[Bug 246003] em(4): Intel I219-V6 on NUC8i5BEH randomly loses carrier or fails over to 100Mbit

From: <bugzilla-noreply_at_freebsd.org>
Date: Thu, 17 Feb 2022 17:50:33 UTC

Joshua Kinard <kumba@gentoo.org> changed:

           What    |Removed                     |Added
         Resolution|Overcome By Events          |Not A Bug

--- Comment #8 from Joshua Kinard <kumba@gentoo.org> ---
Just to add a final resolution and to mark this as a hardware issue and not a
bug in FreeBSD, the problem ultimately turned out to be the 1ft CAT6A Monoprice
SlimRun cable I was using to connect the NUC to my 24-port switch.

I learned awhile ago that swapping to a different cable made the problem go
away, but I didn't know why.  After finally finding where my cable tester was
hidden, I was able to resolve that the slimrun cable had a faulty ground
connection between the two ends that was probably creating a ground loop
between the switch and the NUC, causing the switch to overreact and either
disable the port or drop it to 100mbps, which probably wasn't handled well by
the em(4) driver on the NUC.

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