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rfc:rfc2796

Network Working Group T. Bates Request for Comments: 2796 Cisco Systems Updates: 1966 R. Chandra Category: Standards Track E. Chen

                                                      Redback Networks
                                                            April 2000
                       BGP Route Reflection -
                  An Alternative to Full Mesh 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 (2000).  All Rights Reserved.

Abstract

 The Border Gateway Protocol [1] is an inter-autonomous system routing
 protocol designed for TCP/IP internets. Currently in the Internet BGP
 deployments are configured such that that 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
 AS. This represents a serious scaling problem that has been  well
 documented with several alternatives proposed [2,3].
 This document describes the use and design of a method known as
 "Route Reflection" to alleviate the the need for "full mesh" IBGP.

1. Introduction

 Currently in the Internet, BGP deployments are configured such that
 that 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 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 todays internet networks.

Bates, et al. Standards Track [Page 1] RFC 2796 BGP Route Reflection April 2000

 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 "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
 transitive BGP attributes to prevent loops in routing updates.
 This document is a revision of RFC1966 [4], and it includes editorial
 changes, clarifications and corrections based on the deployment
 experience with route reflection. These revisions are summarized in
 the Appendix.

2. Design Criteria

 Route Reflection was designed to satisfy the following criteria.
    o  Simplicity
       Any alternative must be both simple to configure as well as
       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 non compliant IBGP peers to continue 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.

3. Route Reflection

 The basic idea of Route Reflection is very simple. Let us consider
 the simple example depicted in Figure 1 below.

Bates, et al. Standards Track [Page 2] RFC 2796 BGP Route Reflection April 2000

                 +-------+        +-------+
                 |       |  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 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.

Bates, et al. Standards Track [Page 3] RFC 2796 BGP Route Reflection April 2000

4. 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 a RR are divided into two groups:
         1) Client Peers
         2) Non-Client Peers
 A RR reflects routes between these groups, and may reflect routes
 among client peers.  A 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.
               / - - - - - - - - - - - - -  -
               |           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

Bates, et al. Standards Track [Page 4] RFC 2796 BGP Route Reflection April 2000

5. Operation

 When a 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 the 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. A RR treats other RRs just
 like any other internal BGP speakers. A RR could be configured to
 have other RRs in a Client group or Non-client group.
 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 a 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 co-exist. Conventional BGP speakers
 could be either members of 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.

6. Redundant RRs

 Usually a cluster of clients will have a single RR. In that case, the
 cluster will be identified by the ROUTER_ID 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.

Bates, et al. Standards Track [Page 5] RFC 2796 BGP Route Reflection April 2000

7. Avoiding Routing Information Loops

 When a route is reflected, it is possible through mis-configuration
 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 a RR
 in reflecting a route.  This attribute will carry the ROUTER_ID 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
 which recognizes the ORIGINATOR_ID attribute should ignore a route
 received with its ROUTER_ID as the ORIGINATOR_ID.
 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. It is encoded as follows:
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Attr. Flags  |Attr. Type Code|   Length      | value ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Where Length is the number of octets.
 When a 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 is looped back to the same cluster due to mis-
 configuration. If the local CLUSTER_ID is found in the cluster-list,
 the advertisement received should be ignored.

8. 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 potential 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 potential result in routing loops.

Bates, et al. Standards Track [Page 6] RFC 2796 BGP Route Reflection April 2000

9. 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.
 Both MEDs and 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 which 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.
    o  or by making sure the AS-path lengths from different ASs are
       different when the AS-path length is used as a route selection
       criteria.
    o  or by configuring community based policies using which the
       reflector can decide on the best route.
 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 congruent with the network topology when
 there exist multiple paths for a prefix. One commonly used approach
 is the POP-based reflection, 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

Bates, et al. Standards Track [Page 7] RFC 2796 BGP Route Reflection April 2000

 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.

10. Security Considerations

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

11. Acknowledgments

 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, and John Scudder on Section 9, and
 from Bruce Cole.

13. References

 [1]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
      RFC 1771, March 1995.
 [2]  Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh
      routing", RFC 1863, October 1995.
 [3]  Traina, P., "Limited Autonomous System Confederations for BGP",
      RFC 1965, June 1996.
 [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.

Bates, et al. Standards Track [Page 8] RFC 2796 BGP Route Reflection April 2000

14. Authors' Addresses

 Tony Bates
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 EMail: tbates@cisco.com
 Ravi Chandra
 Redback Networks Inc.
 350 Holger Way.
 San Jose, CA 95134
 EMail: rchandra@redback.com
 Enke Chen
 Redback Networks Inc.
 350 Holger Way.
 San Jose, CA 95134
 EMail: enke@redback.com

Bates, et al. Standards Track [Page 9] RFC 2796 BGP Route Reflection April 2000

Appendix Comparison with RFC 1966

 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 2796 BGP Route Reflection April 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
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 followed, or as required to translate it into languages other than
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Bates, et al. Standards Track [Page 11]

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