7.3 Static Routes and Gateways of Last Resort
7.3.2 Using default and static routes in complicated networks
In this section, several scenarios are presented using static routes and gateways of last resort. These concepts are some of the most difficult for those new to the nuances of IP routing to understand. To truly understand routing, you must have a solid grasp on the fundamentals of local domains versus nonlocal domains, gateways of last resort, summarized routes, and redistribution of static routes and default metrics.

People who are fully conversant in IP routing can analyze any routing table and determine what will happen when a packet to any possible destination is encountered.

Using Static Routes

In the scenario shown in Figure , you can see what happens when an improper static route has been entered to fix a routing problem.

In Figure , the router A routing table has been edited to show what happens when an improper static route is added to a router. Router A has a static route to the major network of 168.71.0.0 via 168.71.6.3. Although this will enable router A to forward packets to 168.71.8.0, router A cannot reach 168.71.9.0. In fact, if router A received a packet for 168.71.9.1 on its Ethernet interface, it would forward it to router B. The proper way of doing this is shown in Figure .

In Figure , the routing table has been fixed to allow full connectivity to the subnets shown. Router A now has access to all local subnets of 168.71.0.0. It also has access to a gateway of last resort and a pointer via the summarized route that extends from 168.71.0.0 via 179.12.9.2 to other unknown subnets of 168.71.0.0 that can be found somewhere in the rest of the world.

In Figure , the routing table has been changed to show what would happen if IGRP were used in the local domain to allow full connectivity to the subnets shown and to allow static routes to all other subnets of 168.71.0.0 and other nonlocal domains. It is easier to use a dynamic protocol than static routes because they are easier to maintain.

Router A now has access to all local subnets of 168.71.0.0. Router A also has a gateway of last resort and a pointer via the summarized route that extends from 168.71.0.0 via 179.12.9.2 to other unknown subnets of 168.71.0.0.