Simple definition
A BGP community is an optional route tag used to apply routing policy across routers and networks.
Purpose and use
A BGP community labels a route so a policy can recognize it, for example by origin or an export instruction.
How does it work?
A standard community occupies 32 bits and is often displayed as ASN:value. Some values, including NO_EXPORT, have standardized meaning.
Example and verification
For a provider exchange, verify the expected value, whether it reaches the neighbor, and the action that its policy actually applies.
Points to watch
An operator-defined value has no universal meaning. Adding a label triggers no action unless a device interprets it.
In short
The community carries policy information; a filter or route-map determines how that information affects routing.