After the Diameter application is notified by the Diameter base that it is unable to deliver the message to the destination, the Diameter application can invoke a server failover procedure. A server failover procedure is enabled through configuration in the Diameter application policy (failure handling).
The server failover procedure on the Diameter application level provides the opportunity for one last retransmission of the original Diameter request messages, but this time with the destination-host AVP cleared in the retransmitted message. The T-bit in the retransmitted messages is set.
Clearing the destination-host AVP allows delivery of the application message to any server in the realm supporting that application. The retransmitted application request message is successfully processed on such server only if the applications servers are redundant and synchronized.