. Route Reflection ................................................3 5. Terminology and Concepts ........................................4 6. Operation .......................................................5 7. Redundant RRs ...................................................6 8. Avoiding Routing Information Loops ..............................6 9. Impact on Route Selection .......................................7 10. Implementation Considerations ..................................7 11. Configuration and Deployment Considerations ....................7 12. Security Considerations ........................................8 13. Acknowledgements ...............................................9 14. References .....................................................9 14.1. Normative References ......................................9 14.2. Informative References ....................................9 Appendix A: Comparison with RFC 2796 ..............................10 Appendix B: Comparison with RFC 1966 ..............................10

1. Introduction

Typically, all BGP speakers within a single AS must be fully meshed and any external routing information must be re-distributed to all other routers within that AS. For n BGP speakers within an AS that requires to maintain n*(n-1)/2 unique Internal BGP (IBGP) sessions. This "full mesh" requirement clearly does not scale when there are a large number of IBGP speakers each exchanging a large volume of routing information, as is common in many of today's networks. This scaling problem has been well documented, and a number of proposals have been made to alleviate this [2,3]. This document represents another alternative in alleviating the need for a "full mesh" and is known as "route reflection". This approach allows a BGP speaker (known as a "route reflector") to advertise IBGP learned routes to certain IBGP peers. It represents a change in the commonly understood concept of IBGP, and the addition of two new optional non-transitive BGP attributes to prevent loops in routing updates. This document obsoletes RFC 2796 [6] and RFC 1966 [4].

2. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [7].
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3. Design Criteria

Route reflection was designed to satisfy the following criteria. o Simplicity Any alternative must be simple to configure and easy to understand. o Easy Transition It must be possible to transition from a full-mesh configuration without the need to change either topology or AS. This is an unfortunate management overhead of the technique proposed in [3]. o Compatibility It must be possible for noncompliant IBGP peers to continue to be part of the original AS or domain without any loss of BGP routing information. These criteria were motivated by operational experiences of a very large and topology-rich network with many external connections.

4. Route Reflection

The basic idea of route reflection is very simple. Let us consider the simple example depicted in Figure 1 below. +-------+ +-------+ | | IBGP | | | RTR-A |--------| RTR-B | | | | | +-------+ +-------+ \ / IBGP \ ASX / IBGP \ / +-------+ | | | RTR-C | | | +-------+ Figure 1: Full-Mesh IBGP In ASX, there are three IBGP speakers (routers RTR-A, RTR-B, and RTR-C). With the existing BGP model, if RTR-A receives an external
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   route and it is selected as the best path it must advertise the
   external route to both RTR-B and RTR-C.  RTR-B and RTR-C (as IBGP
   speakers) will not re-advertise these IBGP learned routes to other
   IBGP speakers.

   If this rule is relaxed and RTR-C is allowed to advertise IBGP
   learned routes to IBGP peers, then it could re-advertise (or reflect)
   the IBGP routes learned from RTR-A to RTR-B and vice versa.  This
   would eliminate the need for the IBGP session between RTR-A and RTR-B
   as shown in Figure 2 below.

                  +-------+        +-------+
                  |       |        |       |
                  | RTR-A |        | RTR-B |
                  |       |        |       |
                  +-------+        +-------+
                        \            /
                    IBGP \   ASX    / IBGP
                          \        /
                           +-------+
                           |       |
                           | RTR-C |
                           |       |
                           +-------+

                Figure 2: Route Reflection IBGP

   The route reflection scheme is based upon this basic principle.

5. Terminology and Concepts

We use the term "route reflection" to describe the operation of a BGP speaker advertising an IBGP learned route to another IBGP peer. Such a BGP speaker is said to be a "route reflector" (RR), and such a route is said to be a reflected route. The internal peers of an RR are divided into two groups: 1) Client peers 2) Non-Client peers An RR reflects routes between these groups, and may reflect routes among client peers. An RR along with its client peers form a cluster. The Non-Client peer must be fully meshed but the Client peers need not be fully meshed. Figure 3 depicts a simple example outlining the basic RR components using the terminology noted above.
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                 / - - - - - - - - - - - - -  -
                 |           Cluster           |
                   +-------+        +-------+
                 | |       |        |       |  |
                   | RTR-A |        | RTR-B |
                 | |Client |        |Client |  |
                   +-------+        +-------+
                 |       \           /         |
                    IBGP  \         / IBGP
                 |         \       /           |
                           +-------+
                 |         |       |           |
                           | RTR-C |
                 |         |  RR   |           |
                           +-------+
                 |           /   \             |
                  - - - - - /- - -\- - - - - - /
                     IBGP  /       \ IBGP
                  +-------+         +-------+
                  | RTR-D |  IBGP   | RTR-E |
                  |  Non- |---------|  Non- |
                  |Client |         |Client |
                  +-------+         +-------+

