Patent ID: 8233383

Claim:
A network relay device that pairs with a specific network device to relay data in a network, the network relay device comprising: a control module that sends and receives control frame signals regularly to and from the specific network device, selects either of a master mode and a backup mode based on the control frame signals sent from the specific network device, and changes a working state according to a result of the selection; and a communication module that is under control of the control module to open a port and enable data transmission via a line linked to the port in the case of setting a master state to the working state, and to block off the port and disconnect the line linked to the port in the case of setting a backup state or another non-master state to the working state, the control module changing the working state among a first intermediate state, a second intermediate state, the backup state and the master state, wherein the first intermediate state is a working state to wait for a notification of a working state from the specific network device and to which the working state is transitioned when changing the working state from the backup state or the second intermediate state to the master state, and wherein the second intermediate state is a working state to wait for reception of the control frame signal and to which the working status is transitioned when the control frame signal cannot be received in the first intermediate state or the backup state, and the control module changing the working state from the backup state to the second intermediate state in the event of failure to receive the control frame signal for a preset time period, then selecting the working state from either the master state or the backup state upon reception of the control frame signal, changing the working state from the second intermediate state to the first intermediate state, in the event of the master state being selected, and changing the working state from the second intermediate state to the backup state in the event of the backup state being selected, the control module changing the working state from the first intermediate state to the second intermediate state in the event of failure to receive the control frame signal for a preset time period, then confirming whether the specific network device is in the backup state based on the control frame signal upon reception of the control frame signal, in the event it is confirmed that the specific network device is in the backup state, changing the working state from the first intermediate state to the master state, and the control module changing the working state from the second intermediate state to the master state, in response to an externally given transition instruction to the master state.