BGP prefix filter list

William Herrin bill at
Thu May 30 17:42:17 UTC 2019

> On Thu, May 30, 2019 at 10:11 AM Mel Beckman <mel at> wrote:
> > Are your sure about your Error #2, where you say "Prefixes from the
same AS are not required to have direct connectivity to each other and many
do not."?
> >
> > From BGP definitions:
> >
> > The AS represents a connected group of one or more blocks of IP
addresses, called IP prefixes, that have been assigned to that organization
and provides a single routing policy to systems outside the AS.

>From -what- BGP definitions? This one?

Lots of things get claimed in books and CS courses that are neither
reflected in the standards nor match universal practice. Heck, most
networking courses still teach class A, B and C... definitions which were
explicitly invalidated a quarter of a century ago.

Even where authors are knowledgeable, they're constrained to present
approximate explanations lest the common use get lost in the minutiae. When
you want to act on the knowledge in an unusual way, you do not have that
luxury. The experts in the IRTF Routing Research Group spent something like
6 years trying to find a way to filter the BGP RIB in the middle without
damaging the Internet. They came up with zip. A big zero. They all but
proved that it's impossible to build a routing protocol that aggregates
anything anywhere but at the edges while still obeying the most basic
policy constraints like not stealing transit. Forget getting BGP to do it,
they couldn't come up with an entirely new protocol that did better.

Bill Herrin

William Herrin
bill at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the NANOG mailing list