[zeroimpl]

Routers know how to figure out what route to use for a given IP.

If they apply this algorithm to the source IP and find that the optimal route to the source is a different interface than it was received on, that’d potentially be a red flag. But if the optimal route to the source is the same as the optimal route to the destination, that’d be a huge red flag.

[toast0]

That really only works if you assume routing symmetry and a single optimal route.