GossipingPropertyFileSnitch 

Automatically updates all nodes using gossip when adding new nodes and is recommended for production.

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.

The cassandra-rackdc.properties file defines the default datacenter and rack used by this snitch:
Note: datacenter and rack names are case-sensitive.
dc=DC1
rack=RAC1

To save bandwidth, add the prefer_local=true option. This option tells Cassandra to use the local IP address when communication is not across different datacenters.

To allow migration from the PropertyFileSnitch, the GossipingPropertyFileSnitch uses the cassandra-topology.properties file when present.

The location of the cassandra-rackdc.properties file depends on the type of installation:
Package installations /etc/cassandra/cassandra-rackdc.properties
Tarball installations install_location/conf/cassandra-rackdc.properties