How do I create failover in my Router?

I currently have Cisco 2600XM Routers with two ethernet interfaces – one connected to my DSL modem and the other to the LAN on the two sites of my point-to-point link. I desire to add a microwave link as an online backup by adding an additional ethernet port ot each router on which I hope to connect the Radio equipment.

My question is how do I configure the router to automatically reroute packets through the microwave link when the primary link (DSL) goes down or fails?

Any lead will be much appreciated.

myke