automatic back up for ethernet WAN

Hi all,

I have two WANs, which are both ethernet based interface connected to my cisco router. I need to use one ethernet WAN as the main link, and the other as automatic fall back. As it is an ethernet interface, it is not going to go down, even when the physical layer(WAN) is down. Can any one help out with a configuration that would automatically switch to the back up ethernet line, in case the primary link goes down.