ether1 flapping and 1 Gbit negotiation fails on LHGG LTE6 with verified cabling

RouterOS General
1

Hey folks,
I’m slowly losing my mind over this one and would really appreciate any input.
I’ve got a real-world deployment with a MikroTik RBLHGGR unit:

:white_check_mark: The Setup:
• Router: MikroTik LHGG LTE6
• RouterOS: 7.19.1
• Power: Passive PoE (tested with the original and multiple other 24V injectors)
• Cable: Fixed ~20 m CAT7, in-wall, certified and tested by an electrician
• Switch: TP-Link smart-managed 1 Gbit switch (also tested with others)
• Install: Router is on the roof, PoE and switch are in the basement

:red_exclamation_mark: The Problem:
Every time I install the device:
• It boots, negotiates 1 Gbit/s on ether1
• After some time (minutes or hours), link flaps begin and it eventually falls back to 100 Mbit/s
• Once it’s at 100 Mbit – it stays there
• Log shows interface down/up flapping
• LTE interface is stable, no drops there
• Issue happens with two different devices

:test_tube: What I’ve tested:
• Swapped devices (same model): same problem
• Fresh config from scratch: still flaps
• No VLANs, no firewall, no queueing
• Multiple PoE injectors tested (24V passive)
• Cabling was verified – no damage, no attenuation issues
• Tried different switches – same result

:weary_face: I’m out of ideas.
I’ve tried everything short of ripping out the in-wall CAT7 cable (which I’d really prefer not to do).
Did anyone ever encounter this kind of “Ether1 starts at 1 Gbit and slowly dies back to 100 Mbit” scenario?
Any help is appreciated – I’m happy to share exports, config. I logged a ticket with Support a few seconds ago, too. Maybe you guys have some ideas too

As a “dirty-fix” I created a small script to just reset the interface - because afterwards it always synchronizes to Gbit.

Thanks in advance,
Florian

[admin@mt-01] > /log/print  where topics~"interface|script"    
 2025-06-11 16:54:04 interface,info lo link up
 2025-06-11 16:54:04 interface,info back-to-home-vpn link up
 2025-06-11 16:54:13 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 16:54:40 interface,info lte1 link up
 2025-06-11 18:49:20 interface,info ether1 link down
 2025-06-11 18:49:23 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:09:30 interface,info ether1 link down
 2025-06-11 20:09:32 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:10:32 interface,info ether1 link down
 2025-06-11 20:10:36 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:10:37 interface,info ether1 link down
 2025-06-11 20:10:43 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:10:54 interface,info ether1 link down
 2025-06-11 20:11:01 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:11:22 interface,info ether1 link down
 2025-06-11 20:11:25 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:11:36 interface,info ether1 link down
 2025-06-11 20:11:39 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:11:40 interface,info ether1 link down
 2025-06-11 20:11:50 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:23:02 interface,info ether1 link down
 2025-06-11 20:23:05 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:56:51 interface,info ether1 link down
 2025-06-11 20:56:54 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:56:55 interface,info ether1 link down
 2025-06-11 20:56:58 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:56:59 interface,info ether1 link down
 2025-06-11 20:57:01 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:57:02 interface,info ether1 link down
 2025-06-11 20:57:04 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 20:57:50 interface,info ether1 link down
 2025-06-11 20:57:53 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 21:36:03 interface,info ether1 link down
 2025-06-11 21:36:05 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 21:36:08 interface,info ether1 link down
 2025-06-11 21:36:10 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 21:36:11 interface,info ether1 link down
 2025-06-11 21:36:14 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 21:36:15 interface,info ether1 link down
 2025-06-11 21:36:20 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 21:36:21 interface,info ether1 link down
 2025-06-11 21:36:23 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-11 21:36:24 interface,info ether1 link down
 2025-06-11 21:36:26 interface,info ether1 link up (speed 100M, full duplex)
 2025-06-12 05:30:49 script,warning Resetting interface ether1 due to degraded state...
 2025-06-12 05:31:04 interface,info ether1 link down
 2025-06-12 05:31:08 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 05:31:09 script,warning Interface ether1 reset completed.
 2025-06-12 06:14:20 script,warning Resetting interface ether1 due to degraded state...
 2025-06-12 06:14:35 interface,info ether1 link down
 2025-06-12 06:14:38 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:14:40 script,warning Interface ether1 reset completed.
 2025-06-12 06:16:22 interface,info ether1 link down
 2025-06-12 06:16:25 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:20:31 interface,info ether1 link down
 2025-06-12 06:20:34 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:20:36 interface,info ether1 link down
 2025-06-12 06:20:39 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:20:55 interface,info ether1 link down
 2025-06-12 06:20:58 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:21:10 script,warning Resetting interface ether1 due to degraded state...
 2025-06-12 06:21:25 interface,info ether1 link down
 2025-06-12 06:21:29 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:21:30 script,warning Interface ether1 reset completed.
 2025-06-12 06:26:18 interface,info ether1 link down
 2025-06-12 06:26:21 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:26:26 interface,info ether1 link down
 2025-06-12 06:26:28 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:26:47 interface,info ether1 link down
 2025-06-12 06:26:49 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:31:30 interface,info ether1 link down
 2025-06-12 06:31:33 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:46:59 interface,info ether1 link down
 2025-06-12 06:47:01 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:47:13 interface,info ether1 link down
 2025-06-12 06:47:15 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:47:25 interface,info ether1 link down
 2025-06-12 06:47:27 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:47:40 interface,info ether1 link down
 2025-06-12 06:47:43 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:47:45 interface,info ether1 link down
 2025-06-12 06:47:51 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:47:55 interface,info ether1 link down
 2025-06-12 06:47:57 interface,info ether1 link up (speed 1G, full duplex)
 2025-06-12 06:48:01 interface,info ether1 link down
 2025-06-12 06:48:03 interface,info ether1 link up (speed 1G, full duplex)
2

Usually when I see similar behavior, it’s 95 out of 100 times … cable.
See if you can use another cable (run it separate on the floor or so just to test).

