BGP prefix filter list

William Herrin bill at
Thu May 30 16:54:14 UTC 2019

On Thu, May 30, 2019 at 8:30 AM Robert Blayzor <rblayzor.bulk at>

> On 5/24/19 2:22 PM, William Herrin wrote:
> > Get it? I announce the /24 via both so that you can reach me when there
> > is a problem with one or the other. If you drop the /24, you break the
> > Internet when my connection to CenturyLink is inoperable. Good job!
> It would be dropped only if the origin-as was the same. Your AS and your
> carriers aggregate announcement would be from two different origin AS.
> At least that's the gist of it...

Hi Robert,

Error #1: section 4.

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

Error #2: An AS is an informative handle, not a route. In routing research
parlance, an identifier not a locator. Prefixes from the same AS are not
required to have direct connectivity to each other and many do not. The
origin AS could solve this by disaggregating the announcement and sending
no covering route, but that's exactly what you DON'T want them to do.

Bill Herrin

William Herrin
bill at
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