Donboy, I will indeed check out post on Expert's Exchange. I am a bit unclear on how my mail server now knows to keep using eth0 as the external interface, for example. Your suggestion sounds good. As for setting up my server as a router, that is exactly what I am attempting to do. Interestingly, I run squid on the server so even though the router setup is not yet working, I am still able to surf on the DSL connection from the windows boxes by setting the browser's proxy to the server's address. Or, don't use the proxy and surf on the cable connection.

TeddyMills: As for "AUTONOMOUS single tree", I will need to think about this more, but if I understand correctly (and maybe I do not) then I have accomplished the same thing as you are suggesting. My Router3 is essentially the other "side-by-side server".

I hate getting hit on the head with a power bar. Please explain the "AUTONOMOUS single tree". Is that what I will have once I get the server box functioning as a router? I think not, but maybe I am wrong.

Automatic failover: no, my setup does not automatically switch to the other ISP. My server needs to stay on the DSL line (where IP address is static and servers are allowed). As for browsing ... windoze boxes will be on the cable connection normally. If cable goes down, I switch router1's gateway from router3 to eth1 and then I'm surfing on the DSL line. (... at least I think it's that easy). Or, use my proxy server setup mentioned earlier.

you shouldn't be able to ping eth0 from the window's boxes, as eth0 is on a different network than the other pc's. you'd need to set up iptables to relay networks, but there's no need to since you'll be able to access the sever via eth1

Yeah, but if I set the server up as a router then I should be able to ping both eth0 and router1, correct?

And you are correct, I can access the server via eth1 -- it currently works. However, I want also to be able to access the DSL line from the windows boxes, if the cable goes down. I realize this will require a manual configureation change -- namely changing router2's gateway from router3 (which allows access to cable) to eth1 (which should allow access to the DSL line via the server acting as a router, once that's working).

Don't ask why I am bothering to do all this. I might use this redundancy feature once per year. Mostly I'm just doing this for the challenge, learning, and because I am now obsessed, etc.

... and I do appreciate all the input ... a lot.

Update:

After thinking about this, I have concluded there is little or nothing to be gained by setting the server up as a router. By running the proxy server (squid) on the server, I have access to the DSL line from behind router2 (windoze boxes) when/if I want it.

I am done and happy.

Thanks for your help.
Apollo