Chapter 2. Introduction to Border Gateway Protocol 4

This chapter covers the following key topics:

• Classless Interdomain Routing ” This section introduces CIDR and discusses both its advantages and its shortcomings.

• Who Needs BGP? ” This section examines several inter-AS scenarios, with an eye to where BGP is necessary and where it is not.

• BGP Basics ” This section discusses the fundamentals of the Border Gateway Protocol, including message types and path attributes.

• IBGP and IGP Synchronization ” This section presents the issues surrounding synchronization between IBGP and the IGP within an AS, why synchronization is required by default, and how synchronization problems can be avoided.

• Managing Large-Scale BGP Peering ” This section presents four tools for controlling large-scale BGP implementations .

• BGP Message Formats ” This section examines the details of the various BGP messages.

Border Gateway Protocol (BGP) is a particularly important topic for any CCIE, and you can expect your knowledge of it to be thoroughly challenged in the CCIE lab.

You learned in Chapter 1, "Exterior Gateway Protocol," that the architects of the ARPANET began recognizing in the early 1980s that autonomous systems, and an inter-AS reachability protocol, were necessary to maintain manageability of the fast-growing Internet. Their original solution, Exterior Gateway Protocol (EGP), was adequate for the backbone-based ARPANET, but from the beginning, the architects understood the necessity of moving to a meshed inter-AS topology. They further understood that EGP was not capable of efficiently routing in such an environment because of its inability to detect loops , its very slow convergence time, and its lack of tools to support routing policies.

Attempts were made to enhance EGP, but in the end, an entirely new inter-AS protocol, a true routing protocol rather than a mere reachability protocol such as EGP, was called for. That inter-AS routing protocol, first introduced in 1989 in RFC 1105[1], is BGP. The first version of BGP was updated exactly one year later in RFC 1163[2]. BGP was upgraded again in 1991 in RFC 1267[3], and with this third modification, it became customary to refer to the three versions as BGP-1, BGP-2, and BGP-3, respectively.

The current version of BGP, BGP-4, was introduced in 1995 in RFC 1771[4]. BGP-4 differs significantly from the earlier versions. The most important difference is that BGP-4 is classless, whereas the earlier versions are classful. The motive for this fundamental change goes to the very heart of the reason exterior gateway protocols exist at all: to keep routing within the Internet both manageable and reliable. Classless interdomain routing (CIDR) ”originally introduced in RFC 1517[5] in 1993, finalized in RFC 1519[6] in the same year as a standard proposal, and amended by RFC 1520[7] ”was created for this purpose, and BGP-4 was created to support CIDR.