What is Exterior Gateway Protocol (EGP)?

The Exterior Gateway Protocol (EGP) is a defunct routing protocol that was used to exchange routing information between autonomous systems on the early Internet. EGP served as the first standardized exterior gateway protocol before being replaced by the Border Gateway Protocol (BGP) in the late 1980s.

EGP was defined in RFC 904, published in April 1984, and was widely used by research institutes, universities, government agencies, and commercial organizations to interconnect autonomous systems during the Internet's formative years.

How EGP Works

EGP operates through a simple mechanism of poll-response exchanges and periodic message polling to maintain neighbor reachability. The protocol consists of three main functions:

• Neighbor Acquisition − Establishing relationships with adjacent routers in different autonomous systems

• Neighbor Monitoring − Continuously checking the reachability of neighboring gateways

• Update Exchange − Sharing routing information through update messages

Unlike interior gateway protocols that operate within a single domain, EGP enabled communication between routers in different autonomous systems. The protocol was primarily used by ARPANET core routers to exchange reachability information with external networks.

Key Characteristics

• Reachability-only protocol − EGP only indicates whether networks are reachable, without using metrics for optimal path selection

• Tree topology requirement − Designed for hierarchical, tree-like network structures

• No loop prevention − Lacks mechanisms to detect or prevent routing loops

• Centralized architecture − Assumes a central core network (ARPANET) with peripheral networks

Advantages and Disadvantages

Why EGP Became Obsolete

EGP's limitations became apparent as the Internet evolved from a centrally managed research network to a distributed commercial infrastructure. The protocol's inability to handle complex topologies, prevent routing loops, or scale beyond tree structures made it unsuitable for the modern Internet's mesh-like architecture.

The Border Gateway Protocol (BGP) replaced EGP by providing path-vector routing, loop prevention, policy-based routing, and support for complex inter-domain relationships required by today's Internet.

Conclusion

EGP served as a crucial stepping stone in Internet routing history, enabling the first inter-domain routing capabilities. While obsolete today, EGP laid the foundation for modern exterior gateway protocols like BGP that power today's global Internet infrastructure.