One of my jobs as a systems engineer is to support corporate customers. One of the customers I support is a textile factory. Recently, one of this organization's engineers reconfigured the network, and users subsequently couldn't log on to the server. The server was a Compaq ProLiant 5500 with five 18GB RAID 5 hard drives, 1GB of RAM, and a dual-port NIC. The server was running Windows NT Server 4.0 Service Pack 6a (SP6a) and Oracle8. The server also acted as the PDC. The company didn't have a BDC. The company had another Compaq server, but that server was only a member server.

I checked NT Event Viewer and saw the error message The Alerter service depends on the Workstation service which failed to start because of the following error: A duplicate name exists on the network (System log event ID 7001). I tried to find the second machine with the duplicate name but couldn't.

I unplugged all the network cables and restarted the server; the Event Viewer error message didn't appear again. I reattached one network cable and rechecked Event Viewer—all was well. But when I tried to reattach the second network cable, the error message again appeared. I physically traced the ends of both cables and found that they were plugged into the same switch. Because the company's dual-port NIC didn't combine logically, each port had its own IP address. The problem arose because both of the NIC's UTP cables were plugged into the same switch.

After I determined the cause of the problem, I had two choices for solving it. I could combine the dual-port NIC into one logical port, or I could divide the Virtual LAN (VLAN) on the switch into two VLANs. I discussed these options with the company's Electronic Data Processing department and decided to divide the VLAN. The server now works smoothly. I was glad that Event Viewer gave me a clear picture of the problem so that I was able to easily solve it.