Generally, datacenter failures are either temporary or permanent. With a permanent failure, such as an event that has caused the permanent destruction of a primary datacenter, there’s no expectation that the primary datacenter will be activated. However, with a temporary failure (for example, an extended power loss or extensive but repairable damage), there’s an expectation that the primary datacenter will eventually be restored to full service.

The process of restoring service to a previously failed datacenter is referred to as a switchback. The steps used to perform a datacenter switchback are similar to the steps used to perform a datacenter switchover

It’s important that switchback not be performed until the infrastructure dependencies for Exchange have been reactivated, are functioning and stable, and have been validated. If these dependencies aren’t available or healthy, it’s likely that the switchback process will cause a longer than necessary outage, and the process could fail altogether.

Click here for Exchange Datacenter Switchover

Datacenter Switchback

1. The first step is to put the Mailbox servers in the restored primary datacenter into a started state and incorporate them into the DAG.
   • Turn on All Exchange Server in the Main Site and Ensure Cluster service has been started on Primary Datacenter Server
     

     Net start clussvc

   • Reset DAG ( From Primary mailbox)
     

     Start-DatabaseAvailabilityGroup –identity “DAG Name” –ActiveDirectorySite “Main AD Site Name” Set-DatabaseAvailabilityGroup –Identity “DAG Name”

2. Do the AD Replication and Wait for 15-30 Minutes. This is very important.
   •  During that make sure most of the databases is healthy in the main site

After a majority of the databases are in a healthy state in the primary datacenter

1. Reset the File Share Witness:

   • Set-DatabaseAvailabilityGroup –Identity “DAG Name” –WitnessServer FQDN-Main -WitnessDirectory “DAGFileShareWitnesses-Main”

2. Dismount Mailbox Database in secondary site:

   • Get-MailboxDatabase | Dismount-Database

     or

     DisMount-Database -Identity “DB Name”

3.  Activate in the primary datacenter

   • Set-ADServerSettings –PreferredServer “Maine Site DC”

   • Move-ActiveMailboxDatabase -Server “MBX-DR Site” -ActivateOnServer “MBX-Main Site”

4. Restart the cluster service in the DR Site

   • net stop clussvc

   • net start clussvc /forcequorum

5. Mount Database in primary data centre:

   • Get-MailboxDatabase “Main AD Site Name”| Mount-Database

     Or

     Mount-Database -Identity “DB Name”

6. Changing the DNS record for the URLs to point to the Client Access server or array in the primary datacenter.

   • Get-ClientAccessArray -Identity mail | fl site #(Exchange 2010)

   • Set-ClientAccessArray -Identity “mail(FQDN)” -Site “Main AD Site Name” #(Exchange 2010)

   • Switch Exchange DNS To point to HUB/CAS server IP in Main Site
     Mail.domaine.com
     Autodiscover.domaine.com
   • Move OAB to the DR site
   • Change the send connector source servers to Main Site
7. Block Automatic Activation on DR site

   • Suspend-MailboxDatabaseCopy -Identity “DB Name”\“MBX-DR Site” -ActivationOnly

Perform forest-wide Active Directory Replication