Dos and don'ts of SAN cabling

Fiber optic cable is one of the simplest parts of a SAN and one of the most time-consuming to troubleshoot. Here's how to insall and troubleshoot it.

Fiber optic cable is one of the simplest parts of a SAN and one of the most time-consuming to troubleshoot when something goes wrong. It's better to make sure nothing goes wrong by taking care in the installation and handling of your SAN cabling.

First, choose the right cable and know which kind you've chosen. This can be a problem because the common types of fiber cable are physically indistinguishable and some SANs mix single and multimode fiber depending on the traffic on the different parts of the SAN. The Fibre Channel standard is that multimode cable has an orange jacket while single mode is yellow. It can save a lot of future problems if you purchase cable that follows the specification. It's also important to get the right kind of multimode cable. The Fibre Channel SAN standard is based on 50-micron cable for multimode use, and many SAN vendors only support 50-micron products.

Cable termination is also important. Fibre Channel cable comes with two types of connectors: SC and ST. The ST connectors are becoming more common, but there are still SC products in the market. You can also get hybrid cables with both kinds of connectors depending on your need.

These are some basic caveats that will help you when speccing out your SAN installation. But no matter how good your specifications are, the fact is that most cable gets installed. Some care and forethought here will prevent a lot of problems.

The first rule of a good installation is to document everything. That includes cable runs, connectors, and which cables are connected to which pieces of equipment. Ideally the documentation should be kept in both on-line and paper form, with the notebook containing the documentation in an easily accessible (and easy to remember) place. Documentation includes clearly labeling the cables in the installation in a permanent fashion in locations where the labels are easy to read.

Whilefiber cable is fairly rugged, it should be handled carefully. Don't pull the cables too tight when installing them, and be sure there is strain relief on each cable. If you're going to have to turn or shift equipment to work on it, make sure the cable is long enough or that the cable is disconnected before the equipment is moved. Don't pull cable ties too tight and don't try to bend the cable too sharply. Cables shouldn't be left hanging unsupported. And don't have cable dragging on the floor where it can be stepped on or tripped over, even during installation.

IBM discusses cable installation in a note titled "SAN - Good Cabling, Bad Cabling" available from the IBM Web site.


For more on SAN cabling, see also:
 

This was first published in October 2003

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