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.