BGP Community - AS0 is de-facto "no-export-to" marker - Any ASN reserved to "export-only-to"?'

Chriztoffer Hansen ch at
Tue Sep 8 18:44:47 UTC 2020


On Tue, 8 Sep 2020 at 17:55, Douglas Fischer via NANOG <nanog at> wrote:
> Most of us have already used some BGP community policy to no-export some routes to somewhere.
> On the majority of IXPs, and most of the Transit Providers, the very common community tell to route-servers and routers "Please do no-export these routes to that ASN" is:
>  -> 0:<TargetASN>
> So we could say that this is a de-facto standard.
> But the Policy equivalent to "Please, export these routes only to that ASN" is very varied on all the IXPs or Transit Providers.
> With that said, now comes some questions:
> 1 - Beyond being a de-facto standard, there is any RFC, Public Policy, or something like that, that would define 0:<TargetASN> as "no-export-to" standard?
> 2 - What about reserving some 16-bits ASN to use <ExpOnlyTo>:<TargetASN> as "export-only-to" standard?
> 2.1 - Is important to be 16 bits, because with (RT) extended communities, any ASN on the planet could be the target of that policy.
> 2.2 - Would be interesting some mnemonic number like 1000 / 10000 or so.

Please see:

If you use large communities (yes, I know the standard is NOT 100%
unilaterally supported by all vendors just yet).

Using the combination of RS${ASN}:0:0 (Don't announce to any peer) and
RS${ASN}:1:${PEERAS} (Advertise to PEERAS) you can do what you are
asking for. Announcing routes to only select peers.

Most major IXP's will support most of the mentioned large
communities. For ISP's in general. It's thou a different story that is
not mine to speak about.

Using 2-byte communities in today's age of explosive "assignment" of
4-byte ASN's is similar to the price-hike of IPv4 space. In the long
term. Standard BGP communities and IPv4 will not be worth the required
effort/investment (unless you want to "cripple" yourself from the
get-go). And IPv6 and Large BGP Communities is "slowly" gaining traction
as the way to go.



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