BGP prefix filter list

William Herrin bill at
Thu May 30 17:48:21 UTC 2019

On Thu, May 30, 2019 at 10:43 AM Robert Blayzor <rblayzor.bulk at>
> On 5/30/19 12:54 PM, William Herrin wrote:
> > It's permissible to announce to your transits with a private AS which
> > they remove before passing the announcement to the wider Internet. As a
> > result, the announcement from each provider will have that provider's
> > origin AS when you see it even though it's actually from a downstream
> > multihomed customer.
> If you were a multi-homed customer the route would still originate from
> two different AS's, both of your upstream ISP's. If it's the same ISP,
> then that would not apply. But then again, if it were the same ISP they
> could filter the more specific and learn downstream customer routes
> either by IGP, filtering or tagging the /24 with no-export

Hi Robert,

Figured someone would stumble in to this one.

1. What happens to the packets when the /24 gets filtered from one source
(in favor of an aggregate) but not from the other?

2. In exchange for this liability, did you gain any capacity in your router?

Bill Herrin

William Herrin
bill at
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