Network Working Group                   Dimitry Haskin
Internet-Draft                          Bolt Beranek and Newman Inc.
                                        August 1991
                                        
                      Default Route Advertisement In
                       The Border Gateway Protocol
	       
1. Status Of This Memo

 This memo will be submitted to the RFC editor as the technical 
 recommendation of the BGP Working Group on the Border Gateway
 Protocol support of the default route advertisement.  

 Distribution of this memo is unlimited.

 Please send comments to the BGP mailing list (iwg@rice.edu).

2. 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 hosts, 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.

3. 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.