BGP communities are extra attributes you can attach to an IP route carried by BGP. You can use communities to indicate which routes should be propagated or filtered (for example, the well-known NO_EXPORT community signifies that the route it’s attached to shall not be sent outside of the local AS), to influence route selection on remote routers or to trigger other BGP-dependent IOS features (for example, quality-of-service marking based on BGP).

Each BGP community is a 32-bit value. The best practice dictates that the top 16 bits should be the AS number of the network defining the community meaning and the bottom 16 bits are defined by the network administrator.

For example, if you use BGP communities to control QoS marking within your network, the top 16 bits should be your AS number. If you’re using a BGP community to mark backup BGP routes before they are sent to your ISP, you should use the BGP community defined by your ISP (and thus the top 16 bits are the ISP’s AS number).

Full article here