TCP/IP Exterior Gateway Protocol (EGP)
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Routing in the early Internet was done using a small number of centralized core routers that maintained complete information about network reachability on the Internet. They exchanged information using the historical interior routing protocol, the Gateway-to-Gateway Protocol (GGP). Around the periphery of this core were located other non-core routers, sometimes standalone and sometimes collected into groups. These exchanged network reachability information with the core routers using the first TCP/IP exterior routing protocol: the Exterior Gateway Protocol (EGP).
Like its interior routing counterpart GGP, EGP was developed by Internet pioneers Bolt, Beranek and Newman (BBN) in the early 1980s. It was first formally described in an Internet standard in RFC 827, Exterior Gateway Protocol (EGP), published in October 1982. This draft document was superseded in April 1984 by RFC 904, Exterior Gateway Protocol Formal Specification. Like GGP, EGP is now considered obsolete, having been replaced by the Border Gateway Protocol (BGP). However, also like GGP, it is an important part of the history of TCP/IP routing, so it is worth examining briefly.
EGP is responsible for communication of network reachability information between neighboring routers, which may or may not be in different autonomous systems. The operation of EGP is somewhat similar to that of BGP. Each EGP router maintains a database of information regarding what networks it can reach and how to reach them. It sends this information out on a regular basis to each router to which it is directly connected. Routers receive these messages and update their routing tables, and then use this new information to update other routers. Information about how to reach each network propagates across the entire internetwork.
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