A network system includes a maintenance terminal and a switch that transfers a packet. Upon receiving a link-trace message packet, the switch transfers, when links to the destination constitute a link aggregation, the link-trace message packet to all physical links constituting the link aggregation. In response to the link-trace message packet, the switch receives a link-trace reply packet from each of the physical links, and generates link-trace result data. the link-trace result data is output to the maintenance terminal.