1Gbps ethernet speed changed to 100Mbps

What you described is indeed quite annoying, especially considering that you tried various fixes to no avail.

What speed does your Ethernet device show (look at the screenshot below)?

A probably silly attempt, anyway I'd give a try:

Turn off your iMac.

Unplug everything from it (power cable, ethernet cables, USB etc.)

Press the power button (yes, without any power cable in – I know, this is the silliest part)

Connect everything back and turn on the Mac

Hi All,

I had this exact issue with my MBP mid 2015. Ethernet connection via Apple's thunderbolt adapter would only connect at 100Mbs. bringing my internet speeds to a miserable 80Mbs when I get 275Mbs on my 5G wifi connection. If I would switch the adapter hardware settings to manual and set it to 1000baseT it wouldn't even connect. After having my ISP come here and look at my router, all checked out well with them. So no issues with the router. Now I call Apple and get to top level tech and we try all sorts of things and nothing is helping.

What I noticed while waiting on hold, I pulled out the ethernet cable from the adapter and noticed that one of the gold pins in the adapter was slightly bent down (for me it was the 4th pin from the left). It wasn't bent very much but I could see a difference from the other pins. So I grabbed a sewing needle and was able to gently get the needle under the one pins and carefully bent it back into place and BOOM! It worked at 1000baseT again.

In short, check the pins in your adapter see if any of them look out of place even just a little from the other pins. Try to bend them back with a needle but be careful they can break very easily. If it breaks, you will have to purchase another adapter.

This more than likely happened by being thrown into my backpack with other adapters and pens etc. Im guessing the adapter and a pen got into a fight and the adapter survived with minor injuries this time. I will keep them separated now on since they can't get along together.

Apple did a great job trying to troubleshoot and were top class even though it ended up being an adapter issue.

I know you have now solved the problem but the most common cause I have seen is a cabling issue.

1000Base-T needs a minimum of CAT5 cable with all eight wires (four pairs) correctly connected.

100Base-T only needs a minimum of CAT3 cable with just four wires (two pairs) correctly connected.

Therefore if your cable is/was CAT3 this would at best cause intermittent problems and possibly always just 100Base-T, if one of the four extra wires became disconnected or was loose this would also cause a similar situation.

Glad to hear you fixed it. I cannot afford to lose my Thunderbolt ports, so I was forced to continue hunting.

The following solved the issue for me:

System Preferences > Network > Select Ethernet on the left side > Select Advanced > Select Hardware Tab

Change settings to following:

Configure: Manually

Speed: 1000baseT

Duplex: full-duplex, energy-efficient-ethernet

MTU: Custom - 3000 and above worked for me

AVB/EAV Mode: Checked

These settings were figured out through trial and error on all the permutations essentially. The only thing that I found affected speeds were the Speed (must be 1000baseT) and the MTU it needed to be set to a number approx 3000 or higher. I'm not a network engineer so I don't know the problems i'm giving myself here by upping the MTU, so perhaps I'll have to change that back later, but as of right now everything is working perfectly.

iMac 21.5 inch, late 2013 running High Sierra.

I'm also experiencing this. I noticed my download speeds seemed to have been capped in the 90s after months of ~850-900 (Verizon FIOS). I don't know for certain, but this seems to have started since installing MacOS Mojave.

Similar to others, the hardware seems to be fixed at 100. Despite others, however, manual configuration doesn't seem to help. I've verified my cable; its the same one I've been using for months at a gigabit speeds before this update. I've also tried some of the reset options but to no avail.

Just to say ...

2019-02-27 as dated...

Mac OSX Mojave 10.14.3 on iMac

I had this problem too.

I tried all the settings in Network Preferences/Hardware to no avail.

I thought it may have been an issue with the new NetGear GS105 10/100/100bt Switch. It was not.

It was simply faulty ethernet cables.

Try this: System Preferences -> Network -> select Ethernet in the sidebar -> remove the service (minus icon on the bottom) -> add a new Ethernet service again (plus icon on the bottom).

Another thing: open System Information (Option +  -> System Information). What do you have listed in your Ethernet Controller under Hardware -> Ethernet Cards?

OK. Check if the detailed informations about Broadcom 57766-A1 match, among other things, these infos:

Firmware version: 57766a-v1.15

Location: /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleBCM570 1Ethernet.kext

Version: 10.3.1

Another test:

Right after booting your iMac, execute the following terminal command:

sudo dmesg

You'll be asked for your system password.

In the output look if it's present something related to AppleBCM5701 and/or Ethernet.

Recently (well, 2 years ago) some users experienced broken Ethernet after updates. I mean, completely broken – not capped like in your case. The solutions was to execute the following terminal command:

sudo softwareupdate --background

I'd give a try even though it's not exactly your issue.

Here's a thread about this: https://www.reddit.com/r/apple/comments/47tfos/warning_software_update_03151913_ breaks_ethernet/

I don't know, probably it's related to the Ethernet Kext (kernel drivers) or even the iMac firmware.

If you have a spare external disk you can install an older macOS version on it and boot it to check if it works without touching your current working configuration.

You said that you swapped the cables with a Windows machine and even installed a USB-Ethernet adapter. When you did this did you keep the same switch/router port? Have you already tried to use physically another port on your switch/router?

Ethernet hardware is rated in megabits per second (Mbps), and the true theoretical maximum that you will experience is megabits divided by 8-bits per byte, or 125 Megabytes (MB) per second on a 1GigE interface, with CAT5E LAN cable. Due to network collisions, and other protocol traffic on your LAN, you will be lucky to achieve 100MB/s traffic on your local LAN. Less if your LAN traffic is passing through older 100BaseT switches.

The fiber speed to your location from the ISP has no bearing on your real world internal LAN speed per the first paragraph's limitations. That 125MB/s is the gas pedal all the way down, and there is no more.

Apple's Network Utility only reports the interface rating as a constant, not the actual instantaneous LAN performance.

I got similar issue, my iMac mid 2017 shows cable unplugged. However, when change hardware setting to automatic, it's connected but in 10baseT/Full duplex only. Other setting will be disconnected with cable unplugged message. After SMC/NVRAM, reinstall o/s, install older version o/s on external hard disk still in vain. Contacted Apple support online also no help at all. So decided to use USB-C to Giga Ethernet instead of built-in network port.

If anyone have same issue, pls post here, to let apple solve it asap.

Thanks