Power over Ethernet (PoE) · Topic 1 of 3

RouterOS PoE Modes and Compatibility

Learn the PoE out modes in RouterOS, how 802.3af and 802.3at differ from passive PoE, and how to avoid damaging devices by choosing the wrong mode.

5 min

In a nutshell

802.3af supplies up to 15.4 W, and 802.3at supplies up to 30 W at the port.
Standard PoE detects and classifies the device before it powers it.
Passive PoE sends a fixed voltage without any detection.

What it is

Power over Ethernet sends electrical power to a device over the same cable that carries its data.

A switch or router with PoE out ports is the power source. The device at the other end, such as an access point or an IP camera, is the powered device. This saves a separate power cable and a nearby socket.

PoE comes in two families. Standard PoE negotiates with the device before it sends power, and passive PoE sends power without any check.

How it works

The port mode decides whether the router checks the device before it powers it.

The PoE standards

Two IEEE standards are common.

Standard	Name	Power available at the port
802.3af	PoE	Up to 15.4 W
802.3at	PoE+	Up to 30 W

Both use detection. The power source first checks that a compatible device is attached, then it classifies how much power the device needs, and only then it turns on power. A device that does not ask for PoE never receives it, so ordinary laptops and printers are safe.

Passive PoE

Passive PoE has no detection and no classification.

The source just sends a fixed voltage over the cable, normally over the spare wire pairs.

The powered device must be built for exactly that voltage. Many MikroTik devices use this older style, and some accept both styles. Always read the input specification of the device before you connect it.

The RouterOS PoE out setting

Each PoE capable port has a poe-out setting, found under /interface ethernet poe.

/interface ethernet poe print
/interface ethernet poe set ether2 poe-out=auto-on
/interface ethernet poe set ether3 poe-out=forced-on
/interface ethernet poe set ether4 poe-out=off

Mode	Behaviour
`off`	No power is sent
`auto-on`	Power is sent only after a device is detected
`forced-on`	Power is sent at all times, with no detection

auto-on is the safe choice. forced-on exists for passive devices that the detection cannot see.

Watch out

forced-on powers the port even if the device cannot take it. A device that is not made for PoE can be damaged, so use it only with a device you know is passive.

Matching device and port

Check four things before you power a device.

Does the port supply power in the style that the device needs, standard or passive?
Is the voltage within the input range of the device?
Is the power enough for the device, including a heater or extra radios?
Is the cable good enough, with all pairs connected?

A mismatch gives a device that never turns on, or that restarts under load. Long or poor cables lose voltage along the way, so a device far from the switch can fail when a short cable works.

Models differ

Not every port or model supports PoE out, and not every model supports every mode.

Some models can only power with one style. Some let you choose between voltage levels on a port.

Some have a total power budget shared by all PoE ports. The product documentation lists these facts for each model, so check it first.

Powering from a router

A router or switch can also be the powered device, if it has a PoE in port.

It then takes power from the upstream device. This is common for small access points that hang on a ceiling. Make sure that the upstream port provides what the device accepts.

Checking

/interface ethernet poe print
/interface ethernet poe monitor ether2

The print shows the mode of each port. The monitor shows what the port is doing now, which the monitoring page covers.

Exam tip

Start with auto-on for every PoE port, and use forced-on only after detection has failed on a known passive device.

What to remember for the exam

Seven facts cover most questions on this topic.

802.3af supplies up to 15.4 W, and 802.3at supplies up to 30 W at the port.
Standard PoE detects and classifies the device before it powers it.
Passive PoE sends a fixed voltage without any detection.
poe-out takes the values off, auto-on and forced-on.
auto-on powers the port only after a device is detected.
forced-on powers the port with no check, and can damage a device.
Check the model documentation for supported modes and the power budget.

Common mistakes

These four mistakes appear often.

Using forced-on on a normal device. A device that is not made for PoE can be damaged.

Expecting a passive device to work with auto-on. Detection may not see it, so it can stay without power.

Ignoring the power budget. Several hungry devices can exceed what the switch can supply.

Blaming the device for a cable fault. A long or poor cable lowers the voltage at the far end.

Check yourself

Work through these three questions.

What is the difference between auto-on and forced-on?
auto-on powers the port only after detecting a device, while forced-on powers it at all times with no check.
Why is standard PoE safe for a laptop?
The port detects whether a compatible device is attached, and sends no power if it is not.
How much power does 802.3at supply at the port?
Up to 30 W.

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