Networks ignoring prepends?

Wed Jan 24 06:12:33 UTC 2024

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.

It also says that BGP implementations are -allowed- to use other
selection criteria. And there are many situations where doing so is
well advised and improves the result. But AS path length is
unambiguously the default, off which a user has to move it.

Regards,
Bill Herrin

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