DOMAIN SERVERS

THE GOAL IS TO HAVE REDUNDANCY DOMAIN SERVER..I HAVE GOT PRIMARY AND SECONDARY(BACKUP DOMAIN CONTROLLER)DOMAIN CONTROLLER..CLIENTS ARE CONNECTED TO THE PRIMARY DOMAIN CONTROLLER AND CAN DETECT BOTH PRIMARY AS WELL AS SECONDARY..IF PRIMARY DOMAIN CONTROLLER IS DISCONNECTED CLIENTS CANNOT DETECT SECONDARY DOMAIN..
WHAT AM I SUPPOSED TO DO?
PLEASE POST YOUR ANSWER