I\'ve just started working with Cassandra (datastax), version 2.1.3 and cqlsh version 5.0.1.
Cassandra starts up fine and the cluster is operational instantly.
Check for correct IP address in the cassandra.yaml file. Majority of times the error is due to incorrect IP address of your system also the username and password too.
After doing so initiate cqlsh by the command :-
cqlsh 10.31.79.1 -u cassandra -p cassandra