Functionally, routing protocols are divided into two types: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). Interior gateway protocols are used on routers within the same autonomous network, the same administrative domain, or the same intranet. Exterior gateway protocols are used to route traffic between routers in different autonomous systems; for example, between Internet service providers (ISPs) or between a single corporate net and two or more ISPs. You might also use an exterior protocol on a single large network that is so widely distributed that it can't be managed as a single unit (for example, a corporate network that spans several continents).
Put simply, internal protocols are intended for routing within an organization, and external protocols are intended for routing between unrelated organizations.
Interior gateway protocols, such as RIP, IGRP, EIGRP, and OSPF, are easier to configure and are designed to handle routing on a smaller scale than exterior gateway protocols. If you have a network with three routers and one connection to the Internet, RIP and a few static routes are all you need. Large networks, or networks with multihomed Internet connections (i.e., more than one ISP) might require a more sophisticated interior protocol, such as EIGRP or OSPF, and may force you to use an exterior protocol for routing to the Internet or between your own autonomous systems.
BGP is the only exterior gateway protocol that is widely used. EGPs are much more complicated than IGPs because they handle more routing information while performing better route summarization. BGP comes at a price in configuration, processing, and the size of the routing table. When configuring BGP for Internet use, you may need a router with sufficient memory to support a full Internet routing table, which contains over 70,000 routes.