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Cisco 831 and multiple WANs?

This Cisco 831 is confusing me.

It has 3 ethernet but 4 fast ethernet interfaces. I've discovered that the 4 fast interfaces are not actually configurable like I'd expect, but are done through the configuration of e0 e1 and e2.

e0 is the WAN, okay.

e2 is a DMZ for FE 4. okay.

But here's what I'm wanting to do with this, and not sure how.

I'd like this router to simply segment two networks, let's say the LAN is and the rest of the network that it should talk to is connected via the WAN port (e0) configured at

That might be easily enough done using the WAN port and maybe e1 (Fa1, Fa2, Fa3) as the LAN.

However, what I'd like is to somehow create a way for this network to survive if the WAN side is cutoff. If becomes unreachable, I'd like to be able to have someone either swap a cable or plug in a cable somewhere else to go use the backup cable modem.

Any idea if this is possible with this router? Suggestions on config examples that might be similar?


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