Autonomous systems

An autonomous system is a group of networks and gateways for which one administrative authority has responsibility.

Gateways are interior neighbors if they reside on the same autonomous system and exterior neighbors if they reside on different autonomous systems. Gateways that exchange routing information using EGP are said to be EGP peers or neighbors. Autonomous system gateways use EGP to provide access information to their EGP neighbors.

EGP allows an exterior gateway to ask another exterior gateway to agree to exchange access information, continually checks to ensure that its EGP neighbors are responding, and helps EGP neighbors to exchange access information by passing routing update messages.

EGP restricts exterior gateways by allowing them to advertise only those destination networks reachable entirely within that gateway's autonomous system. Thus, an exterior gateway using EGP passes along information to its EGP neighbors but does not advertise access information about its EGP neighbors outside its autonomous system.

EGP does not interpret any of the distance metrics that appear in routing update messages from other protocols. EGP uses the distance field to specify whether a path exists (a value of 255 means that the network is unreachable). The value cannot be used to compute the shorter of two routes unless those routes are both contained within a single autonomous system. Therefore, EGP cannot be used as a routing algorithm. As a result, there will be only one path from the exterior gateway to any network.

In contrast to the Routing Information Protocol (RIP), which can be used within an autonomous system of Internet networks that dynamically reconfigure routes, EGP routes are predetermined in the /etc/gated.conf file. EGP assumes that IP is the underlying protocol.