BGP Overview, History, Standards and Versions

As I describe briefly in the overview of routing protocol concepts, the way that routers were connected in the early Internet was quite different than it is today. The early Internet had a set of centralized routers functioning like a “core” autonomous system. These routers used the Gateway-to-Gateway Protocol for communication between them within the AS, and the aptly-named Exterior Gateway Protocol (EGP) to talk to routers outside the core.

Read more here.