A distributed, multistage agent for monitoring, diagnosis and maintenance of network devices comprises an input/output interface for receiving input monitoring and diagnostic data relevant to operation of a first neighboring agent and transmitting output monitoring and diagnostic data relevant to operation of the host network device to a second neighboring agent. An agent process analyzes the input data and generates processed data characterizing operation of the host network device, using a local database which stores expected performance data on operation of said network device. The processed data is correlated with the expected data any error is signaled to the network operator. The agents may be designed as a multiple stage process, the stages being triggered by the data output by an earlier stage only when needed.