Two simple unmanaged switches have died in the same cabinet in two years, one at 18 months and one just today at six months.

Both times we gradually experienced increasing intermittent verified connectivity failures for all equipment (two linux boxes and a Debit/credit terminal) except the uplink (port 1) and the fourth item, a cisco phone connected to the switch via a PoE injector in port 2. Connectivity would eventually return with or without power cycling the switch. At the onset of symptoms power cycling reestablishes connectivity, but eventually it does not.

Link lights remain on and flickering regardless of interrupted connectivity. Interrupted connectivity initially occurs on a port-by-port basis but ends up affecting all ports except the PoE-phone port. Earlier today I, in an attitude of stupefied mystification sat there and plugged each item into port 2 previously inhabited by the PoE-phone and watched it work just perfectly while not working in ports 3/4/5.

The deceased are a D-Link DSS-5+ and a DES-105G. The switches took wall power, but all else, including the PoE injector, are powered through a UPS.

It sure looks like the PoE injector is somehow frying the switches, but 1) does that make any sense, and 2) what's the reasonable workaround?

Attentions to my little mystery much appreciated!

  • 2
    Cheap switches generally won't do so well over time. If you are skimping on switches, is it reasonable to assume that your PoE injectors are of similar consumer level quality?
    – Rex
    Oct 18, 2014 at 4:26
  • Yes, the new switch will be quality, the PoE is just what came with the Cisco phone and wasn't installed by me, tbh I know next to nothing about them. Are they generally safe for "upstream" equipment? Oct 18, 2014 at 23:27

2 Answers 2


For curiousity you could measure the PoE injector´s input port for voltage coming back on pins 4/5 and 7/8 - see this pic: http://beyond-wifi.com/poe/images/table-th.jpg .

But I doubt this is the problem since I would expect the pins related to PoE are not wired in the switch at all... they are actually not even used in the cable probably, except you use fully wired 10GB cables...

BR Florian

  • 1
    Copper Gigabit Ethernet (1000BASE-T) already uses all four pairs of a cable.
    – Lukas
    Oct 20, 2014 at 13:55
  • @Lukas: Uh yes, now that you name it... anyway, PoE injector which is feeding voltage back on the switch side would be considered either a very bad design or it has some kind of fault, so better check with measurement...
    – flohack
    Oct 22, 2014 at 21:16

At this point, try to use slightly better switch equipment than what you've been using. Consider HP ProCurve web-managed or unmanaged swtiches, for instance... They're inexpensive and have lifetime warranties. That does make a difference and helps protect the investment. It's a visible piece of equipment and there isn't much of a reason to skimp.

Protect your equipment with surge protectors, even basic ones. Don't go direct to the wall. You don't do that with your TV, video and computer equipment at home, right? Why do that in a business?

How's the weather? Is the electrical in the facility properly grounded? I've been in situations where lightning and poor electrical design, coupled with PoE caused massive damage; so it's possible. But that's an edge case.

But take the two-pronged approach and protect your gear while stepping up to a better class of equipment.

  • Hmmm Good point, we're in the basement of a medical facility and while our grounding is excellent our power is three-phase, i.e. the difference between "live" and "neutral" is normal, but the difference between "neutral" and "ground" is depending on the circuit not necessarily what it usually is. I will move the new (managed) switch to the UPS if only to guarantee the consistency of voltages (I.e. that all the equipment in the cabinet is using exactly the same "live" and "neutral.") I think you may have hit the nail on the head there, thanks! Oct 18, 2014 at 23:26
  • @WhilomChime Definitely go that way if you're unsure that you're getting clean power. But a surge protector can be handy, too.
    – ewwhite
    Oct 18, 2014 at 23:28
  • :) roger wilco! Oct 18, 2014 at 23:30

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