Category Archives: PoE

Another Example of How Important Wire is to Wireless

A house built on a shaky foundation cannot endure. And a WLAN built on a shaky wiring foundation likewise cannot endure, I tellya. My friends, is your foundation shaky? Is it? CHECK YOUR FOUNDATION NOW. (I happen to sell foundation-strengthening herbal supplements on the side, if you need that sort of thing…)

I’ve long been a proponent of recognizing installed UTP as a vital component in the networking ecosystem. Too many people take Layer 1 for granted, and forgivable sins of of our 10 Mbps and Fast Ethernet pasts won’t fly in a Gig world. Toolmakers like Fluke Networks sell cable certification testers that take the guesswork out of whether a given cable run can be relied on to perform as expected. Don’t use one of these testers at time of cable installation, and you are only assuming you have a good station cable.

I just had an interesting situation come up that I helped a very skilled field tech with. He was working in several different small buildings, each serviced by a Cisco Catalyst Switch and a handful of 3802 802.11ac access points. The switches and cable had been in place for years, and the APs for many months, all with no issues whatsoever.

Then, we changed out the old 3560X switches for shiny new 3650s (curse you Cisco for your bizarre fascination with part numbers so close together), and suddenly some APs weren’t working any more. Between us, we checked all switch settings, POST reports, CDP tables, logs, etc- everything you can dream up on the switch. We put the APs that weren’t working back on the old switches, and they came right up. Hmmm… thoughts turned to PoE/code bugs, but then I went a-Googlin’ before consulting TAC.

I found this document that put me on the path to righteousness. Though we weren’t having “PoE Imax Errors”, a couple of nuggets jumped out at me about our new switches.

PoE Imax

Holy guacamole- We got us a situation! But wait… THERE’S MORE!

PoE Imax2

Shazam! Which, of course, translates in Esperanto to “maybe your cable is actually kind of iffy, and all the CDP stuff that happens at the milliwatt level before PoE gets delivered worked OK with your old switch but not with the new one that has the enhanced PoE controller”.

If you don’t know that the newer switch does PoE differently, you might wrongly assume that your cabling is “good” because the APs worked on it when those APs used the old switches connected to that wiring. By now, you can probably guess where I’m headed…

Our tech tested the cabling on the new-switch-problem APs and in each case found that they needed help to work with the new switch. He re-terminated and tested each, with the APs then coming up with no issues. I have no doubt that this cable was certified 10-12 years ago, but in that time a lot can happen to either end of those cables depending on the environments where they are used.

Live and learn!