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RB4011 PoE in 802.3af

Posted: Sat Nov 21, 2020 9:08 pm
by Slash

I accidently plugged my RB4011+RM with Port 1 (PoE in) with my Dlink PoE switch and my RB4011 booted and was recognized in the switch and all worked fine. In the datasheet it says the RB4011 only accepts passive PoE in. Is it save to operate the RB4011 with a PoE 802.3af switch? Is it an undocumented feature?
BTW, my WAP AC connected to PoE out also booted up without problems.

Re: RB4011 PoE in 802.3af

Posted: Sat Nov 21, 2020 9:32 pm
by mkx
It is safe from RB4011's point of view, it can accept anything between 18V and 57V ... 802.3 af/at is 48V. 802.3 af/at also use same pairs of wires and same polarity. What passive PoE misses is ability for handshake between PC and PD. If PC is willing to blindly provide power (which actually violates 802.3 af/at), then RB4011 (as PD) will happily hum along.

But then ... is it possible that your switch actually supports passive PoE? Although passive PoE implementations are all proprietary but there are only so many ways of doing it with two spare pairs if wires.

Re: RB4011 PoE in 802.3af

Posted: Sun Nov 22, 2020 11:04 am
by Slash
Its is a D-Link DGS-1210, which only supports active PoE. I hooked up my HAP AC to the same switch and it didn't boot. The RB4011 seems to register itself in the switch as a class 4 device meaning 802.3at.

Re: RB4011 PoE in 802.3af

Posted: Sun Nov 22, 2020 12:18 pm
by mkx
This is then a hidden functionality, 802.3 af/at isn't mentioned nether in product page nor in product brochure. Hopefully some MT staffer can explain it here ... or you could ask them at (and post their answer here).

Re: RB4011 PoE in 802.3af

Posted: Sun Nov 22, 2020 12:49 pm
by mkx
Mikrotik PoE manual actually almost explains the mistery. For passive PoE to work, PoE in device has to present a 3kΩ to 26.5kΩ load between power pairs of wires. Initial stage of 802.3 af/at handshake requires PD to present a 19 kΩ to 26.5 kΩ load (and seems like RB4011 is in this range). The continuation of 802.3 af/at handshake, however, requires PD to change resistence according to handshake stage and during those resistance should be around 500 Ω for class 4 PD which is way less than anything specified for MT's passive PoE. If resistance on PoE in doesn't change, PSE might classify PD as class 0 device with power consumption of up to 15.4 W (which is slightly below rated RB4011 power but might be enough).

Re: RB4011 PoE in 802.3af  [SOLVED]

Posted: Mon Nov 23, 2020 8:53 am
by Elans
802.3af/at standard has max current limit 350/600 mA, but this device may consume more, therefore we cannot declare that this device has af/at full support. Due to that, device is not fully tested for this standard and we can not guarantee that it will work stable with every PSE which provides only 802.3af/at.
This is not anything harmful, you can use such setup if it suits your needs and the device is powered.

@mkx has provided all information which can and do affect the device behaviour.