Re: What's up with BGP communities?

[Tom Beecher via NANOG <nanog () lists nanog org>]

Many ASNs will publish some of their communities , along with their meaning
or usage. There are generally two types.

1. Information communities. "This route came from FOO". FOO could be
country, city, exchange, anything. You could use that information to make
your own decisions about a route via routing policy,
2. Action communities. "If you add community FOO, I will take action BAR."
This is a community you could add to an advertisement to a given ASN to
tell them to take some specific action. Don't announce it here, black hole
this, etc.

Not every community that is in use has a publicly published meaning. There
are tons of them that ASNs will use for their own purposes. It's usually
good hygiene NOT to send those out to the DFZ, but sometimes it happens. (
Or is unavoidable for reasons. )

Sometimes the meaning of those communities can be inferred, sometimes
asking the right person can give you an answer. But there will always be
some out there that you won't ever know what they mean unless you work for
that AS. You could ignore them, or strip them off when they enter your
network, whatever makes the most sense in your case.

On Thu, Jan 22, 2026 at 4:57 PM Ronan Pigott via NANOG <
nanog () lists nanog org> wrote:

> January 22, 2026 at 2:24 PM, "William Herrin" <bill () herrin us> wrote:
>
> > 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."
>
> Hi Bill,
>
> Thanks for your response. That's helpful, but I think I'm still confused.
>
> IIUC, a "route map" is a configuration local to the BGP router. So, an
> operator may make some decision about the meaning of a community and then
> attach it to routes advertised to their peers, but no peer can reasonably
> act on the community information without understanding the meaning intended
> by the owner, right? So if I operated a network and my BGP peer advertises
> routes that belong to a bunch of communities, how can I possibly learn the
> intended meaning of those communities to configure a sensible route map for
> my router?
>
> E.g. given "If community X, then I know the route came from that place",
> how
> did I learn "the route came from that place" is the meaning of "community
> X"?
> I gather this meaning is purely the decision of the network operator, so we
> have to communicate that somehow if I am ever to act on the community
> information. Otherwise, why bother to advertise the community at all?
>
> It appears bgp.tools has some knowledge of the community meanings, but I
> don't
> understand how they learned that, or how it works between real networks.
>
> Cheers,
>
> Ronan
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