Loop prevention in route reflectors
Removing the full mesh requirement in an IBGP topology introduces the potential for routing loops. RFC 1966 introduces two other BGP route reflector-specific attributes to prevent loops.
ORIGINATOR_ID is an optional non-transitive BGP attribute created by the first route reflector. This attribute sets the value to the router-id of the router that injected/advertised the route into the AS. If a router receives an NLRI with its router-id in the Originator attribute, the NLRI is discarded.
CLUSTER_LIST is a non-transitive BGP attribute updated by the route reflector. This attribute is appended by the route reflector with its cluster-id. By default this ID is the BGP router-id. The cluster-id can be set with BGP configuration command
bgp cluster-id
<cluster-id>
. If a router receives an NLRI with its cluster-id in the cluster list attribute, the NLRI is discarded.