Re: What's up with BGP communities?

[Bruce Wainer via NANOG <nanog () lists nanog org>]

I believe the bigger question here is:
How does bgp.tools know what the specific communities shown on the page,
e.g. https://bgp.tools/communities/13335 mean? I now have this question
also.
Some copied examples:
  Community Description
  13335:10106 PoP: bos01
  13335:10358 PoP: cwb03
  13335:10712 PoP: den04
  13335:10766 PoP: maa05
  13335:10920 PoP: zrh02

And here's some other copied examples from AS174 Cogent
https://bgp.tools/communities/174 :
  Community Description
  174:21000 Route is learned from NA (North America) non-customer
  174:21001 Route is NA internal or customer route
  174:21100 Route is learned from EU (Europe) non-customer
  174:21101 Route is an EU internal or customer route
  174:21200 Route is learned from AP (Asia Pacific) non-customer
  174:21201 Route is an AP internal or customer route
  174:22009 Italy
  174:22010 Netherlands
  174:22011 Portugal
  174:22012 United Kingdom
  174:22013 United States
  174:22014 Sweden

As Ronan stated, I'm not aware either of a way this is communicated by BGP
itself, so somehow this documentation must exist somewhere publicly - is it
only known by bgp.tools when they specifically have a route peering
connection with an AS and are given the community usage details?

-Bruce Wainer


On Thu, Jan 22, 2026 at 4:25 PM William Herrin via NANOG <
nanog () lists nanog org> wrote:

> On Thu, Jan 22, 2026 at 12:38 PM Ronan Pigott via NANOG
> <nanog () lists nanog org> wrote:
> > I want to know, where does this supplementary information about the
> communities come from, in this case the "PoP: blah" bits? I don't think it
> is communicated by BGP directly, so is there some standard out of band
> mechanism to describe the communities? Or am I just ignorant of this BGP
> feature?
>
> Hi Ronan,
>
> A  BGP community is an arbitrary label attached to a route. It means
> whatever the person who wrote the label wants it to mean.
>
> A BGP router has "route maps" which can use or set these "labels" on
> routes that it has received. For example, a route map may say, "If
> community X then make this route a lower priority than others." Or it
> might say, "If community Y, discard and don't use this route."
>
> Route maps can also set communities. For example, they can say: "If
> the route came from this place, set community X." Then someone else on
> a different router can say, "If community X then I know the route came
> from that place and I want to do something non-default with it, such
> as discarding it."
>
> Like so many things in routing, the use of the terminology "community"
> is weird and confusing. It's just an arbitrary label that means
> whatever the person who defined it wants it to mean.
>
> Make sense?
>
> Regards,
> Bill Herrin
>
>
> --
> For hire. https://bill.herrin.us/resume/
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