Dynamic Routing Protocols

Chapter Description

This sample chapter from CCIE: Routing TCP/IP Volume I shows how routers can discover how to correctly switch packets to their respective destinations automatically and share that information with other routers via dynamic routing protocols.

Static or Dynamic Routing?

When reading (or being lectured about) all the glorious details of dynamic routing protocols, it's hard not to come away with the impression that dynamic routing is always better than static routing. It's important to keep in mind that the primary duty of a dynamic routing protocol is to automatically detect and adapt to topological changes in the internetwork. The price of this "automation" is paid in bandwidth and maybe queue space, in memory, and in processing time.

A frequent objection to static routing is that it is hard to administer. This criticism may be true of medium to large topologies with many alternative routes, but it is certainly not true of small internetworks with few or no alternative routes.

The internetwork in Figure 4.14 has a hub-and-spoke topology popular in smaller internetworks. If a spoke to any router breaks, is there another route for a dynamic routing protocol to choose? This internetwork is an ideal candidate for static routing. Configure one static route in the hub router for each spoke router and a single default route in each spoke router pointing to the hub, and the internetwork is ready to go. (Default routes are covered in Chapter 12, "Default Routes and On-Demand Routing.")

Figure 4.14 This hub-and-spoke internetwork is ideal for static routing.

When designing an internetwork, the simplest solution is almost always the best solution. It is good practice to choose a dynamic routing protocol only after determining that static routing is not a practical solution for the design at hand.

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