1. Introduction

This memo describes the use of the Border Gateway Protocol (BGP) [1] in the Internet environment. BGP is an inter-Autonomous System routing protocol. The network reachability information exchanged via BGP provides sufficient information to detect routing loops and enforce routing decisions based on performance preference and policy constraints as outlined in RFC 1104 [2]. In particular, BGP exchanges routing information containing full AS paths and enforces routing policies based on configuration information.

As the Internet has evolved and grown over in recent years, it has become painfully evident that it is soon to face several serious scaling problems. These include:

• Exhaustion of the class-B network address space. One fundamental cause of this problem is the lack of a network class of a size which is appropriate for mid-sized organization; class-C, with a maximum of 254 host addresses, is too small while class-B, which allows up to 65534 addresses, is too large to be densely populated.

• Growth of routing tables in Internet routers are beyond the ability of current software (and people) to effectively manage.

• Eventual exhaustion of the 32-bit IP address space.

It has become clear that the first two of these problems are likely to become critical within the next one to three years. Classless inter-domain routing (CIDR) attempts to deal with these problems by proposing a mechanism to slow the growth of the routing table and the need for allocating new IP network numbers. It does not attempt to solve the third problem, which is of a more long-term nature, but instead endeavors to ease enough of the short to mid-term difficulties to allow the Internet to continue to function efficiently while progress is made on a longer-term solution.

BGP-4 is an extension of BGP-3 that provides support for routing information aggregation and reduction based on the Classless inter- domain routing architecture (CIDR) [3]. This memo describes the usage of BGP-4 in the Internet.

All of the discussions in this paper are based on the assumption that the Internet is a collection of arbitrarily connected Autonomous Systems. That is, the Internet will be modeled as a general graph whose nodes are AS's and whose edges are connections between pairs of AS's.

The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS and using an exterior gateway protocol to route packets to other AS's. Since this classic definition was developed, it has become common for a single AS to use several interior gateway protocols and sometimes several sets of metrics within an AS. The use of the term Autonomous System here stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other AS's to have a single coherent interior routing plan and presents a consistent picture of which destinations are reachable through it.

AS's are assumed to be administered by a single administrative entity, at least for the purposes of representation of routing information to systems outside of the AS.