Transfer device, communication system, and roundabout path detecting method

A communication system comprising:

• a gateway that relays a frame related to communication between an internal device that is installed in a network that includes a plurality of unit networks and an external device that is installed outside the network;
• and
• a transfer device that is installed in a unit network in which the gateway is installed, wherein
• the transfer device includes
• a sending/receiving unit that sends/receives the frame to/from a transfer device that is installed in another unit network in the network,
• a determining unit that determines, based on destination address or source address in a packet in the frame sent/received by the sending/receiving unit, whether the packet is related to the communication between the internal device and the external device, and
• a notifying unit that notifies, when the determining unit determines that the packet is related to the communication between the internal device and the external device, the gateway that a transfer path for the packet is a roundabout path, and
• the gateway includes
• a switch instructing unit that instructs, when a notification that the transfer path for the packet is a roundabout path is received from the notifying unit, a transfer device that is a source of the packet to switch a gateway.

A roundabout path detecting method performed by a transfer device that is installed in a unit network that includes a gateway, the roundabout path detecting method comprising:

• sending/receiving a frame to/from a transfer device that is installed in another unit network in a network that includes the unit network;
• determining, based on destination address or source address in a packet in the frame, whether the packet is related to communication between an internal device that is installed in the network and an external device that is installed outside the network;
• and
• notifying, when it is determined at the determining that the packet is related to the communication between the internal device and the external device, the external device of an instruction to switch a gateway.