This is going to be a tricky one!
So we're deploying c881 with GSM to our stores and the idea is that if the DSL wan link goes down the GSM radio interface will kick in.
The first obvious problem is that a router can only have one default gateway right? The second is the implementation of VRF and perhaps SAA. I've heard that VRF doesn't work well with NAT. I can see myself spending a lot of time on the phone with TAC so. Does anyone in here have any experience with a similar setup?
Thanks in advance.
A router can have several default gateways. In your situation, just as an example, you could define a static route pointing to the wireless interfaces
ip route 0.0.0.0 0.0.0.0 250
This default route has an administrative distance (AD) of 250 which makes it less preferred than routes (default and other) from other routing protocols (which all have an AD of 200 or less).
You would configure the 3G card to be used as dial-backup should the DSL fail. Typical dial-backup configurations will bring the 3G connection up, once the primary connection fails, and route traffic down the 3G card using a "backup" static route as mentioned above (called a "floating static"), or a dynamic routing protocol over the 3G link to the other end of the dial-up connection. See e.g. http://www.cisco.com/en/US/docs/routers/access/1800/1861/software/feature/guide/3ghwic.html#wp1237893
for some configuration examples.
I am not sure about the NAT and the VRF. Where do you use a VRF, and where NAT?