BGP prefix filter list

Robert Blayzor rblayzor.bulk at
Thu May 30 17:43:40 UTC 2019

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.


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