What is a BGP community?

A BGP community is an optional transitive attribute attached to a route advertisement. It acts as a label that carries information or triggers an action on the neighbour's side. On its own it does not change routing, but operators agree on what each value means, so a single route can carry instructions such as 'do not advertise this route in a given region' or 'lower its local-pref'. Communities are a core BGP traffic-engineering tool because they let networks signal policy without manually reconfiguring filters on both ends of a session.

Standard, extended and large communities

The classic standard community (RFC 1997) is a 32-bit value written as an asn:value pair, for example 64500:100. It works well for 16-bit AS numbers but becomes awkward for 32-bit ASNs, because the AS number no longer fits in a 16-bit half of the field. Large communities (RFC 8092) were introduced for that reason: 96 bits in total, written as three 32-bit components asn:function:parameter. Extended communities (RFC 4360) also exist, used mainly in MPLS/VPN, such as Route Target and Route Origin.

• Standard community (RFC 1997): 32 bits, format asn:value, e.g. 64500:666.
• Large community (RFC 8092): 96 bits, format asn:function:parameter, fully supports 32-bit ASNs.
• Extended community (RFC 4360): 64 bits, used in MPLS VPNs as Route Target and Route Origin.
• Well-known communities such as NO_EXPORT, NO_ADVERTISE and NO_EXPORT_SUBCONFED have globally defined meaning.
• A route can carry several communities at once, and policies process each one independently.

Informational and action communities

In practice communities have two roles. Informational communities describe a route's origin, for example which city or internet exchange it was learned at, and whether it came from a customer, a peer or transit. Action communities are instructions that the receiving router interprets through its own route-maps and turns into a concrete operation, such as changing local-pref, prepending the AS path towards selected neighbours, blackholing a prefix, or selectively announcing or suppressing a route. The meaning of action values is not universal, so each operator publishes its own community table in PeeringDB or in its documentation.

How communities influence route selection

Communities are not an attribute in the BGP best-path algorithm, but they act as a trigger that influences that selection indirectly. A customer sends a route with a tag attached, the operator's policy matches that tag and sets an attribute that affects the decision, most often local-pref or the number of prepends. This lets a customer steer outbound and inbound traffic without access to the provider's network configuration. Communities are only carried onward when policy permits it, so a neighbour usually filters foreign communities and keeps only the ones it knows how to act on.

BGP communities in the AS202520 SkyPass network

At AS202520 SkyPass, communities are the standard way IP transit and peering customers steer their own traffic without contacting our NOC. We publish a set of action communities for changing local-pref, prepending the AS path towards selected upstreams and triggering blackholing as part of DDoS protection, along with informational communities that mark whether a route came from THINX, TPIX, WRIX, 1-IX or from transit. We support both standard and large communities, which simplifies integration for networks with 32-bit AS numbers using our PoPs in Warsaw and Wrocław.