Dynamic routing protocols are generally classified as an exterior gateway protocol or an interior gateway protocol (IGP). An exterior protocol carries routing information between two independent administrative entities, such as two corporations or two universities. Each of these entities maintains an independent network infrastructure and uses an EGP to communicate routing information to the other. Today, the most common exterior protocol is the Border Gateway Protocol (BGP). It is the primary exterior protocol used between networks connected to the Internet, and was designed specifically for such purposes.
In contrast, an interior protocol is used within a single administrative domain, or among closely cooperating groups. In contrast to the exterior protocols, IGPs tend to be simpler and to require less overhead in a router. Their primary drawback is that they can't scale to extremely large networks. The most common interior protocols in IP networks are the Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and the Enhanced Interior Gateway Routing Protocol (EIGRP). The first two are open standards adopted or developed by the Internet community, while the third is a proprietary protocol designed by Cisco Systems for use on their routers.
While it is possible to use an interior protocol as an exterior protocol, and vice versa, it is seldom a good idea. Exterior protocols are designed to scale to the largest of networks, but their inherent complexity and overhead can quickly overwhelm a small or medium-sized network. On the other hand, while interior protocols are fairly simple and have little inherent overhead, they don't scale well to larger networks. Because of the difference in focus between interior and exterior protocols, I will not discuss exterior protocols in this chapter. Instead, I will reserve our discussion of exterior protocols to later chapters where we will be discussing connections to the world outside of your organization.
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