Computer networking is rapidly expanding, and the number of problems occurring in networks is also increasing. Many of these problems have special features which make them difficult to troubleshoot. This paper describes a LAN diagnostic expert system called LODES, which has 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. This paper describes the configuration of LODES and the knowledge and control methods for cooperative diagnosis, especially the method for cooperative diagnostic task scheduling.