BCP38 For BGP Customers

Tue Nov 8 20:01:49 UTC 2022

On Tue, Nov 8, 2022 at 8:40 AM Grant Taylor via NANOG <nanog at nanog.org> wrote:
> Maybe it's the lack of caffeine, but would someone please remind /
> enlighten me as to why uRPF is a bad idea on downstream interfaces?

Hi Grant,

Two words: asymmetric routing.

If the downstream network is architected in such a way that there's
more than one path in and out of their network then there's no way to
guarantee that any particular router believes the forward path to that
network goes to a particular next hop. It can currently map to any
next hop that goes in the direction of one of the valid paths. That
routing is completely independent of how next hops are selected in the
other direction. Packets can travel in one path and out another.

Reverse path filtering literally says don't accept a packet from
somewhere that isn't currently the next hop for that packet's source
address. When it's possible for the forward route to point a different
direction than the one from which valid packets are received, that is
the wrong thing to do. In fact, it breaks the network.

Useful automated reverse path filtering can ONLY be used when there is
exactly ONE valid path to which and from which packets can be
received. This is where strict mode uRPF actually works.

> N.B. Maybe I'm thinking more a varient of uRPF wherein if I have a route
> to the source from the interface in question.  Thus not uRPF-strict
> (active route) nor uRPF-loose (route on any interface).

Even if a particular router happens to have RIB entries for all the
valid paths to a host (a sketchy proposition at best), only one such
entry will be stored in the FIB where uRPF looks to make its filtering
decision.

As for loose mode, it's basically useless in a BCP38 discussion. Loose
mode only filters bogons. It doesn't prevent impersonation of any IP
address currently routed in the system and doesn't do anything at all
on a router with a default route.

Regards,
Bill Herrin

-- 
For hire. https://bill.herrin.us/resume/