                     Figure 3: RR Components

6. Operation

When an RR receives a route from an IBGP peer, it selects the best path based on its path selection rule. After the best path is selected, it must do the following depending on the type of peer it is receiving the best path from 1) A route from a Non-Client IBGP peer: Reflect to all the Clients. 2) A route from a Client peer: Reflect to all the Non-Client peers and also to the Client peers. (Hence the Client peers are not required to be fully meshed.) An Autonomous System could have many RRs. An RR treats other RRs just like any other internal BGP speakers. An RR could be configured to have other RRs in a Client group or Non-client group.
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   In a simple configuration, the backbone could be divided into many
   clusters.  Each RR would be configured with other RRs as Non-Client
   peers (thus all the RRs will be fully meshed).  The Clients will be
   configured to maintain IBGP session only with the RR in their
   cluster.  Due to route reflection, all the IBGP speakers will receive
   reflected routing information.

   It is possible in an Autonomous System to have BGP speakers that do
   not understand the concept of route reflectors (let us call them
   conventional BGP speakers).  The route reflector scheme allows such
   conventional BGP speakers to coexist.  Conventional BGP speakers
   could be members of either a Non-Client group or a Client group.
   This allows for an easy and gradual migration from the current IBGP
   model to the route reflection model.  One could start creating
   clusters by configuring a single router as the designated RR and
   configuring other RRs and their clients as normal IBGP peers.
   Additional clusters can be created gradually.

7. Redundant RRs

Usually, a cluster of clients will have a single RR. In that case, the cluster will be identified by the BGP Identifier of the RR. However, this represents a single point of failure so to make it possible to have multiple RRs in the same cluster, all RRs in the same cluster can be configured with a 4-byte CLUSTER_ID so that an RR can discard routes from other RRs in the same cluster.

8. Avoiding Routing Information Loops

When a route is reflected, it is possible through misconfiguration to form route re-distribution loops. The route reflection method defines the following attributes to detect and avoid routing information loops: ORIGINATOR_ID ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type code 9. This attribute is 4 bytes long and it will be created by an RR in reflecting a route. This attribute will carry the BGP Identifier of the originator of the route in the local AS. A BGP speaker SHOULD NOT create an ORIGINATOR_ID attribute if one already exists. A router that recognizes the ORIGINATOR_ID attribute SHOULD ignore a route received with its BGP Identifier as the ORIGINATOR_ID.
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   CLUSTER_LIST

   CLUSTER_LIST is a new, optional, non-transitive BGP attribute of Type
   code 10.  It is a sequence of CLUSTER_ID values representing the
   reflection path that the route has passed.

   When an RR reflects a route, it MUST prepend the local CLUSTER_ID to
   the CLUSTER_LIST.  If the CLUSTER_LIST is empty, it MUST create a new
   one.  Using this attribute an RR can identify if the routing
   information has looped back to the same cluster due to
   misconfiguration.  If the local CLUSTER_ID is found in the
   CLUSTER_LIST, the advertisement received SHOULD be ignored.

9. Impact on Route Selection

The BGP Decision Process Tie Breaking rules (Sect. 9.1.2.2, [1]) are modified as follows: If a route carries the ORIGINATOR_ID attribute, then in Step f) the ORIGINATOR_ID SHOULD be treated as the BGP Identifier of the BGP speaker that has advertised the route. In addition, the following rule SHOULD be inserted between Steps f) and g): a BGP Speaker SHOULD prefer a route with the shorter CLUSTER_LIST length. The CLUSTER_LIST length is zero if a route does not carry the CLUSTER_LIST attribute.

