Case Study: Dial Backup with a Single Router
While BGP is capable of conditional advertisement, most other routing protocols aren't. You need to find a way to advertise backup links only under certain conditions, particularly if they are dial-on-demand, such as ISDN.
Figure 3-17 depicts a common scenario; Router B has a point-to-point link through Serial 0 to Router A, and a dial-on-demand backup link through BRI 0 to Router C. The routing protocol is EIGRP, and Router B is only receiving 0.0.0.0/0 advertised from Router A (the default route). The network administrator doesn't want the ISDN link up unless the serial link fails.
Figure 3-17. ISDN Dial-on-Demand
There are two possibilities for bringing the ISDN dial-on-demand link up when ...