Expiration Date: March 22, 1993


Network Working Group                   Dimitry Haskin
Request for Comments: DRAFT             Bolt Beranek and Newman Inc.
					September 1992

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

Status Of This Memo

 This document is an Internet Draft.  Internet Drafts are
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 Drafts.

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Abstract

 This document specifies the recommendation of the BGP Working Group on
 default route advertisement support in BGP2 and BGP3 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 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.




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RFC DRAFT (Expiration Date: March 22,  1993)


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 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.



























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