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RaivisR
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Joined: Wed Mar 31, 2021 11:15 am

Flash corruption when powered over PoE

Wed Mar 31, 2021 11:46 am

Hi,

My cAP ac that is powered over PoE forgot what it was supposed to do over the winter. It is located in unheated premises with elevated humidity. That is important detail, because apparently both RJ45 jacks got oxidized over the course of two years the device has been in service.

Symptoms, device would attempt to boot, power light would soon go out and device would fail to complete the boot sequence.

What I believe happened - due to oxidation of RJ45 jack contacts resistance rose and power delivery was very marginal, the moment device reboots, because power delivery is on edge, for some reason it tries to write to the flash, probably at the same time powers up radio or does something like that and current consumption rapidly rises, which leads to elevated current consumption over PoE, but because of higher resistance voltage sags and switching PSU in the device is unable to deliver required power, brownout circuit kicks in, CPU gets reset and flash is in the middle of write operation. Writing to flash requires sectors you are writing to to be erased first, so flash modify would look like read the sector, erase sector, change required bytes, write back sector, which means that one or more sectors of flash at some point during that operation are erased and they as well may contain configuration or parts of RouterOS itself.

In my case only configuration was damaged, but I got reports from two other users with two different devices who had to use netinstall in similar scenarios to revive the devices. One user had his device powered from PoE, the other one had self made solar panel+battery setup, which he admitted often was not delivering enough power.

What users can do to prevent this from happening - make sure your connectors are firmly seated, contacts clean and if used in environment where corrosion/oxidation is likely to happen, use some corrosion prevention spray.
What Mikrotik can do to fix this - don't write to flash, at least not until device if fully operational or partition the flash and only write to parts that are not essential for device boot.

I might be wrong and there is some totally different reason for this happening and it might be known issue with a fix that has been long ago applied, but I could not find any information on that, so decided to write it up here.

RR

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