BGP prefix filter list

Mel Beckman mel at
Thu May 30 17:10:51 UTC 2019


Are your sure about your Error #2, where you say "Prefixes from the same AS are not required to have direct connectivity to each other and many do not."?

From BGP definitions:

The AS represents a connected group of one or more blocks of IP addresses, called IP prefixes, that have been assigned to that organization and provides a single routing policy to systems outside the AS.

“...a connected group..." implies that all the prefixes in an AS must have direct connectivity to each other (direct meaning within the IGP of the AS). I realize that some AS’s have hot backup facilities that they advertise with heavy prefixing, but in my experience, the backup facility must still be interconnected with the rest of the AS, because prefixing doesn’t guarantee no packets will its that border router.


On May 30, 2019, at 9:54 AM, William Herrin <bill at<mailto:bill at>> wrote:

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

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<mailto:bill at>

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