Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 136
Static versus Dynamic Routing

The last topic of discussion is the ability of routing protocols to accept information for their tables from two sources: the network or a user.

Although the RIP protocol allows for automatic updates for routing tables, manual entries are still allowed and are known as static entries. These entries must be entered manually. A default route is a static entry. An endstation that is configured with a default router is said to have a static route entry. Static routes can be configured to be included or not included in a dynamic update. Static routes refer to the process of manually placing an entry in the table. For any given router, the network administrator may update that table with a static route. Static routes override dynamic routes.

Static tables have many disadvantages. First, as discussed earlier, static tables are not meant for large networks that incur many changes such as growth. As the topology changes, all the tables must be manually reconfigured. Second, in the case of router failure, the tables have no way of updating themselves. The route will be disabled, but no alternative route is put in place. Cisco employs a concept called a floating static route that allows for this. Dynamic tables overcome the disadvantages of static entries.

The primary advantage that a static entry may have is for security, for static tables can be configured not to broadcast their routes to other routers. In this way, users can customize their routers to become participants on the network without their network identity being broadcast to other routers on the network. Static routes

Static entries are also used in various IP topologies. For example, in a hub-and-spoke topology where a business has a centralized corporate office and many remote offices (such as a bank), there really is no reason to fully enable RIP at the branch offices. Why not turn RIP supply (the ability to broadcast routes but not listen for any) at the remote branch and add a default route, in the remote branch router, pointing to the upstream router located at the corporate office. In this way, the upstream router dynamically learns about all the remote offices (and learns when they go away) and the branch office has one simple entry in its table (besides its attached subnets): a default route to the upstream router. Since there is no other path besides that one link to the upstream router, the router simply passes on any packets that it receives from its attached workstation on the network. This reduces the amount of memory and processor power needed at the remote branch, enabling a cheaper router to be placed out there.

This is also an example of why OSPF need not be turned on for a complete network. There is no reason whatsoever to run OSPF out at the branch offices; there are plenty of reasons to run it at the corporate offices. OSPF will simply pull in the RIP networks as external networks (but this could possibly build large routing tables).