The NameNode that works and runs in the Hadoop cluster is often referred to as the Active NameNode. Passive NameNode also known as Standby NameNode is the similar to an active NameNode but it comes into action only when the active NameNode fails. Whenever the active NameNode fails, the passive NameNode or the standby NameNode replaces the active NameNode, to ensure that the Hadoop cluster is never without a NameNode.
