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rfc:rfc1397

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

Haskin [Page 1] 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

Haskin [Page 2]

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