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

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: dhaskin@bbn.com




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