Exterior Gateway Protocol (EGP) is a dynamic routing protocol used to exchange routing information between different autonomous systems (ASes) on the Internet. An autonomous system is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. Unlike Interior Gateway Protocols (IGPs), which are used to manage routing within an AS, EGP is designed for routing between ASes.

Note that the name of this protocol is the same as the name of the grouping of exterior gateway protocols to which BGP belongs as well. This can be confusing if it is not clarified. The following diagram shows how EGP is a grouping of protocols, as well as the name of one of those protocols.


EGP, specifically, refers to a protocol that was historically used for this purpose but has since been largely superseded by more advanced protocols, in particular BGP. It was standardized in 1984 in RFC 904 and served as the primary protocol used for routing between autonomous systems in the early stages of the Internet.

Key characteristics of EGP include:

  1. Non-Interior Protocol: EGP is used for routing between autonomous systems, not within them. This is distinct from IGPs like RIP, OSPF, or EIGRP, which are used within an autonomous system.

  2. Policy-Based Routing: EGP allows different autonomous systems to apply their routing policies, determining how traffic is forwarded between them. This is crucial for the management of Internet traffic, allowing network administrators to control traffic flow based on policies rather than just shortest-path routing.

  3. Neighbor Acquisition: EGP uses a neighbor acquisition process to establish and maintain communication between neighboring autonomous systems. This process ensures that routing updates are exchanged only between designated neighbors, reducing the risk of routing loops.

  4. Limitations: The original EGP had several limitations, including its inability to support multiple routes to a destination and its lack of support for route metrics, making it less efficient than modern routing protocols. It was also designed when the Internet topology was more hierarchical, making it less suited for the complex, mesh-like structure of the current Internet.

The primary protocol used today for routing between autonomous systems is the Border Gateway Protocol (BGP). BGP addresses many of the limitations of EGP, offering more flexibility, scalability, and the ability to use multiple metrics for route selection. BGP's support for policy-based routing decisions and its ability to handle the Internet's complex, decentralized architecture have made it the de facto standard for exterior routing among autonomous systems.