rfc3345.txt

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            Re     300 400              3
                 * 200 400   1          2















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      3) Rc and Rd advertise their best paths to each other; Rd selects
         '200 400, 0' because of the MED.

                                  NEXT_HOP
            Router AS_PATH  MED   IGP Cost
            ------------------------------
            Rc   * 200 400   0         50
                   200 400   1         44
            Rd   * 200 400   0         52
                   200 400   1         42
            Re     300 400              3
                 * 200 400   1          2

         Rd has a new best path so it sends an UPDATE to to Re,
         announcing the new path and an UPDATE/withdraw for '200 400, 1'
         to Rc.

      4) Re now selects '300 400' (with no MED) because '200 400, 0'
         beats '200 400, 1' based on MED and '300 400' beats '200 400,
         0' because of IGP metric.

                                  NEXT_HOP
            Router AS_PATH  MED   IGP Cost
            ------------------------------
            Rc   * 200 400    0         50
            Rd   * 200 400    0         52
                   200 400    1         42
            Re   * 300 400               3
                   200 400    0         92

         Re has a new best path and sends an UPDATE to Rd for '300 400'.

   5) Rd selects the '300 400' path because of IGP metric.

                                  NEXT_HOP
            Router AS_PATH  MED   IGP Cost
            ------------------------------
            Rc   * 200 400    0         50
            Rd     200 400    0         52
                 * 300 400              43
            Re   * 300 400               3
                   200 400    0         92
                   200 400    1          2

         Rd has a new best path so it sends an UPDATE to Rc and a
         UPDATE/withdraw to Re for '200 400, 0'.





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      6) Rc selects '300 400' because of the IGP metric.  Re selects
         '200 400, 1' because of the IGP metric.

                                  NEXT_HOP
            Router AS_PATH  MED   IGP Cost
            ------------------------------
            Rc     200 400    0         50
                 * 300 400              45
            Rd     200 400    0         52
                 * 300 400              43
            Re     300 400               3
                 * 200 400    1          2

         Rc sends an UPDATE/withdraw for '200 400, 0' to Rd.  Re sends
         an UPDATE for '200 400, 1' to Rd.

      7) Rd selects '200 400, 1' as its new best path based on the IGP
         metric.

                                  NEXT_HOP
            Router AS_PATH  MED   IGP Cost
            ------------------------------
            Rc     200 400    0         50
                 * 300 400              45
            Rd   * 200 400    1         42
            Re     300 400               3
                 * 200 400    1          2

         Rd sends an UPDATE to Rc, announcing '200 400, 1' and
         implicitly withdraws '300 400'.

      8) Rc selects '200 400, 0'.

                                  NEXT_HOP
            Router AS_PATH  MED   IGP Cost
            ------------------------------
            Rc   * 200 400    0         50
                   200 400    1         44
            Rd   * 200 400    1         42
            Re     300 400               3
                 * 200 400    1          2

         At this point we are back to Step 2 and are in a loop.








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3.2. Potential Workarounds for Type II Churn

   1) Do not accept MEDs from peers (this may not be a feasible
      alternative).

   2) Utilize other BGP attributes higher in the decision process so
      that the BGP decision algorithm selects a single AS before it
      reaches the MED step.  For example, if local-pref were set based
      on the advertising AS, then you first eliminate all routes except
      those in a single AS.  In the example, router Re would pick either
      X or Y based on your local-pref and never change selections.

      This leaves two simple workarounds for the two types of problems.

      Type I:  Make inter-cluster or inter-sub-AS link metrics higher
      than intra-cluster or intra-sub-AS metrics.

      Type II: Make route selections based on local-pref assigned to the
      advertising AS first and then use IGP cost and MED to make
      selection among routes from the same AS.

      Note that this requires per-prefix policies, as well as near
      intimate knowledge of other networks by the network operator.  The
      authors are not aware of ANY [large] provider today that performs
      per-prefix policies on routes learned from peers.  Implicitly
      removing this dynamic portion of route selection does not appear
      to be a viable option in today's networks.  The main point is that
      an available workaround using local-pref so that no two AS's
      advertise a given prefix at the same local-pref solves type II
      churn.

   3) Always compare BGP MEDs, regardless of whether or not they were
      obtained from a single AS.  This is probably a bad idea since MEDs
      may be derived in a number of ways, and are typically done so as a
      matter of operator-specific policy and largely a function of
      available metric space provided by the employed IGP.  As such,
      comparing MED values for a single prefix learned from multiple ASs
      is ill-advised.  This mostly defeats the purpose of MEDs; Option 1
      may be a more viable alternative.

   4) Do not use more than one tier of Route Reflection or Sub-ASs in
      the network.   The risk of route oscillation should be considered
      when designing networks that might use a multi-tiered routing
      isolation architecture.

   5) In a RR topology, mesh the clients.  For confederations, mesh the
      border routers at each level in the hierarchy.  In Figure 3, for
      example, if Rb and Re are peers, then there's no churn.



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4. Future Work

   It should be stated that protocol enhancements regarding this problem
   must be pursued.  Imposing network design requirements, such as those
   outlined above, are clearly an unreasonable long-term solution.
   Problems such as this should not occur under 'default' protocol
   configurations.

5. Security Considerations

   This discussion introduces no new security concerns to BGP or other
   specifications referenced in this document.

6. Acknowledgments

   The authors would like to thank Curtis Villamizar, Tim Griffin, John
   Scudder, Ron Da Silva, Jeffrey Haas and Bill Fenner.

7. References

   [1] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
       1771, March 1995.

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

   [3] Traina, P., McPherson, D. and J. Scudder, J., "Autonomous System
       Confederations for BGP", RFC 3065, February 2001.

   [4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
       Work in Progress.




















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8. Authors' Addresses

   Danny McPherson
   TCB
   EMail: [e-mail]


   Vijay Gill
   AOL Time Warner, Inc.
   12100 Sunrise Valley Drive
   Reston, VA 20191
   EMail: [e-mail]


   Daniel Walton
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   EMail: [e-mail]


   Alvaro Retana
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   EMail: [e-mail]

























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9. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  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
   English.

   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
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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



















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