Let your switch test your cables !

I ran into a problem recently where a server wouldn’t negotiate Gigabit Full Duplex connection with a Cisco 3750 switch. When the problem first showed up, I was assisting someone else onsite trying to fix the problem and forgot about this handy little command.

The command to write down is show cable-diagnostics tdr interface gi1/0/1 (replace gi1/0/1 with the actual port that you need to test). From what I have been able to find out, this command appears to be only on gigabit capable switches. I have only been able to try this on a 3750, so it “might” be available on 3560, 6500 and possibly Nexus switches. There are a couple of things to remember when using this command. You can use this on a cable that is plugged into a switch port and not to a server but the tests may not really tell you anything useful. When using this command on a port that you are having problems with, remember it is an intrusive test, so the connection will temporarily go down while the connection is being tested. You will get a warning on the console when using this command. Unfortunately, you get it after pressing Enter on the command line with no way to stop the test. Maybe in a future release, Cisco will put an additional step in to require you to press enter to proceed with the test and if any other key is pressed, the test will abort.

Normally the test should take less than 15 to 30 seconds from the experience I have had with it. To see the results, use sh cable-diagnostics tdr interface gi1/0/1 (with the earlier command, replace gi1/0/1 with the actual port being tested). While you wont get a lot of detail, the thing to watch for is the results of the tests on the four different cable pairs. If you are having a potential cable problem, one or more of the pairs will show some type of problem, usually with a fail or problem indication on the specific cable pair. Do let the cable length test be definitive as a general rule. With differences in the cabling such as impedance, resistance, connectors, etc, the length of the cable may be a little different from the true cable length. An exception to this would be if one pair shows a significant difference in length. That might indicate a break or other significant problem at a point in the cable prior to terminating at the connector.

While not as detailed as a commercial cable tester, this at least gives you an indication of a problem without having to get equipment on site or with someone who knows how to use it.

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