If the domain controller returns an error that warrants failing over to another domain controller, run the Active Directory Synchronization Failover task from the Task page.
For manual fail-over mode, the fail-over task requests
The name of the downed domain controller or resource
The name of the DC hostname to fail over
The timestamp of the collected HighestCommittedUSN value to use
You also must choose whether to restart Active Sync after the switch to a new domain controller is complete.