Exterior and Interior Gateway Protocols
In order to understand why BGP is designed the way it is, you first need to understand where it fits in the world of routing protocols. Routing protocols can be divided along several axes, the first being Interior Gateway Protocols (IGPs) versus Exterior Gateway Protocols (EGPs). The primary difference between EGPs and IGPs is the place in the network where they provide reachability information; that is, within an administrative routing domain (intradomain) or between administrative routing domains (interdomain).
Exactly what a routing domain is depends primarily on the context. In Intermediate System to Intermediate System (IS-IS) terminology, for instance, a routing domain is the area ...