Routing and routing protocols are not the same thing. All Linux systems make routing decisions, but very few systems run routing protocols. Routing protocols perform two functions: They select the "best" route to a destination, and they communicate that route to other routers on the network. Thus a routing protocol is a technique for defining routes and for disseminating routes.
There are several different routing protocols, and considering the fact that Linux systems are not usually used as routers, a surprising number of these protocols are available for Linux systems. Protocols are differentiated by the metric they use for determining the best route and by the technique they use for distributing routing information.
Routing protocols are divided into interior protocols and exterior protocols. Interior protocols are used inside of a routing domain. Exterior protocols are used to exchange routing information between routing domains. Within your enterprise network, you use an interior routing protocol. It is possible that you will use an exterior routing protocol between your network and your ISP, but even that is unlikely. Most corporate networks are located within the ISP's routing domain, and therefore use an interior routing protocol to talk to the ISP. Following is an overview of three interior protocols—Routing Information Protocol (RIP), Routing Information Protocol Version 2 (RIPv2), and Open Shortest Path First (OSPF)—and one exterior protocol, Border Gateway Protocol (BGP).
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