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Network Working Group                                          D. HaskinRequest for Comments: 1397                 Bolt Beranek and Newman, Inc.                                                            January 1993Default Route Advertisement InBGP2 And BGP3 Versions Of The Border Gateway ProtocolStatus 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 borderHaskin                                                          [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.comHaskin                                                          [Page 2]