History and Development
Overview of Operation
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:
- 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.
- 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.
- 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.
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.
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