BGP default route: Default Route Advertisement In BGP2 and BGP3 Version of The Border Gateway Protocol
How older BGP versions advertise a fallback route
Haskin, D., "Default Route Advertisement In BGP2 and BGP3 Version of The Border Gateway Protocol", RFC 1397, DOI 10.17487/RFC1397, January 1993, <https://www.rfc-editor.org/info/rfc1397>.
@misc{rfc1397,
series = {Request for Comments},
number = 1397,
howpublished = {RFC 1397},
publisher = {RFC Editor},
doi = {10.17487/RFC1397},
url = {https://www.rfc-editor.org/info/rfc1397},
author = {D. Haskin},
title = {{Default Route Advertisement In BGP2 and BGP3 Version of The Border Gateway Protocol}},
pagetotal = 2,
year = 1993,
month = jan,
} RFC text
Contents
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
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
AI ELI5
On this page
§1Default Routes
A default route is a catch-all path that a router uses when it does not have an explicit routing entry for a destination address. Rather than dropping traffic destined for unknown networks, a router can forward it to a default gateway that might know what to do with it.
This is especially useful for routers that cannot maintain a complete routing table—either because the full routing table is too large to fit in memory, or to reduce the processing overhead of consulting a massive table. Instead of storing millions of routes, a router can store only the destinations it uses frequently, and forward everything else to a default gateway.
If a router does not know where a package should go, it sends it to a default gateway that might know better. This saves the router from having to remember the address of every possible destination.
§2Advertising a Default Route in BGP2 and BGP3
To advertise a default route in BGP version 2 or 3, a BGP speaker sends an UPDATE message with the network field set to 0.0.0.0, a special address that indicates a default route. The NEXT_HOP path attribute in the message specifies the IP address of the border gateway that should handle traffic destined for networks not in the router's table.
The ORIGIN attribute should be set to INCOMPLETE (value 2) because the default route is not a known path. If multiple default routes are being advertised, the INTER-AS-METRIC path attribute can express a preference order, allowing BGP speakers to favor one default gateway over another. If a previously advertised default route becomes unreachable, an UNREACHABLE message notifies neighbors.
UPDATE messages containing default routes follow the same rules as conventional route advertisements and are processed according to the receiving router's local policies.
A router sends a special message saying '0.0.0.0 goes to this gateway.' Other routers read this message and know where to send traffic when they do not have a better route. If there are multiple default gateways, the router can say which one is better.
§3Obsolescence in BGP4
RFC 1397 applies only to BGP versions 2 and 3. Starting with BGP version 4 (RFC 4271, the current standard for BGP), default route advertisement capability was built directly into the protocol design. BGP4 routers do not need to follow this recommendation because they have a standardized way of handling default routes as part of their specification.
For modern BGP deployments, default routes are handled by the standard BGP4 protocol and this RFC is Historic, retained as a reference for anyone still maintaining legacy BGP2 or BGP3 implementations.
Newer versions of BGP (BGP4) learned how to advertise default routes automatically, so this paper's advice only matters if you are running an old version.