Ethernet speed stuck at 100 baseT

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Level 1

8 points

Ethernet speed stuck at 100 baseT

For three years I have been using my Ethernet port for my main work computer at 1000 BaseT. It got me 950 MB/sec with ISP ATT fiber. Suddenly it is stuck at 100 Base T. Switching to manual in hardware tab of network settings brings no change. Even switching with my wife's ethernet that is giving 1000 BaseT on her 2015 iMac at her desk, my machine always reverts to 100 BaseT.

It is not the cables. It is not the router. It is not in the preferences. It is not the PRAM. It's not the switches. I am riddled!

iMac Pro, macOS 11.6

Posted on Oct 17, 2021 7:44 AM

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Answer 1

Macwilli Author

Level 1

8 points

Oct 21, 2021 5:09 AM in response to ku4hx

I have tried something else: Using an adapter from USB-C to Ethernet. Same thing - only 100 base T possible. So it is definitely not a hardware problem. Software must be the issue. Will wipe SSD clean and install Monterey once available.

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Answer 2

ku4hx

Level 7

23,694 points

Oct 18, 2021 12:41 PM in response to Macwilli

Have you considered reinstalling your OS from the recovery panel? Settings, documents and personal data will not be disturbed. However, backup first just to be certain.

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Answer 3

Macwilli Author

Level 1

8 points

Oct 18, 2021 12:36 PM in response to ku4hx

This is my iMac Pro, 2 years old. I guess either hardware failure or something deep in the OS got stuck. When negotiating for speed it takes a long time - much longer than before. Even the oldest Mac mini 2009 runs at 1000BaseT. Maybe the new OS Monterey will help.

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Answer 4

ku4hx

Level 7

23,694 points

Oct 17, 2021 10:00 AM in response to Macwilli

Maybe your ISP is meddling with their signal. If it's not your equipment, what's left? Or maybe your network card is failing. How old is your Mac? Age is not always a cause, but failures do tend to follow age in some cases.

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Ethernet speed stuck at 100 baseT

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