NETWORK SYSTEM, LAYER 2 SWITCH, AND NETWORK FAULT DETECTION METHOD

PROBLEM TO BE SOLVED: To detect a one-way path fault between two communication devices in a short period.SOLUTION: A network system comprises: a first communication device being set a first address; and a second communication device connected to the first communication device through a network and being set a second address. The first communication device has a monitoring frame transmission part which generates a monitoring frame, where the second address is set as a destination address and the first address is set as a source address respectively, and outputs them to the network; and a monitoring response frame monitoring part which monitors reception of a monitoring response frame. The second communication device has a monitoring response frame transmission part which generates the monitoring response frame by setting the destination address set in the received monitoring frame as the source address and setting the source address set in the monitoring frame as the destination address in a counterchanged manner respectively; and outputs them to the network.