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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThe Exterior Gateway Protocol (EGP) was an early Internet protocol that let neighboring gateways in different autonomous systems (ASes) tell each other which networks they could reach. It did not calculate routes. Each autonomous system ran its own internal routing method, and EGP carried reachability information across the boundary between them. The name is also used loosely for any protocol that routes between autonomous systems, so the term needs some care.
What EGP Did
EGP handled three jobs between neighboring gateways: establishing a relationship with a neighbor, checking that the neighbor was still reachable, and exchanging information about which networks each side could reach. The formal specification, D. L. Mills’s RFC 904 (April 1984), describes these as neighbor acquisition, neighbor-reachability monitoring, and reachability updates.
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Acquiring and monitoring neighbors
Two neighboring gateways first had to agree to exchange routing information. Once they had, each side kept track of the other’s health with periodic Hello messages and I-Heard-You replies. A neighbor that stopped answering could be detected and treated as unreachable.
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Exchanging reachability updates
Updates were solicited, not simply broadcast. A gateway sent a Poll command, and the neighbor answered with an Update listing the networks it could reach. This request-response pattern kept the exchange under the control of the polling side.
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Why EGP Was Not a Routing Algorithm
The original design split responsibility in two. An autonomous system chose its own interior routing algorithm to move traffic inside its network. EGP handled only the exchange of reachability between exterior neighbors. Eric C. Rosen, author of RFC 827 (October 1982) while at Bolt Beranek and Newman Inc., put it plainly: “The Exterior Gateway Protocol is NOT a routing algorithm.”
This distinction matters when reading older networking material. A reader who expects EGP to compute best paths across the Internet will misread how the protocol was meant to work. It told a gateway which destinations were reachable through a neighbor; it did not, by itself, choose among competing paths.
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Design Limits
EGP was built for a specific topology, and its limits follow from that choice.
Tree-shaped topology and routing loops
The original design expected autonomous systems to be connected in a tree with no cycles. RFC 827 acknowledges that EGP did not carry enough information to prevent routing loops if a cycle existed in that topology. Where the real Internet developed redundant links, this assumption became a weakness rather than a convenience.
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Third-party reachability and the two-level hierarchy
EGP also restricted how far reachability information could travel. RFC 1812 (Frank Baker, editor, June 1995) records that the protocol prevented routers from advertising third-party EGP information. In practice this limited EGP to a two-level hierarchy: a gateway could describe the networks it reached directly, but not pass along what it had learned about networks beyond its neighbors.
Three Meanings of “Exterior Gateway Protocol”
The phrase refers to three different things, and confusing them is the most common error. The table separates them.
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| Usage | What it refers to | Key reference |
|---|---|---|
| The Exterior Gateway Protocol (EGP), or “EGP” | The specific historical protocol for exchanging reachability between neighboring gateways, with the fixed design limits described above | RFC 827 (October 1982) for purpose and architecture; RFC 904 (April 1984) for the formal specification |
| Exterior gateway protocols | A generic category: any protocol that routes between autonomous systems | RFC 1812 (June 1995) uses the broader inter-AS routing concept |
| BGP-4 | A different inter-AS routing protocol, not a second name for EGP | RFC 1812 describes BGP-4 as an inter-AS routing protocol; it lists the historical EGP in an appendix of protocols it does not recommend for the modern Internet’s more complex conditions |
When you write about the subject, reserve “EGP” for the historical protocol, use “exterior gateway protocol” in lowercase for the category, and name BGP-4 explicitly when you mean the modern protocol. Calling BGP-4 “EGP” will mislead readers about how the Internet’s inter-AS routing actually works.
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What the Sources Do and Do Not Establish
The RFCs establish EGP’s purpose, mechanics, and limits. They describe it as a historical protocol and place BGP-4 as the inter-AS protocol that replaced its role in the RFC 1812 framework. They do not provide current deployment statistics, and they do not give a feature-by-feature operational comparison between EGP and BGP-4. Any claim about how widely EGP is used today would need separate, dated evidence that this article does not rely on.
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For the definition itself, the evidence is solid: EGP exchanged reachability between neighboring gateways in different autonomous systems, it did not compute routes, and its tree-shaped design limited loop prevention and third-party propagation.
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