Using isdn as a backup link an ethernet interface

Is it possible to have an ISDN backing up an Ethernet interface ??

In theory, it might be possible to do that since (for example) EIGRP can handle multiple links with different path costs.

But in reality, ISDN is a WAN link while Ethernet is a LAN. I don't see how you would have them connected to the same network in real life, other than an experimental lab.