Introduction

As at press time, the Internet holds well over 90,000 routes, which is far more than Interior Gateway Protocols (IGPs) such as EIGRP or OSPF were designed to handle. Additionally, whereas private internetworks generally have only a few possible paths, with the Internet, there could literally be thousands of ways to arrive at the same destination. EIGRP or OSPF would have major problems under these circumstances.

Clearly, another protocol is needed to deal with the complex routing issues on the public Internet. The answer to this problem is BGP, or, more precisely, BGP 4. Unlike OSPF or EIGRP, which were designed to be intra-AS (autonomous system), BGP was specifically designed to route traffic between autonomous systems, and is therefore called an Inter-AS routing protocol.

This chapter gives some background on the development of this protocol, explains the difference between EBGP and IBGP, explores some of the most popular design concepts, and discusses more complex issues such as route reflectors and confederations.

The History of BGP

Border Gateway Protocol 4 (BGP 4) was preceded by BGP 1, BGP 2, and BGP 3 as standards for use in exchanging TCP/IP routing information between domains. Furthermore, all BGP versions were preceded by the Exterior Gateway Protocol (EGP) as both a standard and a protocol for interdomain routing. The Internet and most of the backbone providers were using EGP exclusively until the mid 1990 s. There was limited use of BGP 3 on any production network, primarily because of the stability of EGP and...