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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)

Enhanced Interior Gateway Routing Protocol (EIGRP) 1 TCP/IP Border Gateway Protocol (BGP/BGP-4)

For ease of administration, routers on large internetworks are grouped into autonomous systems (ASes) that are independently controlled by different organizations and companies. Interior routing protocols such as RIP and OSPF are used to communicate routing information between routers within an autonomous system. Obviously, if interior routing protocols are used within ASes, we need another set of routing protocols to send that information between ASes. These are called exterior routing protocols.

The entire point of autonomous system architecture is conveyed in the meaning of the first word in that phrase: autonomous. The details of what happens within an AS are hidden from other ASes, which allows the administrator of an AS to have the independence to control how he or she runs it, including the selection of one or more from a variety of different interior routing protocols. In contrast, to reliably connect ASes together, it is essential that each one be running the same exterior routing protocol, or the result would be something akin to the Tower of Babel. The result of this is that in TCP/IP there is generally only one exterior routing protocol in widespread use at a given time.

In this section I describe two different TCP/IP exterior routing protocols. The first is the Border Gateway Protocol (BGP), the one used in modern TCP/IP. BGP is very important since it is used on the current Internet and other larger internetworks, so it is covered in considerable detail. The second is the Exterior Gateway Protocol (EGP). This is an obsolete protocol that was used for communication between non-core routers and the router core in the early Internet, and is described briefly for both completeness and historical interest.

Enhanced Interior Gateway Routing Protocol (EIGRP) 1 TCP/IP Border Gateway Protocol (BGP/BGP-4)