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rfc:rfc4456

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.

Bates, et al. Standards Track [Page 1] RFC 4456 BGP Route Reflection April 2006

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].

Bates, et al. Standards Track [Page 2] RFC 4456 BGP Route Reflection April 2006

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

Bates, et al. Standards Track [Page 3] RFC 4456 BGP Route Reflection April 2006

 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.

Bates, et al. Standards Track [Page 4] RFC 4456 BGP Route Reflection April 2006

               / - - - - - - - - - - - - -  -
               |           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.

Bates, et al. Standards Track [Page 5] RFC 4456 BGP Route Reflection April 2006

 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.

Bates, et al. Standards Track [Page 6] RFC 4456 BGP Route Reflection April 2006

 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.

Bates, et al. Standards Track [Page 7] RFC 4456 BGP Route Reflection April 2006

 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].

Bates, et al. Standards Track [Page 8] RFC 4456 BGP Route Reflection April 2006

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.

Bates, et al. Standards Track [Page 9] RFC 4456 BGP Route Reflection April 2006

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.

Bates, et al. Standards Track [Page 10] RFC 4456 BGP Route Reflection April 2006

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

Bates, et al. Standards Track [Page 11] RFC 4456 BGP Route Reflection April 2006

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Bates, et al. Standards Track [Page 12]

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