Modern TCP/IP routing architecture groups routers into autonomous systems (ASes) that are independently controlled by different organizations and companies. The routing protocols used to facilitate the exchange of routing information between routers within an AS are called interior routing protocols (or historically, interior gateway protocols). Since most network administrators are responsible for routers within a particular organization, these are the routing protocols you are most likely to deal with unless you become a major Internet big-shot. J

One of the benefits of autonomous systems architecture is that the details of what happens within an AS are hidden from the rest of the internetwork. This means that there is no need for universal agreement on a single “language” for an internet as is the case for exterior routing protocols. As a network administrator for an AS, you are free to choose whatever interior routing protocol best suits your networks. The result of this is that there is no agreement on the use of a single TCP/IP interior routing protocol. There are several common ones in use today, though as is usually the case, some are more popular than others.

In this section I provide a description of six different protocols used for routing within autonomous systems in TCP/IP. The first two sections provide comprehensive descriptions of two of the most popular TCP/IP interior routing protocols: the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). The third section provides a more brief discussion of two historical interior routing protocols and two proprietary ones developed by networking leader Cisco Systems.