Network relay device and control method thereof

A network relay device, comprising:

• a plurality of ports, each being configured to be connectable with one physical line;
• a virtual line controller configured to treat a plurality of physical lines as a virtual line, the plurality of physical lines are respectively connected with the plurality of ports;
• and
• a status check frame controller configured to send via the virtual line a status check frame for use in checking a status of a network, which the network relay device is connected with via the virtual line, wherein
• the network relay device forms a Ring network,
• the status check frame controller sends the status check frame from a port connected in a first direction of the Ring network and from a port connected in a second direction of the Ring network that is different from the first direction, and
• the status check frame controller changes a frame-sending port to be used to send a next status check frame, in order to avoid continuously using an identical port as both a frame-sending port to send the status check frame and a frame-receiving port to receive the status check frame from another network relay device.