I use a 1841 router with two fastethernet for two different Lan and a serial connection for a Wan connection with another 1841 router. Is it possible to configure the router to use the serial for backup of one fastethernet
Since your ethernet interface wont go down until you physically disconnected the link connected onto it which will have an adverse effect in the routing table(if you are using normal static routes with floating route)you need to use a feature called object tracking to track the remote end and reroute the traffic through the serial interface.
You can verify the below link for achieving the same..
