Fault diagnosis in relaying data among a plurality of networks

A computer-readable storage medium containing a computer program for execution by a computer in a network relay apparatus located at a network node, said relay apparatus having a plurality of input ports and a plurality of output ports, each of said plurality of input ports and said plurality of output ports communicably connecting to a link of said network, and wherein a plurality of paths each connect a respective one of the input ports to at least one of the output ports;

• said computer program, when executed by said computer, causes the network relay apparatus to:
• produce a diagnostic data file having a target address which indicates target output port that is part of a target node of the network to which the diagnostic data file is to be delivered;
• read the target address in the diagnostic data file and select a target output port that is at least one of the output ports through which the diagnostic data file is to be outputted to the target node;
• select one of the paths as a selected path along which the diagnostic data file is to be carried to the target output port;
• send the diagnostic data file to the selected target output port via the selected path;
• and
• detect which one of the output ports the diagnostic data file has arrived at as an actual output port to determine whether or not a fault has occurred in at least one of the port selecting step, the path selecting step, and the plurality of the paths of the network relay apparatus based on consistency of the actual output port with the target output port.