A type of routing protocol used to distribute routing information between different autonomous systems in large internetworks based on the TCP/IP protocol.
The Internet is one example of an Exterior Gateway Protocol (EGP). EGPs specify how networks within an autonomous system are advertised to routers outside the given autonomous system.
EGPs thus facilitate the exchange of inter-autonomous-system routing information between different autonomous systems, independent of whether these autonomous systems employ the same Interior Gateway Protocols (IGPs) within their networks.
The EGP was the original routing protocol developed for communicating routing information between autonomous systems on the Internet. It is no longer used because of its poor support for multipath networking environments, and the Border Gateway Protocol (BGP) has replaced it.
The term “Exterior Gateway Protocol” now refers both to the particular protocol itself and to the class of protocols it describes.
Figure A - Intradomain and Interdomain Routing
Figure A shows two routing domains, D1 and D2, and an overlapping (shaded) region depicting the interconnection between border routers from each domain. In more current routing terminology, a routing domain also is referred to as an autonomous system. An autonomous system is an independent routing domain under the control of a single administrative authority.
An exterior gateway protocol provides the capability for sharing routing information between the two domains.