Loop Prevention in Route Reflectors
Removing the full mesh requirements in an IBGP topology introduces the potential for routing loops. When RFC 1966 was drafted, two other BGP route reflector specific attributes were added to prevent loops.
ORIGINATOR_ID, an optional nontransitive BGP attribute is created by the first route reflector and sets the value to the RID of the router that injected/advertised the route into the AS. If the ORIGINATOR_ID is already populated on an NLRI, it should not be overwritten.
If a router receives a NLRI with its RID in the Originator attribute, the NLRI is discarded.
CLUSTER_LIST, a nontransitive BGP attribute, is updated by the route reflector. This attribute is appended (not overwritten) by the route reflector with its cluster-id. By default this is the BGP identifier. The cluster-id can be set with the BGP configuration command bgp cluster-id cluster-id on IOS and IOS XR nodes. NX-OS devices use the command cluster-id cluster-id.
If a route reflector receives a NLRI with its cluster-id in the Cluster List attribute, the NLRI is discarded.
Example 1-10 provides sample output prefix output from a route that was reflected. Notice that the originator ID is the advertising router and that the cluster list contains two route-reflector IDs listed in the order of the last route reflector that advertised the route.
Example 1-10 Route Reflector Originator ID and Cluster List Attributes
RP/0/0/CPU0:R1-XR# show bgp ipv4 unicast 10.4.4.0/24
! Output omitted for brevity
Paths: (1 available, best #1)
Local
10.1.34.4 from 10.1.12.2 (192.168.4.4)
Origin IGP, metric 0, localpref 100, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 7
Originator: 192.168.4.4, Cluster list: 192.168.2.2, 192.168.3.3