Cisco IOS Access Lists

Implementing route preferences

Earlier in this chapter, I talked about the strategy of implementing route preferences. In this section, I discuss and show examples of how to implement them. I start with the basic example of simply eliminating routes and move on to more complex examples of using offset-list statements to alter routing metrics and altering route administrative distances based on the sources of routing updates.

Eliminating undesired routes

The simplest way to prefer routes is to prevent the routes that are not preferred from being accepted by a router at all. Let’s look at Figure 4.13 for an example.

Figure 4-13. Ignoring routes through an unencrypted path

In this part of an intranet, Routers 1 and 2 send routing updates for the networks 172.18.0.0/16, 172.19.0.0/16, and 10.0.0.0/8 to Router 3 through Router 3’s serial interfaces 0 and 1. Both Router 1 and Router 2 have routes to network 10.0.0.0/8, Router 1 via Path A and Router 2 via Path B. In this intranet, the network administrators try to encrypt all of the serial links between networks wherever they can to safeguard their intranet from eavesdropping. They generally succeed except for the paths leading to network 172.18.0.0/16, such as Path A, which is in one of a number of countries where encryption is heavily controlled. The network administrators accept this fact by setting the following policy:

Only traffic ...