BGP prefix filter list

Mel Beckman mel at
Fri May 31 01:18:55 UTC 2019

No, that's not the situation being discussed. As I've pointed out, a multi homed AS without an IGP connecting all prefixes is non-compliant with the BGP definition of an AS. Your Tokyo/DC example is additionally non-compliant because it doesn't have a single routing policy. It has two policies. That this may work in certain circumstances doesn't make it compliant with the standard.

I can stop a car by throwing out a boat anchor, but that doesn't comply with DOT standards for braking :)

From: Valdis Kletnieks <valdis at> on behalf of Valdis Klētnieks <valdis.kletnieks at>
Sent: Thursday, May 30, 2019 5:58:34 PM
To: Mel Beckman
Cc: Thomas Bellman; nanog at
Subject: Re: BGP prefix filter list

On Fri, 31 May 2019 00:10:42 -0000, Mel Beckman said:
> What are you talking about? Do you use multi homed BGP? If so, I???d expect you
> to know that an organization with multiple sites having their own Internet
> still uses a single AS. They have IGP paths to route traffic between sites
> (e.g., by using dedicated circuits).

The situation being discussed is an organization with multiple sites that *don't*
have a behind-the-scenes dedicated circuit, tunnel, or other interconnect.

For example, XYZ Corp has a POP in Tokyo announcing a /16 to their provider
there, and a POP in DC announcing a different /16 to their North American
provider, using the same ASN for both - but traffic between the two /16s
traverses the commodity Internet.

Or they advertise the same /16 and pray to the anycast gods. :)

(Actually, that's OK too, as long as both Tokyo and DC also announce a second
route (possibly a more-specific, or different address space) for their interconnect
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