What is BGP communities and route filtering?

BGP communities are optional, transitive path attributes that tag routes with additional information, enabling flexible policy application. A community is a 32-bit value written as AS:value. Well-known communities: NO_EXPORT (do not advertise to eBGP peers), NO_ADVERTISE (do not advertise to any peer), LOCAL_AS (do not advertise outside the local confederation). Custom communities: ISPs use them to signal route preferences, triggering upstream policies. For example, attaching community 65000:100 might tell an ISP to set lower local preference for that route. BGP route filtering techniques: prefix-lists (filter by specific prefixes/ranges), AS-path filter lists (filter by regex on AS_PATH), route-maps (set/match multiple attributes, most flexible), community-lists (match by community values). Proper BGP filtering is critical for ISPs and large networks — incorrect BGP announcements can cause major Internet routing incidents.