Tried to post a similar question a few days ago but I don't think it went through. I have an ASA5510, also have a 2851 with 3 multilinked T1's, and a T1 from a separate provider on a 1700. What I would like to do is have the extra T1 as a failover for the multilinked T1's in the (unfortunately rather common) case that they go down.
From what I've read I could do this simply by having the extra T1 on a separate interface and adding an ip route 0.0.0.0 0.0.0.0 with that IP and a higher priority.
However, I also run a vpn tunnel to our datacenter, and I would like this to failover as well on the backup T1.
From what I've read it seems possible to do this with a loopback interface, though I have no clue how to set that up.
Any assistance is much appreciated!