When you use SQL Server Always On Availability Groups, the Always On secondary replica may go into a disconnecting state. Additionally, the following error message is logged in the SQL Server error log:
A connection timeout has occurred while attempting to establish a connection to availability replica availability_replica_name with id availability_replica_id. Either a networking or firewall issue exists, or the endpoint address provided for the replica.
When you try to reestablish the connection, you may receive the following error message:
This secondary replica is not connected to the primary replica. The connected state is DISCONNECTED.
When this behavior occurs, the issue is not fixed until you restart SQL Server Services on the secondary replica. In a rare scenario, you may have to restart SQL Server Services on the primary replica to resume Always On data movement.Note
This problem might occur only on very powerful computers and when SQL Server is very busy. For example, in one scenario, this problem occurred on a very busy system with 24 cores.