Network Working Group                                          D. Haskin
Request for Comments: 1397                 Bolt Beranek and Newman, Inc.
                                                           January 1993


                    Default Route Advertisement In
        BGP2 And BGP3 Versions Of The Border Gateway Protocol

Status of this Memo

  This RFC specifies an IAB standards track protocol for the Internet
  community, and requests discussion and suggestions for improvements.
  Please refer to the current edition of the "IAB Official Protocol
  Standards" for the standardization state and status of this protocol.
  Distribution of this memo is unlimited.

Abstract

  This document specifies the recommendation of the BGP Working Group
  on default route advertisement support in BGP2 [1] and BGP3 [2]
  versions of the Border Gateway Protocol.

  This recommendation only applies to BGP2 and BGP3 versions of the
  Border Gateway Protocol since starting with the BGP4 [3] version a
  default route advertisement capability is built in the protocol.

1. Overview

  The purpose of the default route advertisement capability is to
  advertise the IP address of a border gateway which can be used as the
  default next hop to destinations that are not listed explicitly in
  the BGP peer's routing table.

  This capability will allow routers, that are unable to maintain a
  complete routing table (e.g., due to its size) to learn a border
  gateway that is ready to handle the default traffic.  Also, in
  contrast to static defaults, if there is more than one default
  gateway, this would make it possible for a BGP speaker to express a
  preference for one over the other.  It also reduces the need to
  configure default routes in routers.

2. Default Route Advertisement

  A default route is advertised in an UPDATE message that carries
  reachability information for network 0.0.0.0.  A Network field of
  such an UPDATE message must contain the IP address 0.0.0.0 as the
  indication that it carries a default route.  The NEXT_HOP path
  attribute of such a message provides the IP address of a border



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RFC 1397               Default Routes in Old BGPs           January 1993


  gateway that can be used as a default next hop to destinations that
  are not listed in the BGP peer's routing table.  The value of the
  ORIGIN attribute should be 2 (INCOMPLETE).  The AS_PATH attribute
  should be constructed according to the same rules that apply to a
  conventional network advertisement.

  If multiple default routes are advertised by a BGP speaker,  the
  INTER-AS-METRIC path attribute can be included in the corresponding
  UPDATE messages to express  preference levels for entry points to the
  same AS.

  The UNREACHABLE path attribute is used to indicate that a previously
  advertised default route has become unreachable.

  UPDATE messages containing the default route advertisements should be
  handled according to the rules that apply to all other UPDATE
  messages.  If multiple default route are acquired by a BGP speaker, a
  route is selected according to the local policies adopted by this BGP
  speaker.

References

  [1] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol (BGP)",
      RFC 1163, cisco Systems, T.J. Watson Research Center, IBM Corp.,
      June 1990.

  [2] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
      3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
      Corp., October 1991.

  [3] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
      Work in Progress, T.J. Watson Research Center, IBM Corp., cisco
      Systems, December 1992.

Security Considerations

  Security issues are not discussed in this memo.

Author's Address

  Dimitry Haskin
  Bolt, Beranek & Newman
  150 Cambridge Park Drive
  Cambridge, MA 02140

  Phone: 617-873-8609
  Email: [email protected]




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