Cassandra cluster configuration
Before adding new datacenter in production environment, the first step should be to prevent the client from connecting to the new datacenter and to ensure reads or writes does not query the new datacenter unless it is completely ready to serve since it may result into latency and timeout errors.

These are few point should follow while adding a new datacenter.


Make sure to change QUORUM consistency level to LOCAL_QUORUM and ONE to LOCAL_ONE so your driver wont hit new data center until it is not ready to serve.
you have to Alter Keyspaces to use network topology if not using (Do not mention new datacenter in Alter Keyspace command at this point).
if using PropertyFileSnitch, cassandra-topology.properties file should be updated with new node's IPs on all the existing nodes.


After adding datacenter successfully.
1.alter Keyspace to set replication factor in new datacenter and mention the number of replica as well.