If you search on the Cisco web page, you'll probably find it under "reverse-path" or "reverse-path forwarding" - only works for singly homed customers.
Then it is merely similar, and not identical, to what I described. What I described included the notion that the source address must be in the set of all (not just the best) routes back. The theory on this is that even most asymmetric routing will have a lesser route that would be symmetric. Of course some really goofed assymetric routes do often appear and those may be trouble. Still, the place where ISPs should apply it is all the interfaces _other_ _than_ their border interfaces. They will have control over their own routing symmetry (hopefully) and the logic (at least as I describe it) applies to packets coming _in_ on an interface, not those going out. -- -- *-----------------------------* Phil Howard KA9WGN * -- -- | Inturnet, Inc. | Director of Internet Services | -- -- | Business Internet Solutions | eng at intur.net | -- -- *-----------------------------* philh at intur.net * --