We hired a new coworker and learned that he needed a Cisco VOIP phone. I had one spare unit left, an older Cisco 7912 unit. I went to go plug it in, and the POE over Ethernet simply wasn’t registering on the phone. I knew for a fact that the phone itself was fine, and the switch I was plugging the phone into was functioning well. I also knew that my station cables were working fine, so I used my Fluke LinkRunner to test the cables and the port. Everything checked out; the Fluke indicated proper POE, however, when I plugged the phone in, nothing at all.

I knew that this port had a history of being troublesome, but previously to this I had a Cisco 7940 phone working well in this spot, so it was a mystery as to why a 7912 wasn’t also working. I tested the port a few times, each time seeing proper POE voltage and wattage. Even the switch itself noticed my Fluke tester and was registering that a device was consuming POE supply on the port in question. I couldn’t understand why a phone that works well in one place doesn’t work in another when everything is equal. Obviously, not everything was as equal as I thought. Something had to be wrong.

I looked at the Fluke LinkRunner, it listed POE as coming in on pairs 1 and 2 for the positive circuit and 3 and 6 for the negative circuit. So then I took the Fluke to my testing lab and looked at POE coming from a Cisco Catalyst 3560 switch. The Fluke indicated that 3 and 6 were positive, and 1 and 2 were negative. I immediately figured out what the issue was. Ethernet jacks can conform to T568A or T568B, the difference is subtle and is a flipped pair of conductors. I did a little desk diving and popped the cover off the jack in the wall, everything that I deal with is always T568B. Always. The jack in the wall? T568A. So armed with what I knew, I tugged the old keystone jack out and replaced it with the last good one that I have. Punched it down, and tested it again. The Fluke indicated POE, 3-6-1-2, I plugged in the phone and pop! The phone came to life!

So, just when you think you can just get on with things, always check the standards. You always have to assume that nobody else is. What a mess. But at least it was an easy fix.