Exterior Gateway Protocol (EGP)

Exterior Gateway Protocol (EGP) is used to exchange net-reachability information between Internet gateways belonging to the same or different autonomous systems.

EGP was developed by Bolt, Beranek and Newman in the early 1980s. It was first described in RFC 827 and formally specified in RFC 904 (1984).

Applications of Exterior Gateway Protocol (EGP):
During the early days of the Internet, EGP was frequently used by research institutes, universities, government agencies and private organizations, to interconnect autonomous systems but was replaced by Border Gateway Protocol (BGP).

How it works?
EGP was originally designed to communicate reachability to and from the Advanced Research Projects Agency Network (ARPANET) core routers. Information was passed from individual source nodes in distinct Internet administrative domains called autonomous systems (ASs) up to the core routers, which passed the information through the backbone until it could be passed down to the destination network within another AS.

Unlike most other protocols, EGP is focused on network reachability and does not consider any metrics to calculate the best path.



EGP has three major functions:

  1. Establish a set of neighbours
  2. Check status of neighbours(if they are alive/reachable)
  3. Inform neighbours the networks that reachable within their AS’s

Message formats:

  1. Neighbor Acquisition: To establish/de-establish neighbourship
  2. Neighbor Reachability: To check if neighbors are alive
  3. Poll: To ensure reachability of a particular network
  4. Routing Update: To share network reachability information
  5. Error: To inform the error occurred

Advantages and Disadvantages:
EGP was the first exterior gateway protocol that gained widespread acceptance in the Internet. The routing table is stable with minimal changes as the protocol does not react to issues within the Autonomous system.

EGP is a simple reachability protocol, and, unlike modern distance-vector and path-vector protocols, it is limited to tree-like topologies and does not support multipath networking environments, making it less efficient.

Since this routing protocol is designed to be centrally controlled, it reduces the scalability which is a major drawback in today’s fast growing Internet.

Another reason is that no central authority controls the commercialized Internet. The Internet is composed of many equal networks. In a distributed architecture, the autonomous systems require routing protocols, both interior and exterior, that can make intelligent routing choices. Because of this, EGP is no longer popular.

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