The author describes a LAN diagnostic expert system called the Large Internetwork observation and Diagnostic Expert System (LODES), which has transport=control=protocol/internet=protocol-(TCP/IP-) related and troubleshooting knowledge, and can handle problems occurring in a complex and heterogeneous network environment. Each constituent network has its own expert system containing management information about the local network. TCP/IP-related problems are thus resolved singly or cooperatively, depending on their type. This system also includes a network observer, which monitors packets flowing in the network and can find a number of problems and problem indicators for automatic problem detection and troubleshooting. The configuration of LODES is described, as well as the knowledge and control methods for cooperative diagnosis, especially the method for cooperative diagnostic task scheduling.