Failover and Fallback feature is not enable by default. We have to enable this feature and has to configure the Primary Peer and Secondary Peer for an Entity.
Failover: When a Primary Peer is not up (Primary Peer: Where the message was intended to reach or the actual destination) then the Pending messages will be forwarded to the Secondary Peer. This procedure is called Failover.
Fallback: After failover if the Primary Peer is again got up so the messages flow should again change towards the Primary peer. This procedure is called Fallback.
Note: It is only possible when the Destination Peer AVP is not present in the message.