BGP Cluster ID
A BGP speaker is a BGP enabled router. By default BGP speakers do not advertise iBGP-learned prefixes to iBGP peers - this is done to maintain loop prevention. RFC 4456 introduces the route reflection feature which removes the need of full mesh between iBGP speakers. When a route reflector reflects a prefix, it creates/modifies an optional non-transitive attribute called CLUSTER_LIST by adding its own cluster ID to it.
This attribute is used for loop prevention: when a router receives an update whose CLUSTER_LIST contains the router's own cluster ID, this update is discarded.
By default cluster ID is set to the BGP router ID value, but can be set to an arbitrary 32-bit value.