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It was not in fact the motherboard port... :( Ok, the warnings are because of the WinBox itself. But what is up with all the link up and downs? This is after switching to an SFP28 port on a ConnectX-4 PCIe NIC.

Decided to run some simple connectivity tests with cnping, and my god, what is this. Intermittent failures all around, usually for exactly one second. I don't think I've ever seen this before... Well, at least it looks like the failure has a distinct signature.

So. Intermittent 1 second long connectivity failures. Affects something between PC <-> switch <-> test-host. Not PC port/NIC, not test-host, these were all swapped. Probably not cables/SFP modules. There's only one problematic part left to test... Yep, failure only happens to the PC. It's software.

(a day later) I was drafting this late yesterday and shelved until today. Now, no issues at all for like 12h+ already. Was it software? Was it an SFP module? I don't think anything changed besides the weather outside... Anyway, now I switched to an SFP28 DAC for better reliability, just in case.

And as I discovered in the original post, the switch only officially supports 4 SFP+ to RJ-45 modules (probably bc power), so swapping one of the modules for a DAC is a good idea all around. Now I'm at exactly 4 active modules. Was that the problem? This fixed itself before the swap, so no...? 🤷