hdfs data node disconnected from namenode

Question

I get from time to time the following errors in cloudera manager:

This DataNode is not connected to one or more of its NameNode(s). 

and

Answer 1

Hadoop uses specific ports to communicate between the DataNode and the NameNode. It could be that a firewall is blocking those specific ports. Check the default ports in the Cloudera WebSite and test the connectivity to the NameNode with specific ports.