Case Study: Dial Backup
One of the problems you face when using dial backup in OSPF is where the router dials into versus where the area borders are. Figure 5-15 will be useful in seeing what the problems are and in considering some solutions.
You want Router F to dial up to some other router when Router E loses its connection to Router C. You can either dial into Router D, Router A, or Router B; but the question is which one?
The immediate choice would be to configure Router F to dial into Router D if the remote segment loses all connectivity through Router E, but this still leaves a single point of failure at Router A. The single point of failure could be solved by moving the link between Routers D and A so that the link runs between Routers ...