With a tie on the Router ID, it falls to the minimum cluster list length to try break the deadlock.
The step uses the cluster list to locate the path that has travelled the least number of iBGP advertisement hops.
The cluster list is a non-transitive BGP attribute that is appended by a router reflector with it’s cluster ID.
Cluster IDs are used by route reflectors as an anti-loop mechanism to prevent loops.
This Cluster ID is only locally significant and not advertised between AS numbers.