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Power to the sniffers

How to power protocol analyzers that don't support PoE.

You really need to have a troubleshooting toolkit with things like a little hub, some extra cables that you know are good, and of course, a quality analyzer. I'm a big fan of protocol analyzers, or sniffers, for a lot of reasons. I'm also a big fan of preparing your network for troubleshooting before problems occur. Prepping includes doing things like strategically placing passive taps in your fiber optic cables and critical server connections, gathering critical information, and possibly configuring "mirror" or "span" ports on user switches.

The preferred method of using the best analyzers is to insert them between two devices. They have two ports, an MDI and MDIX, and you connect one to a switch and one to a computer, so that the analyzer sits between them and doesn't require a hub or a span port. A hub will drop the link to half-duplex, while a span port will introduce a lot of unnecessary variables into your troubleshooting. For example, if you're troubleshooting jitter using a span port, how much of the jitter is in "the network" and how much is caused by the span port?

But there's a gotcha: VoIP has thrown this method a curve called Power over Ethernet, (PoE), something that might not occur to you until you go to troubleshoot a problem.

Here's what could happen: You insert your analyzer between your switch and an IP Phone and... nothing. You look at the phone... it's dark. Then you realize that, of course, the analyzer won't pass the signal back to the switch to tell it to provide power to the phone. That's actually a good thing, because if the analyzer did pass that signal, the power wouldn't reach the phone, and would likely fry the sensitive electronics inside your analyzer.

So what should you do? First, get a spare power supply for your phones and add it to your toolkit (remember your toolkit?) so you have it when you need it. With this, you can still insert your analyzer and power your phone separately. Your protocol analyzer isn't going to help you with PoE anyway, so you're not losing any functionality. Next, bug your vendor about adding PoE support to your analyzer.


Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.


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