Re: What's up with BGP communities?

[Ronan Pigott via NANOG <nanog () lists nanog org>]

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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