BGP community
A BGP community, defined in RFC 1997, is an optional 32-bit value a router can attach to a BGP announcement as a tag, conventionally written AS:VALUE, that signals routing policy intent to the networks receiving that announcement -- prefer this route, de-prioritize it, don't re-announce it further, or route it to a null interface -- without the two networks needing to coordinate a bespoke prefix-list filter for every case. A community is carried as an optional transitive path attribute, meaning it can propagate across multiple networks along with the route itself, so an operator several hops away from where a community was set can still match on it and act accordingly, provided every network in between preserves it. Because the original 32-bit format splits evenly into two 16-bit halves, it ran out of room to name autonomous systems once 32-bit ASNs became common, which RFC 8092 addresses with Large BGP Communities: a 96-bit value split into three 32-bit fields, large enough to hold a full 32-bit ASN. Communities are also the mechanism behind remote-triggered black holing, where a network operator tags a route with a community its upstream has published specifically to mean "drop all traffic to this prefix," letting a DDoS target signal its own upstream to null-route an attack without a phone call.
By AdminKit team, Netbase JSC · Updated