What is EGP?

An EGP (Exterior Gateway Protocol) is a routing protocol used between autonomous systems; today that means BGP.

Routing protocols fall into two families. An IGP, such as OSPF or IS-IS, runs inside one organisation's network and is built to find the fastest path quickly, trusting every router involved. An exterior protocol runs between separately run networks, each its own autonomous system, so it has different priorities: scaling to the whole internet's routing table, preventing loops between networks, and letting each operator apply its own policy about which routes to accept and announce. BGP does this by carrying the AS path with every route instead of a simple metric.

The name is confusing because it is also the name of a specific, now obsolete protocol, the Exterior Gateway Protocol of the early 1980s, which connected networks to the early internet's core. It could only handle a simple tree-shaped topology and was replaced by BGP around the start of the 1990s. Its one lasting trace is in BGP itself: the ORIGIN attribute has a value e meaning "learnt via EGP", which you may still see in the codes legend of BGP output but will almost never see on a real route. So when someone says "the EGP" they usually mean BGP in the exterior role, and "EGP" on its own in a modern design document means the same. The BGP Fundamentals lesson explains how BGP fills that role between autonomous systems.