RFC 1397

okay bestie, first the lowdown 📋✨ this is RFC 1397, called "Default Route Advertisement In BGP2 And BGP3 Versions Of The Border Gateway Protocol" 💖. It's from the Network Working Group, written by D. Haskin of Bolt Beranek and Newman, Inc., and it's dated January 1993 📅. The category is an IAB standards track protocol 🦋. The title block doesn't list anything it obsoletes or updates, so no drama there 🙈.

Status of This Memo 📜

okay so this RFC is an IAB standards track protocol for the whole Internet community 🌐, and it's literally asking y'all for discussion and suggestions 🫶. For the official standardization state and status, check the current edition of the "IAB Official Protocol Standards" 📚. Distribution is unlimited, so share away 💅.

Abstract 🦋

The BGP Working Group put together a recommendation about default route advertisement support in BGP2 [1] and BGP3 [2] versions of the Border Gateway Protocol 🌸.

This recommendation only applies to the BGP2 and BGP3 versions, because starting with the BGP4 [3] version, a default route advertisement capability is built into the protocol 🧚.

1. Overview ✨

The whole point of the default route advertisement capability is to advertise the IP address of a border gateway that can work as the default next hop for destinations that aren't listed explicitly in the BGP peer's routing table 🗺️.

This lets routers that can't keep a complete routing table (because it's huge 🐘) learn about a border gateway that's ready to handle the default traffic 🚦. Unlike static defaults, if there's more than one default gateway, a BGP speaker can 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. The Network field of such an UPDATE message MUST contain the IP address 0.0.0.0 as the sign that it's carrying a default route 🚩. The NEXT_HOP path attribute of that message gives the IP address of a border gateway that can be used as a default next hop to destinations that aren't 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 using the same rules that apply to a conventional network advertisement ✨.

If a BGP speaker advertises multiple default routes, 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 routes are acquired by a BGP speaker, a route is selected according to the local policies adopted by this BGP speaker 🧚.

References 📚

Here are the receipts 🧾

[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. ngl this is the scariest sentence in the whole doc 😭

Author's Address 🎀

Dimitry Haskin
Bolt, Beranek & Newman
150 Cambridge Park Drive
Cambridge, MA 02140

Phone: 617-873-8609
Email: dhaskin@bbn.com