BGP communities
Prepending is the blunt instrument everyone reaches for. Communities are how you actually steer traffic.
What a community is
A tag attached to a BGP announcement. It carries no meaning in itself — the receiving network decides what to do with it, and publishes a list of what it supports.
So the first step is always: ask your upstream for their community list. Ours is available on request; open a ticket when you set up the session.
The well-known ones
- NO_EXPORT (65535:65281)
- Accept this prefix but do not pass it on. Useful for a more specific route you want your upstream to use internally without announcing it to the world.
- NO_ADVERTISE (65535:65282)
- Do not pass it to any peer at all.
- BLACKHOLE (65535:666)
- Discard traffic for this prefix at the edge. The standard way to ask for remotely triggered blackholing — see null routes and blackholing.
What provider communities give you
Most transit providers publish communities in the shape
ASN:action:target that let you:
- Prepend once, twice or three times to one specific peer rather than to everyone
- Not announce to a particular peer or exchange at all
- Not announce in a particular region or continent
- Set local preference inside their network
That is the real argument over prepending: prepending is global and crude, and every network on the internet sees it. A community reaches only the network you are asking.
A worked example
Traffic from one carrier takes a poor path to you and you would rather it came via the other. Prepending your prefix globally pushes everyone away, including networks that had a good path. A community that prepends to that one peer leaves the rest untouched.
Same for a regional problem: suppress the announcement in one region and let the rest of the internet keep the short path.
Check what actually happened
Changes take effect in seconds, and then you want to see them from outside. Use a public looking glass or a route collector to confirm the prefix is seen the way you intended, from more than one vantage point.
Communities are stripped by some networks and ignored by others, so always verify rather than assume. And remember the AS path you are influencing is only half the story — see reading a traceroute for why return paths matter.
With us
You need your own AS number and a session before any of this applies — see running BGP with us and multihoming. Once the session is up, ask for the community list and we will tell you exactly which values do what.
And if a path is bad and you are not running BGP yourself, just tell us. The routing is ours, and moving your traffic onto a better path is often something we can do from our side — see routing problems.
Still stuck? Mail support@novogara.com — an engineer answers, at any hour. Back to the knowledge base