Add or Replace a Database Mirroring Witness (SQL Server Management Studio)
Applies to: 
SQL Server
If the database mirroring endpoints use Windows Authentication,, you can use SQL Server Management Studio to add or replace a witness. Adding a witness in Management Studio also changes the operating mode to high-safety mode with automatic failover.
Note
We strongly recommend that the witness reside on a separate computer from either of the partners. The service account used by the witness must be in the same domain as the service accounts used by the principal and mirror server instances, or it must be in a trusted domain.
To Add or Replace a Witness
After connecting to the principal server instance, in Object Explorer, click the server name to expand the server tree.
Expand Databases, and select the principal database of the session to which you are adding or replacing a witness.
Right-click the database, select Tasks, and then click Mirror. This opens the Mirroring page of the Database Properties dialog box.
Click Configure Security.
If the Configure Database Mirroring Security Wizard welcome screen appears, click Next.
In the Include Witness Server dialog box, click Yes, and then click Next.
In the Choose Servers to Configure dialog box, the Witness server instance check box is automatically checked. Click Next.
On the Principal Server Instance dialog box, keep the existing port and endpoint. Click Next.
On the Witness Server Instance dialog box, click Connect.
In the Connect to Server dialog box, specify the witness server instance in the Server name field, and use Windows Authentication (the default). Click Connect.
Once a connection is established, the listener port and database mirroring endpoint of the witness server instance are displayed on the Witness Server Instance dialog box. Click Next.
The Service Accounts dialog box contains fields for the domain service accounts of the principal, mirror, and witness server instances.
If the server instances all use the same service account, leave the fields blank.
If the witness server instance uses a different service account from either of the partners, fill in the Principal, Mirror, and Witness fields with the account name:
DOMAINNAME \ username
The domain name must be in upper case.
Click Next.
On the Complete the Wizard summary screen, optionally, verify the witness configuration, and then click Finish.
On finishing, the wizard returns you to the Database Properties dialog box where the server network address of the witness now appears in Witness field. Also, High-safety mode with automatic failover (synchronous), which is required with a witness, is automatically selected.
To enable the witness and change the session to high-safety mode with automatic failover, Click OK.
See Also
Database Mirroring Witness
Database Mirroring (SQL Server)
Database Properties (Mirroring Page)
Establish a Database Mirroring Session Using Windows Authentication (SQL Server Management Studio)
Database Mirroring Witness