An expert system for diagnosing a data communication network is discussed. The expert system includes a knowledge base comprising a plurality of rules pertaining to reactive and proactive diagnosis of the network. The expert system also includes a plurality of modules for processing network-related questions, such as a rule module which uses forward rules to hypothesize whether network problems exist, and which uses backward rules to verify whether the hypothesized network problems exist. The expert system further includes a facilitator for selecting modules to process the questions. The facilitator operates by sending a poll request associated with one of the questions to the modules, analyzing estimates from the modules to identify a module which can most efficiently and effectively process the question, and instructing the identified module to process the question. The rule module also operates to proactively diagnose the data communication network by identifying in the knowledge base a rule pertaining to proactive diagnosis, formulating questions based on the rule, and circulating the formulated questions within the expert system.