A communication system comprising: a plurality of switches (20a, 20b, 20c, 20d); and a control unit that controls each of the switches, said communications system being characterized in that the control unit includes a reply request transmitting means that transmits a reply request to the switch, a reply receiving means that receives a reply including information on ports provided at the switch, from the switch, a topology information creating means that creates topology information, based on the reply transmitted from each switch, the topology information including port interconnection information relative to ports provided at the control unit and each port of the switches, and a control channel determining means that determines a control channel for control messages, using a communication network that the switches use to transfer packets, based on the topology information; and the switch includes a transfer means that transfers the reply request from a port other than that through which the reply request has been received, when the reply request is received, and a reply transmitting means that returns the reply including information on the ports of the switch, the reply being transmitted through a path for the control unit, when the reply request is received; and the control unit transmits control messages to each switch and receives control messages from each switch, via the control channel that is determined using the communication network. a plurality of switches (20a, 20b, 20c, 20d); and a control unit that controls each of the switches, said communications system being characterized in that the control unit includes a reply request transmitting means that transmits a reply request to the switch, a reply receiving means that receives a reply including information on ports provided at the switch, from the switch, a topology information creating means that creates topology information, based on the reply transmitted from each switch, the topology information including port interconnection information relative to ports provided at the control unit and each port of the switches, and a control channel determining means that determines a control channel for control messages, using a communication network that the switches use to transfer packets, based on the topology information; and the switch includes a transfer means that transfers the reply request from a port other than that through which the reply request has been received, when the reply request is received, and a reply transmitting means that returns the reply including information on the ports of the switch, the reply being transmitted through a path for the control unit, when the reply request is received; and the control unit transmits control messages to each switch and receives control messages from each switch, via the control channel that is determined using the communication network. a transfer means that transfers the reply request from a port other than that through which the reply request has been received, when the reply request is received, and a reply transmitting means that returns the reply including information on the ports of the switch, the reply being transmitted through a path for the control unit, when the reply request is received; and the control unit transmits control messages to each switch and receives control messages from each switch, via the control channel that is determined using the communication network.