A communication device for use in a network relay system including the communication device and at least one other communication device, each of the communication devices (1000-1, 1000-2) comprising: a transfer processing unit configured to transfer an input packet according to routing information of the network relay system; and a control unit configured to be set in operational or in standby, the control unit including a processing unit configured to learn the routing information and output the routing information to a repeater of the control unit, wherein: when the control unit is set in operational, the repeater of the control unit is configured to: receive the routing information from the processing unit, set the routing information for the transfer processing unit of the subject communication device, and transmit the routing information to the repeater of the other communication device, and when the control unit is set in standby, the repeater of the control unit is configured to: receive the routing information from the repeater of the other communication device, and set the routing information for the transfer processing unit of the subject communication device; wherein the transfer processing unit and a transfer processing unit of the other communication device are configured to realize a non-blocking communication with each other. a transfer processing unit configured to transfer an input packet according to routing information of the network relay system; and a control unit configured to be set in operational or in standby, the control unit including a processing unit configured to learn the routing information and output the routing information to a repeater of the control unit, wherein: when the control unit is set in operational, the repeater of the control unit is configured to: receive the routing information from the processing unit, set the routing information for the transfer processing unit of the subject communication device, and transmit the routing information to the repeater of the other communication device, and when the control unit is set in standby, the repeater of the control unit is configured to: receive the routing information from the repeater of the other communication device, and set the routing information for the transfer processing unit of the subject communication device; wherein the transfer processing unit and a transfer processing unit of the other communication device are configured to realize a non-blocking communication with each other. when the control unit is set in operational, the repeater of the control unit is configured to: receive the routing information from the processing unit, set the routing information for the transfer processing unit of the subject communication device, and transmit the routing information to the repeater of the other communication device, and receive the routing information from the processing unit, set the routing information for the transfer processing unit of the subject communication device, and transmit the routing information to the repeater of the other communication device, and when the control unit is set in standby, the repeater of the control unit is configured to: receive the routing information from the repeater of the other communication device, and set the routing information for the transfer processing unit of the subject communication device; receive the routing information from the repeater of the other communication device, and set the routing information for the transfer processing unit of the subject communication device; wherein the transfer processing unit and a transfer processing unit of the other communication device are configured to realize a non-blocking communication with each other.