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To force or not to force, that is the question |
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Wednesday, 28 December 2005 |
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Speed and Duplex are something that Network Engineers have to deal with on a daily basis. Its also something that can cause plenty of headaches if it isn't done properly. Before I get to my method I will explain the terms speed and duplex when dealing with Ethernet first. |
| Speed: This is the wirespeed that data can be transmitted in megabits per second across certain types of Ethernet media. Typically this speed will range from 10Mbps - 1000Mbps or 1Gbps (Gigbits per second). Note: The speed will not only depend on the equipments ability to support it but also the media (cabling) that the data is being carried over. Don't try to run Gigabit Ethernet over Category 3 wire! Duplex: This is the method in which the devices send and receive data. In half duplex mode the device can only send data or receive data but can never do both at the same time. In Full duplex operation the device is able to use seperate pairs to send and receive data allowing the process to happen simultaneously while increasing the speed of the connection X2. In full duplex mode the device doesn't need to perform the collision detection process because in a full duplex environment collisions are impossible. |
When it comes to forcing speed and duplex settings on NICs and Switch ports there is a basic rule of thumb that I live by: - A: ALWAYS make sure both sides are setup the same.
IE: If one side has the speed and duplex forced then make sure the other side has the speed and duplex forced to the same settings, if one side is set to auto-negotiate then make sure the other side is also set to auto-negotiate. Most of the time if one side is forced to a particular speed and duplex (specifically full duplex) and the other side is set to auto, the side that is trying to auto-negotiate will generally default to half duplex. This will likely cause frame and CRC (cycle redundancy check) errors and cause the connection to run very slow, with many problems. - B: If the device is considered something more static like another switch, router, server, etc I generally force the speed and duplex on both sides to the highest available and check to insure there are no errors accumulating on the port. If the device is a workstation (PC) or a printer I will generally try to run them at auto-negotiate since they tend to move around a little more.
Sometimes problems can arise based on the age of the NIC driver or the switch and you will need to force the speed and duplex anyway but I would always try to update the driver and or firmware first.
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