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Rather frequently, my iMac with a 1Gb ethernet drops to 100Mb/s connection speed. I can unplug the cable, plug back in a second later, and the speed restores to 1000Mb/s. I recently discovered that I can get the same positive effect by executing:

sudo ifconfig en0 down
sudo ifconfig en0 up

In either case, it's very annoying and not a problem I've experienced on my MacBook or older iMac. I've also tried re-installing the entire OS from scratch, replacing cables, and replacing network switches and routers. So, all things point to the iMac ethernet adapter or macOS. Oh, and restarting the system also corrects it. So, it loses speed over time and I haven't correlated it to any particular application or event.

What could be happening and how do I fix it?

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  • Does this answer your question? macOS artificially restricts speeds to 100 Mbps over Ethernet
    – Alper
    Commented Oct 28, 2021 at 3:49
  • Thanks @Alper, I've installed iperf and when performance drops again, I'll bind to the en0 interface and test between the two iMacs to see if that also drops to 100 Mbps vs. just using internet speed tests. At the moment I'm getting 16.4 Gbits/sec on one interface and 939 Mbits/sec on the problematic one.
    – Michael Prescott
    Commented Oct 28, 2021 at 4:07
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    In case the answers on the other questions don't help to solve the prolem, please edit the attempts you've made into your question here. It will then get put into the review queue for reopenings.
    – nohillside ♦
    Commented Oct 28, 2021 at 5:20
  • 16.4 Gbits/sec !?! Typo?
    – Tetsujin
    Commented Oct 28, 2021 at 7:27
  • No typo. That interface is a thunderbolt connection between the two iMacs. It's really nice.
    – Michael Prescott
    Commented Oct 28, 2021 at 11:47

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