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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)                 9  TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)

TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP) 1 BGP Fundamentals and General Operation

Modern TCP/IP internetworks are comprised of autonomous systems (ASes) that are run independently. Each may use an interior routing protocol such as RIP, OSPF, IGRP or EIGRP to select routes between networks within the AS. To form larger internetworks, and especially the “mother of all internetworks”, the Internet, these autonomous systems must be connected together. This requires use of a consistent exterior routing protocol that all ASes can agree upon, and in today's TCP/IP that protocol is the Border Gateway Protocol (BGP).

In this section I describe the characteristics, general operation and detailed operation of the Border Gateway Protocol (BGP). The discussion is divided into two subsections. The first provides an overview and general look at the operation of BGP, including a discussion of key concepts such as topology, neighbor relationships, route determination and general messaging. The second gives a more detailed analysis of the different message types and how they are used, and describes the format of each message as well.

BGP is another in the rather large group of protocols and technologies that is so complex it would take dozens of topics to do justice. Therefore, I include here my somewhat standard disclaimer that you will find in this section only a relatively high-level look at BGP. You will need to refer to the BGP standards (described in the topic on BGP standards and versions) if you need more details.

TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP) 1 BGP Fundamentals and General Operation