Database Availability Group (DAG), a high availability feature for Mailbox servers in Exchange 2013 requires a witness server to maintain a Quorum. Quorum is used by Windows Failover Clustering feature which uses voter count to sustain/function the cluster. In Exchange 2013, DAG can use two different modes of Quorum, Node and File Share Majority (used when DAG is configured with even number of nodes) or Node Majority (used when DAG is configured with odd number of nodes). You must configure DAG Witness Server in Exchange 2013 to make Mailbox server highly available.
Configure DAG Witness Server in Exchange 2013
You have to configure witness server even though you have odd number of nodes in DAG knowing the witness server won’t be used in this case. Witness server is only used when the cluster needs to maintain the quorum (vote counts). Whether witness server will be used or not, you must configure it. If the Exchange roles are separately installed then, Microsoft recommends to configure CAS server to become witness server as part of DAG configuration. If both roles are installed in same machine, then any other server that’s member of domain controller can be used as witness server. You can use domain controller as witness server but it is not recommended. Similarly, a DAG member can not be configured as witness server.
As shown in figure above, FILE server will be configured as witness server for Mailbox DAG.
Step 1: Make sure file server role is installed in the witness server.
Step 2: Now logon to File server with local Administrator account. Add Exchange Trusted Sub System group to the local Administrator group as shown below.
Step 3: Now create a shared folder. Here, I have created a shared folder called WitnessShare in C drive.
Step 5: Create a DAG. Now, go back to EAC and create a new DAG. You can now specify the witness directory (shared folder) as shown below.
This is how you can configure a witness server and create a DAG.