BGP communities
Labels you attach to BGP routes to group them and signal policy — like tags that routes carry with them between routers and ASes. They let one network tell another how a route should be treated, without having to agree in advance on every single prefix.
Format: a 32-bit value, usually written "AS:value" (e.g. 65000:100); "large communities" exist for 32-bit AS numbers. Use case: signal policy to your operator — many ISPs publish communities that customers can tag their advertisements with, e.g. "set local-pref to backup", "prepend 2 times toward Europe", or "don't advertise this outside your network". Well-known communities: NO_EXPORT (don't advertise further out of the AS), NO_ADVERTISE, and RTBH/blackhole communities (ask the operator to null-route an attacked IP). Win: scalable, automated policy signaling instead of manual per-prefix agreements. An indispensable tool in operator BGP and DDoS mitigation. Related to BGP MED and BGP route reflector.