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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?

Michael Prescott
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  • Does this answer your question? macOS artificially restricts speeds to 100 Mbps over Ethernet – Alper Oct 28 '21 at 03:49
  • 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 Oct 28 '21 at 04:07
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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 Oct 28 '21 at 05:20
  • 16.4 Gbits/sec !?! Typo? – Tetsujin Oct 28 '21 at 07:27
  • No typo. That interface is a thunderbolt connection between the two iMacs. It's really nice. – Michael Prescott Oct 28 '21 at 11:47
  • @Alper 's reference doesn't really address this problem. Good to know about iperf usage, but I don't see anything in my router causing the slow down. I did watch network preference panel for a while, inspired by that issue, and saw the Ethernet drop/deactive and a second later it re-activated negotiated from 1Gbps to 100Mbps. I think the issue is macOS or the adapter. Just don't know how to fix it properly, yet. I can force it to drop again as noted above and it picks up to 1Gbps again, but that is such a kludgy solution. – Michael Prescott Oct 28 '21 at 11:51
  • The thunderbolt connetion is just a red herring & not pertinent to the issue, then. If your hardware interface is dropping to 100BaseT, then suspect physical connectivity issues. Change cables, get contact cleaner on the plugs & sockets & insert/remove a dozen times rapidly [with all power removed from the connectors] – Tetsujin Oct 28 '21 at 14:00
  • I crimped CAT 7 cables myself and they're run through walls and attic, so changing those weren't an option. I tried new patch cables with no resolution. So, I got a new switch. No problems so far. I think that is near the root of the issue. It's interesting that no other 1Gbs adapters have problems with the old switch. Only the iMac with a 10Gbps adapter (re-discovered/reminded of that). Anyhow, seems ok, I hope it lasts. I wonder when 10Gbps switches will become norm. Right now, a copper 10Gbps is way too expensive. – Michael Prescott Nov 02 '21 at 09:19

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