BGP communities are tags you attach to routes. They signal intent or trigger actions.
Format: -bit value, typically (e.g., ).
Use cases:
Internal signaling. Tag routes from customers vs peers. Apply different LOCAL_PREF based on community.
External signaling. Tell upstream to prepend your route in certain regions.
Blackholing. Tag a route with a blackhole community. Upstream drops traffic to that prefix during DDoS.
Well-known: NO_EXPORT, NO_ADVERTISE