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Chapter Summary
Routing protocols are the tools that figure out how to get IP packets from their source to their destination as quickly and efficiently as possible. Generally speaking, routing protocols run on network devices known as routers and communicate continuously with neighboring routers. That communication enables routers to piece together a comprehensive view of the larger network in which they function. Part of that view includes identifying IP network addresses reachable via the network, as well as all the ways to reach those networks. Some logic must be applied to figure out which of those alternative routes to each destination network is best.Each router stores a collection of best routes in a routing table that forwards IP packets efficiently to their destination.You can calculate a route's efficiency two main ways. The first is known as distance-vector routing. Distance-vector routing protocols count the number of routers in each path and choose that with the fewest number of routers between it and the destination network. The second way to calculate routes is kind of the opposite approach. Instead of focusing on the routers, this approach focuses on data that measures the quality of the links interconnecting the routers. This approach is known as link-state routing. A third approach exists, but doesn't calculate routes. Instead, it enables you to develop rules for forwarding IP packets in the aggregate rather than making decisions individually. Policy-based routing, as this approach is known, is only useful for networks that require massive scalability, such as the Internet.
