For network fault management, we present a new technique that is based on on-line monitoring of networks with link state routing protocols, such as OSPF (open shortest path first) and integrated IS-IS. Our approach employs an agent that monitors the on-line information of the network link state database, analyzes the events generated by network faults for event correlation, and detects and localizes the faults. We apply our method to a real network topology with various types of network faults. Experimental results show that our approach can detect and localize the faults in a timely manner, yet without disrupting normal network operations.