7.3 Static Routes and Gateways of Last Resort
7.3.1
Improper use of static routes and gateways of last resort
This section reviews the material just presented. The following section presents a scenario in which an improper use of static routes and gateways of last resort can cause connectivity failures. The main figure shows the network diagram that will be used for this scenario.

Two types of IP network addresses are under considered in this section:

  • Nonlocal domain addresses (a major network address that the router does not have a physical connection to) ---  In  the main figure, router B does not have an explicit route to 168.72.5.0. However, it does have a gateway of last resort of 10.0.0.0. This gateway of last resort is created with a static route to 10.0.0.0 that points at 168.71.6.1 and configures the ip default-network 10.0.0.0 command. This route is sufficient for router B to forward packets to 167.72.5.0 because router B has no local context for 168.72.0.0 (it is not a local domain).
  • Local domain addresses (a major network address that the router does have a physical connection to) --- Router A must have either an explicit route back to 168.71.8.0 or a static route pointing to 168.71.8.0 with a next hop of 168.71.6.3. A gateway of last resort will not allow router A to reach 168.71.8.0 because router A is connected to the major net 168.71.0.0. This connection makes 168.71.8.0 part of the router A local domain. Routers expect to have explicit routes for all subnets in their local domains. It is possible to get around this by installing a static route to the major network alone. However, this can lead to problems when configured improperly.