Network Operators and smurf

Wait; all traffic is coming in one interface. The CEF thing will have no
effect if the spoofed source address is a real network.

However, if it is a completely bogus source address ( or somesuch),
then yes, it does make it a bit easier to filter.

If the spoofer is dialed up to YOUR network, and spoofs the address of
someone else out on the net, then YOUR router should find that the source
interface is not in the list of routes for that address, and discard it.

If the spoofer is attacking YOU, then that means the network the spoofer
is attached to is NOT blocking him by this method, but SHOULD.