EGP (Exterior Gateway Protocol) is an exterior routing protocol that is supported by the TCP/IP software. Exterior routing protocols exchange information between different ASs. The EGP gets the information about its own AS from the IGPs, such as RIP and OSPF. Usually, exterior routing protocols are used only when different companies or a commercial service are being connected.
The information EGP receives from the IGP must be explicitly configured. The exterior routing protocol shares only the information specified in the outgoing route filters. This is desirable because you generally want to limit the information exchanged between different ASs.
EGP exchanges routing information between ASs. As mentioned earlier, most companies and organizations usually group all their routers into one AS, although large companies might use more than one AS. In either case, one or more routers within each AS are chosen to use EGP to communicate with the outside world, usually over the Internet. These EGP routers are called exterior routers.
Routers that belong to the same AS are called interior neighbors, and routers that belong to different ASs are called exterior neighbors.
The first step that EGP takes to establish communication between exterior routers is to perform neighbor acquisition. During neighbor acquisition, one exterior router makes a request to another router to agree that they share reachability information. The next step is that the router continually tests whether its EGP neighbors are responding. Finally, reachability information is exchanged periodically between EGP neighbors through the use of Routing Update messages.
EGP has one major limitation: The distance indicated for a particular destination does not specify the cost to the destination. EGP reports only whether a destination is reachable. Because of this limitation, EGP can be used only in a tree-type network. All routing protocol domains must connect to the same central network, such as the Internet or ARPAnet. Consequently, EGP cannot support a looped topology.
Another limitation is that EGP can advertise only one route to a given network. Therefore, there can be no load sharing for traffic between any particular pair of machines. Also, the use of just one path for a network can result in packets taking nonoptimal paths during some traffic conditions on the network.
Finally, it is difficult for EGP to switch to an alternate route if the primary route fails.