I have a set of 7912G IP Phones, which only accept Cisco pre-standard PoE. To save myself buying a Cisco switch, is it acceptable practice to change the wiring of my ethernet cables slightly and use a standard PoE injector from any manufacturer as detailed in some online tutorials?
Sources
Best Answer
It's your network, do whatever compromise is required by you, your management, and their budget.
I agree with comment above: don't mess with hidden wire, do it close to the phone or in the patch panel. [EDITED re safety] Be aware any legal consequence of non-professionally made cabling, especially as regards fire hazards. But because the power is allowed to be whichever way round (by 802.3ag), that itself isn't an issue: it's only the pre-standard phones which require it to be a particular way. From memory it was a fight between Cisco and another company about which way the polarity was, and the result was a late amendment to permit either polarity.
So 802.3af specifically says that "Alternative B" power (ie, 4+5/7+8) can go either polarity, and that it is up to the Power Sourcing Equipment (PSE) to decide whether to do A or B and which polarity.
So you had better check (empircally) what power your POE-supplying switch actually delivers, and whether it powers the phone -- 802.3af has pretty complex startup behaviour.
If that doesn't work, you could consider "Nasty Cisco Power Patch Panel", which you'll find available as mid-span injectors, or easy to do with a punch down patch panel and a suitable power supply.
My notebook tells me this about 802.3af
Details from standard, originally 802.3af 2003, now folded into 802.3 2012
802.3 2012
Let us know how you get on.
Jonathan.