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

adamv0025 at adamv0025 at
Wed Sep 9 13:34:59 UTC 2020

My advice to “someone who is setting up a new ISP and has a very little clue as where to start” would be just don’t and instead hire someone who’s well versed in this topic.

But I see what you mean, RFC7938 was a good food for thought. But at the same time I’m sceptical, for instance would it help if BCP38 was an RFC? 

Would be nice for instance if the community could put together a checklist of things to consider for ISPs (could be in no particular order) (and actually there are such lists albeit concentrated around security)   




From: Jeff Tantsura <jefftant.ietf at> 
Sent: Wednesday, September 9, 2020 9:52 AM
To: adamv0025 at
Cc: nanog at
Subject: Re: BGP Community - AS0 is de-facto "no-export-to" marker - Any ASN reserved to "export-only-to"?'


I don’t think, anyone has proposed to use ‘’reserved ASNs” as a BCP, example of “ab”use of ASN0 is a de-facto artifact (unfortunate one).

My goal would be to provide a viable source of information to someone who is setting up a new ISP and has a very little clue as where to start. Do’s and don’t’s wrt inter-domain communities use. 


I really enjoyed the difference RFC7938 (Use of BGP for Routing in Large-Scale Data Centers) made, literally 100s of companies have used it to educate themselves/ implemented their DC networking.




On Sep 9, 2020, at 10:04, adam via NANOG <nanog at <mailto:nanog at> > wrote:

I don’t agree with the use of reserved ASNs, let alone making it BCP, cause it defeats the whole purpose of the community structure.

Community is basically sending a message to an AS. If I want your specific AS to interpret the message I set it in format YOUR_ASN:<community value>, your AS in the first part of the community means that your rules of how to interpret the community value apply.

Turning AS#0 or any other reserved AS# into a “broadcast-AS#” in terms of communities (or any other attribute for that matter) just doesn’t sit right with me (what’s next? multicast-ASNs that we can subscribe to?).

All the examples in Robert’s draft or wide community RFC, all of them use an example AS# the community is addressed to (not some special reserved AS#).


Also should something like this become standard it needs to be properly standardized and implemented as a well-known community by most vendors (like RFCs defining the wide communities or addition to standard communities like no_export/no_advertise/…). This would also eliminate the adoption friction from operators rightly claiming “my AS my rules”.   





From: NANOG < at < at> > On Behalf Of Douglas Fischer via NANOG
Sent: Tuesday, September 8, 2020 4:56 PM
To: NANOG <nanog at <mailto:nanog at> >
Subject: BGP Community - AS0 is de-facto "no-export-to" marker - Any ASN reserved to "export-only-to"?'


Most of us have already used some BGP community policy to no-export some routes to some where.

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.



Douglas Fernando Fischer
Engº de Controle e Automação

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