I have a customer who has a t1 plugged into a wic on a 1751 router for internet access. They have purchased a cable modem and this is plugged into an ethernet wic on the 1751 router for backup if the t1 goes down. How can I set up that if the t1 is unavailable users will go out the cable modem?
You could use what is known as floating default route. What you essentially do is to configure two default routes, one with it's default adm dist and the second with a slightly higher adm dist (ip route 0.0.0.0 0.0.0.0 Ethernet0/0 10). While both the interfaces are up, the first default route is used. However, as soon the interface that it is pointing to goes down, the second default routes comes into play and all traffic is routed using this. You could see http://www.cisco.com/en/US/tech/tk648/tk365/technologies_troubleshooting_procedures09186a00800cdf2e.shtml for a similar example.
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