It also seems to be the one thing you did not test yet. So it doesn’t hurt to make sure.
If you have the same problem with several devices and switches and the cable is the one thing which hasn’t been changed yet … chances are high it’s the culprit.

3

If you can reach the device then run a temp loose cable to run out any issues with the cat7, how did the sparks verify the cable? sometimes the testing tools can leave you down

4

Hi,
i had the same thing on my RB4011 and the 1GB connection to my cable modem.
I placed an old dump 5 Port switch in between. And that worked. There was no other solution.
I decided to buy a copper SFP module and use that as WAN port for the 4011 and that worked very well. None of my routers ports worked with my cable modem.

5

Hey guys,

Thanks for your input. To be honest, I also suspect that the cable might be the culprit.

However, the customer already had an electrician check it, and according to his measurements, the cable was deemed to be “in good condition” – which somewhat weakens my concerns. Still, I’ll try to convince the customer to go for a new cable, even though that might be physically tricky. According to the customer, there’s some sort of fire protection inside the existing conduit, which makes it almost impossible to pull an additional cable through without major effort.

The device is installed on the roof of a building, while the rest of the infrastructure is located in the basement – so even using a “standard patch cable” would be quite a challenge in this setup.

Thanks again for your thoughts!

6

P.S.: We even swapped the switch for a basic unmanaged one – no change, the problem remained.

7

I’ve seen a good deal of “qualified electricians” doing disastrous UTP cable work … so I wouldn’t take electrician’s word as pure gold … specially if it was the same electrician pulling the cable and certifying it. So I consider electricians “guilty until proven innocent” :wink:
Sometimes conductivity of individual wires is fine but the frequency characteristics is unfit for anything higher than 100Mbps … any only proper (read: expensive) UTP tester would show that.

8

I agree with @mkx here, don’t trust electrician when they say that they tested UTP cable unless they give you report on each line they tested. First thing they screw up is crimping connectors… So start by checking every connection there is. Also their testing is always with small cable tester which will confirm there is connection (They test with plain DC signal) they don’t tell you if that connection is good.

9

Totally agree – in our case, the guy who pulled the cable also “certified” it :sweat_smile:

Customer now ordered a 50m pre-made patch cable, which will be routed around the building to the roof.
Not pretty, but it should settle the question whether the original cable is the culprit. Looking forward to Tuesday were the cable will arrive🔍

cheers guys - thx for your input!

10

geez this is a no win situation if the culprit was indeed the cable it sounds like there’s a great amount of civil works will going to happen base on your description, if this is not the cable then you are back to drawing board I hope whatever works for you the odds will be on your favor :slight_smile:

11

Hi Guys!

Guess what? This little troublemaker right here → Ubiquiti ETH-SP-G2 :tada:

Turns out it was the sneaky culprit sitting happily on the mounting pole (surely, it was chilling right between the router and the CAT cable). Now that we’ve kicked it out of the setup, we’re not expecting any more link-down drama. Fingers crossed! :grinning_face_with_smiling_eyes:

Cheers!

12

Ah, you never mentioned before it was also part of the equation.
But good thing you found the bad guy !

13

Yep you’re right - because even I did not know :slight_smile:
This is offSite at a customer which even did not mention :stuck_out_tongue:

however - it looks like as fixed

14

I assume that the ETH-SP-G2 was installed as a partial lightning protection. Now there is none at all and the LTE router on the pole is a lightning rod. Your customer may want to install a fiber optic solution to protect the equipment inside the building.

15

You’re absolutely right – now that some time passed - I can safely confirm that this hardware was the source of all the trouble. Zero link issues since its removal.

The customer decided not to pursue any replacement – maybe relying on pure optimism as a defense strategy :wink: Still, I’d be curious: any fibre alternative you’d suggest?

16

I have no suggestions (no experience or specific expertise in this field).

Since the RBLHGGR is PoE powered, I am thinking towards keeping the Ethernet cable to the roof, install a Fiber-to-PoE media converter (powered from a properly protected outlet) indoor and run a fiber optic cable from the converter to the core router/switch for isolation. The converter should probably be installed as close to the LTE router as possible.

I your customer is interested in proper protection, you will probably want to consult an expert in this field.

17

Maybe he’ll be interested as soon a thunerstorm comes over his building :wink:

however - thx for any of your help guys!

18

Try to disable “Energy Efficient Ethernet” in your cable modem. If it’s a device from AVM, this setting is hidden in a support menu.

19

Maybe it was just faulty and a replacement device does fix it as well?

20

Jep - think so - but what should I do if the customer dont want to :slight_smile: