Snitches 

A snitch determines which datacenters and racks nodes belong to.

A snitch determines which datacenters and racks nodes belong to. They inform Cassandra about the network topology so that requests are routed efficiently and allows Cassandra to distribute replicas by grouping machines into datacenters and racks. Specifically, the replication strategy places the replicas based on the information provided by the new snitch. All nodes must return to the same rack and datacenter. Cassandra does its best not to have more than one replica on the same rack (which is not necessarily a physical location).

Note: If you change snitches, you may need to perform additional steps because the snitch affects where replicas are placed. See Switching snitches.