I'm trying to understand why source-address RTBH needs to be coupled with configuring uRPF on the edge ports.
I've read RFC5635 and other vendor papers on this topics but I can't figure out the reason, perhaps I'm missing something obvious.
Why would S/RTBH not work without uRPF? The trigger router would advertise the source-address that needs to be blocked with a specific "evil" community and based on this community the edge routers would install a next-hop of discard. So the source-address would be blocked anyway, so why the need for uRPF?