Exterior Gateway Protocol (EGP)
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Exterior Gateway Protocol (EGP)

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).

History and Development

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 This draft document was superseded in April 1984 by RFC 904, Exterior Gateway Protocol Formal Specification. Like GGP, EGP is now considered obsolete, having been replaced by the Border Gateway Protocol (BGP).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.

Overview of Operation

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.

Routing Information Exchange Process

The actual process of exchanging routing information involves several steps to discover neighbors and then set up and maintain communications. Briefly, the steps are:

  1. Neighbor Acquisition: Each router attempts to establish a connection to each of its neighboring routers by sending Neighbor Acquisition Request messages. A neighbor hearing a request can respond with a Neighbor Acquisition Confirm to say that it recognized the request and wishes to connect. It may reject the acquisition by replying with a Neighbor Acquisition Refuse message. For an EGP connection to be established between a pair of neighbors, each must first successfully acquire the other with a Confirm message.
 
  1. Neighbor Reachability: After acquiring a neighbor, a router checks to make sure the neighbor is reachable and functioning properly on a regular basis. This is done by sending an EGP Hello message to each neighbor for which a connection has been established. The neighbor replies with an I Heard You (IHU) message. These messages are somewhat analogous to the BGP Keepalive message but are used in matched pairs.
 
  1. Network Reachability Update: A router sends Poll messages on a regular basis to each of its neighbors. The neighbor responds with an Update message, which contains details about the networks that it is able to reach. This information is used to update the routing tables of the device that sent the Poll.
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A neighbor can decide to terminate a connection (called neighbor de-acquisition) by sending a Cease message; the neighbor responds with a Cease-ack (acknowledge) message.

As we know, the primary function of the early Internet was to connect peripheral routers or groups of routers to the Internet core. It was therefore designed under the assumption that the internetwork was connected as a hierarchical tree, with the core as the root. EGP was not designed to handle an arbitrary topology of autonomous systems like BGP, and cannot guarantee the absence of routing loops if such loops exist in the interconnection of neighboring routers. This is part of why BGP needed to be developed as the Internet moved to a more arbitrary structure of autonomous system connections; it has now entirely replaced EGP.

Error Reporting

An Error message is also defined, which is similar to the BGP Notification message in role and structure. It may be sent by a neighbor in response to receipt of an EGP message either when the message itself has a problem (such as a bad message length or unrecognized data in a field) or to indicate a problem in how the message is being used (such as receipt of Hello or Poll messages at a rate deemed excessive). Unlike the BGP Notification message, an EGP router does not necessarily close the connection when sending an Error message.

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