BGP originator ID and cluster-list.
Originator ID: to indicate what router originated a reflected route.
The Originator ID is an optional non-transitive attribute. It is set by a route-reflector that reflects a route. A BGP speaking router should ignore routing information when the Originator ID value matches its own BGP identifier.
Cluster-list: a sequence of cluster-ID’s that represents the reflection path.
The cluster-list is an optional non-transitive value. Whenever a route-reflector reflects a route, it has to prepend its own cluster-ID value to the cluster-list. If the cluster-list is empty, it must create a new one. By using this attribute, a route-reflector can detect a loop. Whenever it sees that its own cluster-ID is already present in the cluster-list, the route should be ignored by the route-reflector.