hEX PoE 7.21.5: voltage_too_high

Hardware MikroTik hardware general
1

Hello,

I've deployed in several locations, a central CRS328-24P connecting and powering a couple of hEX PoE, themselves powering two Wifi AP.

PoE settings on CRS328-24P port powering hex PoE device, are:

poe out:   auto_on
poe_voltage: auto
poe out voltage: 52.1V

On hEX PoE, (with 6.49.20), ports connecting AP are set like this:

poe out:   auto_on
poe out voltage: 50.7V

On hEX PoE, (with 7.21.5), I had to set poe_out to off to remove voltage_too_high message and restore connectivity but downstream devices being PoE device, this is not a solution.

Any suggestion ?

best regards

2

This behaviour indicates that the hEX PoE connected APs only support low voltage. Please try setting the PoE-Out voltage to low on the CRS328 port that powers the hEX PoE:

/interface ethernet poe set etherX poe-voltage=low
(replace X with the actual port in use.)

If you still receive a voltage-too-high message, please open a support ticket containing the supout.rif file from the hEX PoE, along with the exact models of the WiFi access points connected to it and their corresponding ports.

3

Only to keep things as together as possible, this happened when upgrading to 7.21.5 from 6.49 which was working:

4

Powered access points are Unifi AC Lite. Usually those consume around 3W, maybe more when booting.

In this case, I've got got several instances of 6.49.20 powered hex PoE, working fine in the same environment. As Jaclaz said, the alarm message was triggered by RouterOS upgrade to 7.21.5 (direct from 6.46.X).

5

Trying low_voltage setting on CRS328-24P port may not hurt but IMHO, having 48V delivery is a very appreciated feature so it very important to either keep it or understand why we can't get it.

6

This one?:
https://techspecs.ui.com/unifi/wifi/uap-ac-lite?subcategory=all-wifi

Which is normally powered at 24V, and should work also ONLY[1] on PoE 24V passive (mode B, just like most Mikrotiks).

Just saying...

[1] I think you are very lucky that 48 V was NOT pushed up to them.

7

Several generations of this AP were sold during its lifetime (EOL-ed now).

If my memory serves me right, the first ones were 24V only while the late ones supported 802.3af (or maybe both).

In this installation, I've got currently several units directly powered with 48V by CRS328-24P.

It would surely help to understand what can trigger a voltage_too_high message ? Maybe this AP advertises it supports both 24 and 48V input and 7.21.5-powered hEX PoE concludes it should warn sysadmin first and cut power to downstream device.

Maybe this AP starts by advertising it supports 24V input which trigger above message display.

8

After double checking connected AP, one of them is an UAP-AC-Lite while the other is an UAP-AC-LR.

The first one seems to work with a 24V input while the other one is unstable.

I think it should if this "regression" or "feature change" was explicitly listed in 7.21.5 Announcement thread.

9

What do you mean "unstable"?

Loosely the idea (in Mikrotik devices) should be the following:

  1. does the device probe on the PoE out port read 3 kOhm resistance or more? Yes, it's PoE. No It is NOT PoE.
    IF Yes:
  2. does the device probe on the PoE out port read 26.5 kOhm or so? Yes, it is high voltage PoE. No, it is NOT high voltage PoE
    IF Yes:
  3. let me try to negotiate with the device 802.3af or 802.3at power requirements.

If you power the hex Poe with 802.3af/at (which implies high voltage) or with passive high voltage you CANNOT power a 24V PoE in device with it.

On the other hand it is entirely possible that one of the detection steps listed above works in one release but it is "off" on another one.

10

By unstable, I mean connection to the AP is up and down continuously (every 90s or so).

11

An interesting note worth mentioning here:

on the same (very remote) location, a sibling hEX PoE with RouterOS 7.20.2 works OK with 2 downstream UAP-AC-Lite.

Maybe replacing a downstream UAP-AC-LR (which requires 48V, I think) with UAP-AC-Lite (which accepts both 24 and 48V) would help.

This wouldn't change the fact this behaviour is triggered by 7.21.5 upgrade but it gives a work around.

12

Unfortunately PoE auto-detection is not an exact science. E.g. no one is exactly sure what resistance should be apparent on a device that supports both 24V passive and af/at.

The "voltage too high" is confusing for some people, but it simply means that the auto-detection came to the conclusion that the device would want 24V, for which the approx. 50V is too high.

Can you not force high voltage PoE on? (Of course only given that you're sure the device can handle it.)