BGP prefix filter list

Mel Beckman mel at
Thu May 30 17:58:35 UTC 2019


Come on now. The definition of an autonomous system is well established in RFC1930, which is still Best Current Practice:

An AS is a connected group of one or more IP prefixes run by one
      or more network operators which has a SINGLE and CLEARLY DEFINED
      routing policy.

This is not an “approximate explanation“. It’s a standard, as strong as any standard that exists for the Internet.

How is your statement "Prefixes from the same AS are not required to have direct connectivity to each other and many do not” supported by the published standard? :-)


On May 30, 2019, at 10:42 AM, William Herrin <bill at<mailto:bill at>> wrote:

> On Thu, May 30, 2019 at 10:11 AM Mel Beckman <mel at<mailto: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<mailto:bill at>
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