Network Working Group                                          T. Bates
Internet Draft                                            Cisco Systems
Expiration Date: September 2004                              R. Chandra
                                                                E. Chen
                                                       Redback Networks

                         BGP Route Reflection -
                    An Alternative to Full Mesh IBGP


1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

2. 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.

Bates, Chandra, Chen                                            [Page 1]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

3. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [7].

4. 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.

   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 non-
   transitive BGP attributes to prevent loops in routing updates.

5. Design Criteria

   Route Reflection was designed to satisfy the following criteria.

      o  Simplicity

         Any alternative must be both simple to configure as well as

      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

Bates, Chandra, Chen                                            [Page 2]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

         routing information.

   These criteria were motivated by operational experiences of a very
   large and topology rich network with many external connections.

6. 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 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

   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.

Bates, Chandra, Chen                                            [Page 3]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

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

                Figure 2: Route Reflection IBGP

   The Route Reflection scheme is based upon this basic principle.

7. 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.

Bates, Chandra, Chen                                            [Page 4]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

                 / - - - - - - - - - - - - -  -
                 |           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

8. 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

   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.

Bates, Chandra, Chen                                            [Page 5]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

   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.

9. 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.

10. 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 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.


Bates, Chandra, Chen                                            [Page 6]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

   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 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.

11. Impact on Path Selection

   The ORIGINATOR_ID (when present) of a path SHOULD be treated as the
   BGP Identifier of the path in the route selection as described in

   If the BGP Identifiers of two paths are equal when compared in the
   route selection, then the path with the shorter CLUSTER_LIST length
   SHOULD be preferred. The CLUSTER_LIST length SHOULD be considered as
   zero for a path that has no CLUSTER_LIST attribute.

12. 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

   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, Chandra, Chen                                            [Page 7]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

13. 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

      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
   are often fully meshed for the purpose of optimal intra-POP routing,

Bates, Chandra, Chen                                            [Page 8]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

   and the intra-POP IGP metrics are configured to be better than the
   inter-POP IGP metrics.

14. Security Considerations

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

15. 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, John Scudder and Bruce Cole.

16. Normative 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.

   [6]  Bates, T., R. Chandra and E. Chen "BGP Route Reflection - An
        Alternative to Full Mesh IBGP", RFC 2796, Arpil 2000.

   [7]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

Bates, Chandra, Chen                                            [Page 9]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

17. Authors' Addresses

    Tony Bates
    Cisco Systems, Inc.
    170 West Tasman Drive
    San Jose, CA 95134


    Ravi Chandra
    Redback Networks Inc.
    300 Holger Way.
    San Jose, CA 95134


    Enke Chen
    Redback Networks Inc.
    300 Holger Way.
    San Jose, CA 95134


18. Appendix A Comparison with RFC 2796

   The impact on route selection is added.

19. Appendix B 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, Chandra, Chen                                           [Page 10]

Internet Draft      draft-ietf-idr-rfc2796bis-00.txt          March 2004

20. Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   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
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

21. Acknowledgement

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

Bates, Chandra, Chen                                           [Page 11]

------- End of Forwarded Message