You can configure Azure AD Connect to use a specific Domain Controller for password sync and password writeback. To do this, perform the following steps…
On the machine where Azure AD Connect is installed:
- Go to Start, look for and open "synchronization service".
- Go to "Connectors" and look for AD.
- Select it and click "properties".
- Go to "configure directory partitions".
- Select the checkbox "only use preferred domain controllers", and click on "configure" to add the DC(s).
- Finally restart the "Microsoft Azure AD Sync" service.