If you've ever signed up for IP transit or peered with an upstream provider, you've probably been handed a long list of "BGP communities" and a vague pointer to "set them on your routes." This guide explains what they actually do, why they matter, and how to use them in production.
What Is a BGP Community?
A BGP community is a tag attached to a route announcement. The tag itself does nothing — it's a number. What matters is that your upstream's routers are configured to look for specific tags and act on them.
Communities are typically written as ASN:value — for example, 64500:100. Some networks use the newer "large communities" format (ASN:value:value) which gives more flexibility for 32-bit ASNs.
Think of communities as a small, structured language between you and your upstream. Without them, you'd have to email the NOC every time you wanted to change how a route is treated.
What You Can Actually Do With Them
1. Prepend (Make a Path Look Worse)
Prepending adds extra copies of your ASN to the AS-path so that a route looks longer to other networks, making it less preferred. Most upstreams expose communities like:
ASN:101— prepend once to a specific peerASN:102— prepend twiceASN:103— prepend three times
This is how you do basic outbound traffic engineering without your provider rewriting their config every time you want a knob turned.
2. Blackhole (Drop Traffic to a Specific IP)
If you're under DDoS attack and one IP is taking the brunt, you can announce that /32 with a blackhole community. Your upstream will null-route traffic to that IP at their edge, sparing the rest of your network.
This is the most important community to know during an incident. DDoS protection at scale uses blackholing as one tool among several.
3. No-Export and Selective Advertisement
Sometimes you want a route to stay inside your upstream's network and not be advertised further. The well-known community NO_EXPORT does exactly that. Selective advertisement communities let you say "advertise this to peers but not to other transits" or "advertise only to customers."
4. Region or POP Scoping
Larger networks expose communities that scope your route to a continent, country, or specific POP. For example, "advertise this prefix only in Europe" or "do not advertise in Asia." Useful for anycast services and for keeping CDN traffic local.
Standard vs Large Communities
Standard BGP communities are 32 bits, usually expressed as two 16-bit numbers. That works fine for old 16-bit ASNs but breaks down for the 32-bit ASNs the RIRs hand out today.
Large communities (RFC 8092) are 96 bits, expressed as three 32-bit numbers. Modern networks expose both. If you have a 32-bit ASN, prefer large communities — they let your full ASN appear in the tag without truncation tricks.
How To Use Communities In Practice
- Get the list. Every transit provider publishes a community guide. Read it once before you need it.
- Test in a maintenance window. Apply the community on a non-critical prefix first and watch the BGP table on a looking glass to confirm the behavior.
- Document what you set. Future-you will need to know why prefix X has community Y attached.
- Have a rollback. Communities are easy to set and easy to forget. Keep a list and review it quarterly.
Common Mistakes
- Prepending too aggressively. Three or four prepends is usually the practical limit. Beyond that, some networks will already deprefer or filter your route, so adding more doesn't help.
- Forgetting blackholes are still there. A blackhole during an attack is a tactical move. Remove it once the attack ends or that IP stays dark forever.
- Mixing standard and large communities inconsistently. Decide on one format per provider and stick with it.
- Assuming all upstreams behave the same. Community semantics are per-provider.
ASN:666is a common blackhole convention but not universal — read the docs.
What Noded Exposes
Noded customers running their own ASN over our IP transit get a documented community list covering prepending, blackholing, region scoping, and selective advertisement. We also expose route-server peering for customers who want to test BGP behavior before committing to a transit session.
If you need IP transit, colocation with BGP, or help bringing up your own ASN, talk to us.
FAQ
Do I need my own ASN to use BGP communities?
To set communities on routes you announce, yes — you need an ASN and a BGP session with your upstream. To benefit from communities your upstream sets on routes coming to you, no.
What's the difference between local-pref and communities?
Local-pref is a value internal to your network that decides which path you prefer for outbound traffic. Communities are tags you exchange with other networks to influence their behavior. They often work together: a community received from a peer might be mapped to a local-pref by your edge router.
Are communities standardized?
The format and a handful of well-known values (NO_EXPORT, NO_ADVERTISE) are standardized. The meaning of provider-specific communities is not — every network defines its own.
Can I see what communities are attached to a route?
Yes — looking glasses, public route servers, and your own router's BGP table will show communities. Filtering routes by community is also a standard operation in modern routers.
How do I learn more?
Read your transit provider's community documentation, RFC 1997 (standard communities), and RFC 8092 (large communities). For hands-on, peer with a route server or set up a lab with two virtual routers and a transit emulator.