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Networking Unlimited, Inc. developed an ISDN dial backup configuration that allowed any remote to call any core router as if all core routers were just different phone numbers on the same interface on the same router. While the configuration looked robust on paper, it turned out to be fairly weak in practice. The weakest link in the chain was the physical 56Kbps link at the spoke into the RBOC frame relay network. As a result, router maintenance at the core was severely constrained, as there was no way to predict when a spoke would lose its frame link. The challenge was to develop a configuration which would allow a spoke to sequentially call all the core routers until a successful ISDN link was established. The key to solving the client's dilemma was to develop a configuration which did not depend upon assigning addresses to the ISDN interfaces. The dial backup was modified so that it was driven by dial on demand routing. That way, when communications were lost the local frame interface remained up, dial backup would occur regardless. All that was required to verify that an ISDN interface was functioning correctly was to ping the "magic" target IP address. The new scheme has been an unqualified success. The network is much more robust, ISDN testing has been automated, and core maintenance is much simpler.
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