A software defined network (SDN) system, controller, and controlling method, where the SDN system includes at least one Nth level controller and at least two (N+1)th level controllers belonging to the Nth level controller, where the (N+1)th level controller is configured to receive a first message sent by a node belonging to the (N+1)th level controller, and when the first message is a cross-domain message according to status information of each node that is managed by the (N+1)th level controller, forward the first message to the Nth level controller to which the (N+1)th level controller belongs, and the Nth level controller receives the first message, and perform decision processing according to status information of the (N+1)th level controller that is managed by and belongs to the Nth level controller and status information of boundary nodes of the (N+1)th level controller belonging to the Nth level controller.