August 17, 2026

BGP communities are not decoration

BGP communities are sticky notes on a route. Well-known ones have RFCs. Private ones have a wiki page that may or may not match production. If you treat an unexplained 3356:123 as geography, you will write a novel that the transit did not sign.

I read communities in the looking glass after I already know origin and path. They tell me whether someone asked for no-export, a blackhole, a prepend, or a region preference. Then I check the transit’s current community cookbook — not a blog from 2014. The ASN explorer is for who they are; the community is for what they asked the path to do.

BGP communities I actually trust

no-export, no-advertise, no-peer. BLACKHOLE (65535:666) when the policy is documented. Large communities when the 16-bit ASN story ran out — RFC 8092. I do not trust a random 4-octet that a slide deck called “Europe” unless the transit’s looking glass still says so this quarter.

203.0.113.0/24
  3356 64500
  communities: 3356:123 3356:575 65535:666

Traffic engineering without folklore

If you want a prepend, prepend. If you want a region, use the community the transit publishes this year. Write the intended outcome in the change ticket, then confirm in the glass from the region you cared about. A community you set and never verified is interior design.

Notes are useful. Notes that nobody reads are litter. Keep a short internal list of the communities you set and the ones you honor. Delete the rest from muscle memory.