Mikrotik RouterBoard RB951Ui-2HnD - frozen red LED 5th port

as wiki informs us:

There is a red LED on port 5, which seems to be some indication of PoE (apparently enabled by default). You can turn the LED off (PoE will continue supplying power over port 5, even without the red LED on) with the following command. You can put it in /etc/rc.local
echo 0 > /sys/class/gpio/gpio2/value

but

# echo 0 > /sys/devices/virtual/gpio/gpio2/value

-ash: can't create /sys/devices/virtual/gpio/gpio2/value: nonexistent directory

used to do the trick, at least until 15.05.1, when i last tried.

Then @amosipov provided a snippet of configuration that fits well under the LED section in the wiki or your /etc/config/system

I would skip the comment though. I'm not sure if it really turns off PoE and there is no PoE device around to check, but surely does the magic and unfreezes the red 5th LED to a green one and blinking, when traffic passes by.

If you share the same considerations

  1. I have no idea if it is ever turned off anyway..
  2. short answer : everything will be fine as mikrotik says.
    and if you like some extra reading

How does POE work?

Network cables, such as Cat 5e and Cat 6, comprise eight wires arranged as four twisted pairs. In 10 and 100BASE-T Ethernet, two of these pairs are used for sending information, and these are known as the data pairs. The other two pairs are unused and are referred to as the spare pairs (Gigabit Ethernet uses all four pairs).

Because electrical currents flow in a loop, two conductors are required to deliver power over a cable. POE treats each pair as a single conductor, and can use either the two data pairs or the two spare pairs to carry electrical current.

Power over Ethernet is injected onto the cable at a voltage between 44 and 57 volts DC, and typically 48 volts is used. This relatively high voltage allows efficient power transfer along the cable, while still being low enough to be regarded as safe.

This voltage is safe for users, but it can still damage equipment that has not been designed to receive POE. Therefore, before a POE switch or midspan (known as a PSE, for power sourcing equipment) can enable power to a connected IP camera or other equipment (known as a PD, for powered device), it must perform a signature detection process.

Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compatible PDs (a 25kOhm resistance). Once this signature has been detected, the PSE knows that higher voltages can be safely applied.

or go for the whole thing. :wink:

A detail about RB951Ui-2HnD voltage from Mikrotik's official site ( https://mikrotik.com/product/RB951Ui-2HnD ) :

PoE output function for port #5 - it can power other PoE capable devices with the same voltage as applied to the unit.

The included power source is a 24 Volt one.

PoE capability tested on port #5 after aplying snippet (red light off) using a Ubiquiti NanoStation M5 (24 Volt passive PoE, 0.5 Amp) : does not power on (no lights at all).

I think we can assume the snippet is correct.

Edited accordingly on [https://openwrt.org/toh/mikrotik/rb951ui#initial_setup](http://Mikrotik RouterBoard RB951Ui-2HnD Initial Setup)