"reachable-via any" means you're only going to drop the packet if you
don't have *ANY* route back to them.
What's a route? An IP RIB instance? A BGP Loc-RIB instance? An IGP LSDB
IP prefix entry? A BGP Adj-RIB-In instance?
I think you mean "if you don't have *ANY* **FIB** entry for the
If I peer with two large providers on the same router and both
have prefix D.1 behind them and advertise the prefix to me, it's
likely that only one of those two paths is going to make it into
the BGP Loc-RIB (and subsequently, the IP RIB then FIB).
If I use ANY FIB entry as proof that it's a valid source then
that only addresses RFC1918ish space and only suggest that I
first need to generate an invalid BGP route for the prefix, then
spoof the packets. This doesn't fix spoofing with global IP
If I use only entries that occur in the RIB and associate them
with the receiving interface and receive a packet with an SA of
D.1 from the peer whose path wasn't installed in the BGP
Loc-RIB then I'll drop it. (And there's nothing broken with
this configuration -- it's why we have routers with 1 million
BGP paths but only 150K routes/fib entries, as I'm sure you
If you're going to do source address validation then you need
to associated all potential valid paths for a given prefix with
the associated ingress interface, else it's mostly useless.