Ubiquiti UniFi AP AC Lite randomly eth0: link down link up

Installing and Using OpenWrt Network and Wireless Configuration
1

I have this randomly link down and up on my Ubiquiti UniFi AP AC Lite that currently run 19.07.2 but this happened also with .0 and .1.
Any idea about why this behavior?

Mon Mar  9 14:09:06 2020 kern.info kernel: [13024.901750] eth0: link down
Mon Mar  9 14:09:06 2020 kern.info kernel: [13024.905282] br-lan: port 1(eth0) entered disabled state
Mon Mar  9 14:09:06 2020 daemon.notice netifd: Network device 'eth0' link is down
Mon Mar  9 14:09:08 2020 kern.info kernel: [13026.983216] eth0: link up (1000Mbps/Full duplex)
Mon Mar  9 14:09:08 2020 kern.info kernel: [13026.988039] br-lan: port 1(eth0) entered blocking state
Mon Mar  9 14:09:08 2020 kern.info kernel: [13026.993478] br-lan: port 1(eth0) entered forwarding state
Mon Mar  9 14:09:08 2020 daemon.notice netifd: Network device 'eth0' link is up
Mon Mar  9 14:10:33 2020 kern.info kernel: [13112.261084] eth0: link down
Mon Mar  9 14:10:33 2020 kern.info kernel: [13112.264615] br-lan: port 1(eth0) entered disabled state
Mon Mar  9 14:10:33 2020 daemon.notice netifd: Network device 'eth0' link is down
Mon Mar  9 14:10:36 2020 kern.info kernel: [13114.342441] eth0: link up (1000Mbps/Full duplex)
Mon Mar  9 14:10:36 2020 kern.info kernel: [13114.347268] br-lan: port 1(eth0) entered blocking state
Mon Mar  9 14:10:36 2020 kern.info kernel: [13114.352712] br-lan: port 1(eth0) entered forwarding state
Mon Mar  9 14:10:36 2020 daemon.notice netifd: Network device 'eth0' link is up
2

Did you make any firewall or interface changes?

3

The configuration is like a Dumb Access Point with firewall, dnsmasq and odhcpd services disabled.
The AP interface are the defaults.

4

...during the time period in the log in question, specifically?

5

I didn't any configuration change during the down up events.
This down up happen randomly 0 time a day, 2 times a day, 5 times a day or more and the time between the event always change 1 min, 1 hour, 3 hours.....
I can't identify any associated event that happen and that may explain the down up.

Router side (ER-X) the log show this

Mon Mar  9 14:09:05 2020 kern.info kernel: [13022.119611] mtk_soc_eth 1e100000.ethernet eth0: port 1 link down
Mon Mar  9 14:09:08 2020 kern.info kernel: [13024.797675] mtk_soc_eth 1e100000.ethernet eth0: port 1 link up
Mon Mar  9 14:10:33 2020 kern.info kernel: [13109.302385] mtk_soc_eth 1e100000.ethernet eth0: port 1 link down
Mon Mar  9 14:10:35 2020 kern.info kernel: [13111.927068] mtk_soc_eth 1e100000.ethernet eth0: port 1 link up
7

As a precaution change the cable.
Is the AP taking power from power supply or PoE?

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8

PoE with the original adapter included with the AP

9

I have noticed some random AP restarts and disconnections on my 2 UAP-AC-Lites. They are on the Unifi firmware though. The unifi software doesn't provide much information either, only that the devices rebooted or that they were disconnected. So I cannot understand what is the reason. They both take power from a different PoE switch, which are connected on UPS.
Do you have any brown-outs in your premises? Unlike black-out that the power goes completely off, in brown-out the voltage drops to a lower level, devices might still function but not at full capacity. You can see it on lightbulb that dims.

10

I guess that I haven't brown-outs considering that I live in city new house but the new LED lightbulb observation may not be a good meter to check this out.
I changed the cable as you suggested, let see if this fix the problem.

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