If you are using Exchange 2013 with a DAG setup you are familiar with the PAM or Primary Active Manager, right? And you are more familiar with SAM – the Secondary Active Manager – aren’t you?
The Active Manager runs within MSExchangeRepl.exe and manages the database copy between the Nodes within the DAG. PAM is the instance which decides which copy of a DAG should be Active.
Be aware: Exchange itself is not a clustered application. It just uses the Windows Failover-Cluster technology to do the job.
Important while do a planned upgrade from Exchange to a new Cumulative Update: Note: If your DAG have more nodes, you need first upgrade all non PAM nodes and then upgrade PAM node.
Details about PAM/SAM and more can be found here: Active Manager
Your EXGuru – aka Peter Forster – aka Satschent Peter