First, 10MB Ethernet is always half duplex. That is what the standard specifies. There are a number of pseudo-standardized 10MB full duplex (3Com has one) solutions, but they only work under very specific conditions. (If you have 10 MB full duplex, you usually have a problem unless you have a specifically matched switch for the network interface card, again a 3Com NIC with a 3Com switch). The most common cause of this problem is a duplex mismatch between the switch and the workstation or router. I would check the port that the router is connected to and make sure that it is 10MB half duplex. If it auto sensing, then set it to 10MB half duplex. When you connect a device to an auto-sensing port, a system goes through a process of "flash testing" each possible connection speed and duplex. This process is less than perfect and sometimes can get it wrong, especially when you have an auto-sensing card that is carrying out the same procedure. Cabling problems
I would replace the cable on the router. Seventy percent of problems like this are caused by a faulty cable. Also change the patch lead on your Linux boxes. Packet analysis
To determine if you have a Layer 2 or Layer 3 broadcast problem, you should get a packet sniffer and have a look at the broadcast packets. Look at the source of the packets to determine what the application is. I use Network Associates Sniffer as it has an intelligent engine that will analyze the capture and report in clear English for most problems. If you have a lot of IP and IPX network broadcasts and you use Microsoft Windows, you should implement a Microsoft WINS server and set the NetBIOS node type to hybrid mode. This stops Windows from broadcasting for name resolution. If you have IPX as well as IP on your workstations, you should disable NetBIOS over IPX to stop name broadcasts. If you have IPX only, you should implement IP to resolve this problem. Otherwise, the Sniffer capture will have to be analysed by an experienced person to determine what the source of broadcasts are. Good Luck.
This was first published in May 2001