You're thinking of "ping based routing" which Cisco implements using their "response time recorder" feature. However, unless you are working with static IP addresses, you will have another problem, which is that Cisco does not clear the NAT translation table unless the interface fails hard. If both paths need NAT but use different NATs (very common), your existing connections will fail and will not come back because the NAT for the dead interface will still be in effect.
Expect to receive suggestions to use a floating static route (only works if the SHDSL interface goes down, which it often does not, and never will if using an external DSL modem) and policy NAT (necessary to set up NAT translations correctly, but only affects the initial definition, not the NAT used once a NAT is defined, even if the interface changes). In other words, be sure to distinguish between suggestions based on what should work and suggestions based on what has actually worked in a real configuration. I've spent days in the lab getting something to work and the results are a hideous hack that only works on 8xx and 17xx routers (and that I've yet to find anyone willing to put into production, it's cheaper just to buy a Linksys or other SOHO router designed to do the job).
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
By the way, any lurkers with a 17xx who want to try it, please contact me by email. I'd love to find out if the syslog event processing hack works in the real world.
