BGP prefix filter list

Mel Beckman mel at
Thu May 30 20:00:54 UTC 2019

Yes, my original quote wasn’t exactly word-for-word from the standard, but it was semantically identical.

I’m sure we can find corner cases, but it’s clear that the vast majority of BGP users are following the standard.  Anycast isn’t a violation of the standards because it’s defined in BGP as a single destination address having multiple routing paths to two or more endpoints.


On May 30, 2019, at 12:48 PM, William Herrin <bill at<mailto:bill at>> wrote:

> On Thu, May 30, 2019 at 10:58 AM Mel Beckman <mel at<mailto:mel at>> wrote:
> > Come on now. The definition of an autonomous system is well established in RFC1930, which is still Best Current Practice:
> >

Your quote wasn't from the RFC. Sorry, my google fu is only good enough to find your actual quote, not the similar one you didn't reference.

> > 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.

Interesting but it bears little resemblance to modern practice. Consider an anycast announcement, for example, where multiple distributed servers at isolated pops terminate the packet. Consider Amazon where both region-local unicast announcements and global anycast announcements all originate from AS 16509. Indeed the whole concept of traffic engineering rests on the premise that an AS' routing policy is NOT the same at every border.

Bill Herrin

William Herrin
bill at<mailto:bill at>
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