It determines by a snitch which datacenters and racks nodes belong to. They inform Cassandra about the network topology so that requests are routed efficiently and allow Cassandra to distribute replicas by grouping machines into data centers and racks. Specifically, the replication strategy places the replicas based on the information provided by the new snitch. All nodes must return to the data center and the same rack. Cassandra does its best not to have more than one replica or extra replica on the same rack or identical rack.