Partial vs Full tables

William Herrin bill at
Wed Jun 10 17:31:47 UTC 2020

On Wed, Jun 10, 2020 at 9:43 AM William Herrin <bill at> wrote:
> The answer is "no," you're not running reverse-path filtering on a BGP
> speaker, not even in loose mode, because that's STUPID.

Sorry, it'd be pre-coffee if I drank coffee and I was overly harsh
here. Let me back up:

The most basic spoofing protection is: don't accept remote packets
pretending to be from my IP address.

Strict mode URPF extends this to networks: don't accept packets on
interfaces where I know for sure the source host isn't in that
direction. It works fine in network segments whose structure requires
routes to be perfectly symmetrical: on every interface, the packet for
every source can only have been from one particular next hop, the same
one that advertises acceptance of packets with that destination. The
use of BGP breaks the symmetry requirement so close to always that you
may as well think of it as always. Even with a single transit or a
partial table. Don't use strict mode URPF on BGP speakers.

Loose mode URPF is... broken. It was a valiant attempt to extend
reverse path filtering into networks with asymmetry but I've yet to
discover a use where there wasn't some faulty corner case. If you
think you want to use loose mode RPF, trust me: you've already passed
the point where any RPF was going to be helpful to you. Time to set it
aside and solve the problem a different way.

Bill Herrin

William Herrin
bill at

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