cassandra-rackdc.properties file 

Configuration file for the GossipingPropertyFileSnitch, Ec2Snitch, and Ec2MultiRegionSnitch.

The GossipingPropertyFileSnitch, Ec2Snitch, and Ec2MultiRegionSnitch use the cassandra-rackdc.properties configuration file to determine which datacenters and racks nodes belong to. They inform the database about the network topology to route requests efficiently and distribute replicas evenly. Settings for this file depend on the type of snitch:

This topic also includes instructions for migrating from the PropertyFileSnitch to the GossipingPropertyFileSnitch.

GossipingPropertyFileSnitch 

This snitch is recommended for production. It uses rack and datacenter information for the local node defined in the cassandra-rackdc.properties file and propagates this information to other nodes via gossip.

To configure a node to use GossipingPropertyFileSnitch, edit the cassandra-rackdc.properties file as follows:

Migrating from the PropertyFileSnitch to the GossipingPropertyFileSnitch 

To allow migration from the PropertyFileSnitch, the GossipingPropertyFileSnitch uses the cassandra-topology.properties file when present. Delete the file after the migration is complete. For more information about migration, see Switching snitches.

Note: The GossipingPropertyFileSnitch always loads cassandra-topology.properties when that file is present. Remove the file from each node on any new cluster or any cluster migrated from the PropertyFileSnitch.
The location of the cassandra-rackdc.properties file depends on the type of installation:

Package installations
Installer-Services installations

/etc/dse/cassandra/cassandra-rackdc.properties

Tarball installations
Installer-No Services installations

installation_location/resources/cassandra/conf/cassandra-rackdc.properties