rfc2796.txt

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



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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




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   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.














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14. Authors' Addresses

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

   EMail: [e-mail]


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

   EMail: [e-mail]


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

   EMail: [e-mail]



























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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.






































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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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Acknowledgement

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



















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