I have cisco 2821 router and I want to monitor an ISDN BRI interface, in case the line goes down. I would then like the router to shut down the ethernet interface automatically, thus taking it of the network. This would allow my server to failover to another Router
if you are using HSRP between RTR1 (with ISDN) and RTR2. you could have HSRP to track ISDN interface and make RTR2 active when ISDN fails, so no need to shut RTR1 Ethernet.
RTR1 & RTR2 needs to be on same VLAN.
There are other ways, but this is the most common one.