Networks ignoring prepends?

Wed Jan 24 15:42:01 UTC 2024

On Wed, Jan 24, 2024 at 5:23 AM Chris Adams <cma at cmadams.net> wrote:
> Once upon a time, William Herrin <bill at herrin.us> said:
> > On Tue, Jan 23, 2024 at 4:00 PM Chris Adams <cma at cmadams.net> wrote:
> > > Once upon a time, William Herrin <bill at herrin.us> said:
> > > > Nevertheless, in the protocol's design, the one expressed in the
> > > > RFC's, AS path length = distance.
> > >
> > > The RFC doesn't make any equivalence between AS path length and
> > > distance.  You are the one trying to make that equivalence,
> >
> > Respectfully Chris, you are mistaken.
> >
> > https://datatracker.ietf.org/doc/html/rfc4271#section-9.1.2.2
> >
> > "a) Remove from consideration all routes that are not tied for having
> > the smallest number of AS numbers present in their AS_PATH
> > attributes."
> >
> > So literally, the first thing BGP does when picking the best next hop
> > is to discard all but the routes with the shortest AS path.
>
> That's literally not the first thing - you skipped section 9.1.1.

Phase 1 is local pref. That's what 9.1.1 says. As implied by the word
"local," it's set locally by the local operator, not by the origin,
though many providers offer haphazard mechanisms that sometimes have
some impact if the origin doesn't mind playing whack-a-mole with BGP
communities.

Unless locally configured to selectively change the local pref off the
default, all routes have the same local pref. So it moves to phase 2
(section 9.1.2). This matches what I've been saying for the entire
thread: unless the operator intentionally makes the route worse, it
follows the shortest AS path. Per the RFC.

> It also literally says nothing about distance.

BGP is a distance-vector protocol. BGP's authors preferred different
terminology so they used different terminology. Nevertheless, BGP is a
distance-vector protocol and when you ask what it uses to determine
distance, the answer is the AS path length because all the other
criteria are policy functions not distance functions.

Want to go another few rounds with pedantry over word choice, or can
we leave it there?

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/