In a bottom up approach to network troubleshooting, the cabling is checked first and then troubleshooting moves...
up the protocol stack. When one user goes down, it is far easier to start at the physical layer and move up. Some idiosyncrasies can develop when EMI and/or environmental concerns are causing the problem. For instance, I had a customer once that had problems with a circuit at 3:00 every few days. The problem would clear up by the next morning. In looking at the trending reports, traffic was increasing at about the same time. No matter where the connection moved in the electronics, the same thing would happen. Tracking down the problem proved to be a nightmare. As it turned out, every few days the sprinklers would turn on and the outside cable was not run in conduit but was laying on the ground at the base of the building (covered in grass, no less). When the water dissipated, the problems went away.
When there are errors, either continuous or intermittent, it is a good idea to look at your physical layer. Field terminated patch cords are a particularly rotten culprit, but other environmental conditions can also be to blame. When walls are moved, cables that were once placed away from fluorescent light fixtures may no longer be outside of acceptable range, new power panels may be located too close, etc. It is important to note that you should not rely on the fact that you have a link light on your switch port to determine if the cable is good or bad. Just like your electronics, there are conditions where you may have a link, but the signal is so degraded from sender to receiver that the packet is useless. Remember the expression "lights are on but no one's home." This is true for copper or fiber.
If you have employed Gigabit Ethernet and your cabling was installed before the new parameters for channel performance were adopted, you will also want to have your cabling recertified for the new parameters. You should note that when equipment is tested for operation with any physical layer media, that this is done in a lab in a pristine environment. Actual installations may vary for a number of reasons. If you are going with the bottom up approach, check all of the physical medium and don't skip this step because you can ping a device or see it's link light. On the other side, if you don't have a link light –- it's obvious.
Then you work your way up –- checking the network card diagnostics, switch port statistics, and work up to the application. If only one application is not working, start at the top. If several are not working or all are not working for one workstation, start your way up from the bottom. And remember, once in a while it will be in the middle or this rule will be backwards. Click here to read Top down approach to troubleshooting
Carrie Higbie, Global Network Applications Market Manager, The Siemon Company
Carrie has been involved in the computing and networking industries for nearly 20. She has worked with manufacturing firms, medical institutions, casinos, healthcare providers, cable and wireless providers and a wide variety of other industries in both networking design/implementation, project management and software development for privately held consulting firms and most recently Network and Software Solutions.
Carrie currently works with The Siemon Company where her responsibilities include providing liaison services to electronic manufacturers to assure that there is harmony between the active electronics and existing and future cabling infrastructures. She participates with the IEEE, TIA and various consortiums for standards acceptance and works to further educate the end user community on the importance of a quality infrastructure. Carrie currently holds an RCDD/LAN Specialist from BICSI, MCNE from Novell and several other certifications.