10. Implementation Considerations

Care should be taken to make sure that none of the BGP path attributes defined above can be modified through configuration when exchanging internal routing information between RRs and Clients and Non-Clients. Their modification could potentially result in routing loops. In addition, when a RR reflects a route, it SHOULD NOT modify the following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED. Their modification could potentially result in routing loops.

11. Configuration and Deployment Considerations

The BGP protocol provides no way for a Client to identify itself dynamically as a Client of an RR. The simplest way to achieve this is by manual configuration. One of the key component of the route reflection approach in addressing the scaling issue is that the RR summarizes routing information and only reflects its best path.
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   Both Multi-Exit Discriminators (MEDs) and Interior Gateway Protocol
   (IGP) metrics may impact the BGP route selection.  Because MEDs are
   not always comparable and the IGP metric may differ for each router,
   with certain route reflection topologies the route reflection
   approach may not yield the same route selection result as that of the
   full IBGP mesh approach.  A way to make route selection the same as
   it would be with the full IBGP mesh approach is to make sure that
   route reflectors are never forced to perform the BGP route selection
   based on IGP metrics that are significantly different from the IGP
   metrics of their clients, or based on incomparable MEDs.  The former
   can be achieved by configuring the intra-cluster IGP metrics to be
   better than the inter-cluster IGP metrics, and maintaining full mesh
   within the cluster.  The latter can be achieved by

      o  setting the local preference of a route at the border router to
         reflect the MED values, or

      o  making sure the AS-path lengths from different ASes are
         different when the AS-path length is used as a route selection
         criteria, or

      o  configuring community-based policies to influence the route
         selection.

   One could argue though that the latter requirement is overly
   restrictive, and perhaps impractical in some cases.  One could
   further argue that as long as there are no routing loops, there are
   no compelling reasons to force route selection with route reflectors
   to be the same as it would be with the full IBGP mesh approach.

   To prevent routing loops and maintain consistent routing view, it is
   essential that the network topology be carefully considered in
   designing a route reflection topology.  In general, the route
   reflection topology should be congruent with the network topology
   when there exist multiple paths for a prefix.  One commonly used
   approach is the reflection based on Point of Presence (POP), in which
   each POP maintains its own route reflectors serving clients in the
   POP, and all route reflectors are fully meshed.  In addition, clients
   of the reflectors in each POP are often fully meshed for the purpose
   of optimal intra-POP routing, and the intra-POP IGP metrics are
   configured to be better than the inter-POP IGP metrics.

12. Security Considerations

This extension to BGP does not change the underlying security issues inherent in the existing IBGP [1, 5].
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13. Acknowledgements

The authors would like to thank Dennis Ferguson, John Scudder, Paul Traina, and Tony Li for the many discussions resulting in this work. This idea was developed from an earlier discussion between Tony Li and Dimitri Haskin. In addition, the authors would like to acknowledge valuable review and suggestions from Yakov Rekhter on this document, and helpful comments from Tony Li, Rohit Dube, John Scudder, and Bruce Cole.

14. References

14.1. Normative References

[1] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

14.2. Informative References

[2] Savola, P., "Reclassification of RFC 1863 to Historic", RFC 4223, October 2005. [3] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 3065, February 2001. [4] Bates, T. and R. Chandra, "BGP Route Reflection An alternative to full mesh IBGP", RFC 1966, June 1996. [5] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998. [6] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796, April 2000. [7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
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Appendix A: Comparison with RFC 2796

The impact on route selection is added. The pictorial description of the encoding of the CLUSTER_LIST attribute is removed as the description is redundant to the BGP specification, and the attribute length field is inadvertently described as one octet.

Appendix B: Comparison with RFC 1966

All the changes listed in Appendix A, plus the following. Several terminologies related to route reflection are clarified, and the reference to EBGP routes/peers are removed. The handling of a routing information loop (due to route reflection) by a receiver is clarified and made more consistent. The addition of a CLUSTER_ID to the CLUSTER_LIST has been changed from "append" to "prepend" to reflect the deployed code. The section on "Configuration and Deployment Considerations" has been expanded to address several operational issues.
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Authors' Addresses

Tony Bates Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 EMail: tbates@cisco.com Ravi Chandra Sonoa Systems, Inc. 3255-7 Scott Blvd. Santa Clara, CA 95054 EMail: rchandra@sonoasystems.com Enke Chen Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 EMail: enkechen@cisco.com
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