A first redundancy device and a second redundancy device connected to the first redundancy device are provided. A third switching device has a first link with a first port group of the first switching device and has no link with the second switching device. A fourth switching device has a second link with a first port group of the second switching device and has no link with the first switching device. Here, a communication between the first redundancy device and the second redundancy device is performed through the first link when the first link has no fault, and the communication is performed through the second link when the first link has a fault and the second link has no fault.