In a multi-chassis link-aggregation device, each of first and second switch devices is provided with a relay condition, which defines prohibition of a predetermined relay path. If there is no link failure in each multi-chassis link-aggregation device, each of the first and second switch devices prohibits forwarding of a frame, which has been received by a bridge port, from each multi-chassis link-aggregation device. On the other hand, if a link failure is present in a second port (second multi-chassis link-aggregation device) of the first and second switch devices permits forwarding of the frame, which has been received by the bridge port, from the second port. In this case, if a destination port of the frame received by a first port includes the second multi-chassis link-aggregation device, the first switch device adds a port identifier representing the received port to the frame and forwards that toward the second switch device.