1Gbps ethernet speed changed to 100Mbps

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.

After a lot of troubleshooting with Apple, my ISP, and my router manufacturer, I was able to determine that it is the connector on the CAT 6 cable going from my router (through my attic)and the the jack on my wall. If you haven't already, try running a cable directly from your router to your iMac to rule out a bad cable or connector.

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.

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

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?

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

My imac always direct connect to the router using amp cat6 cable. And the setup never changed /moved since i bought it on aug17. I also checked the pins in the ethernet port, all are nice and shinny. when using usb-c to ethernet adapter with attach to same cable and same router port. It can reach 1000baseT. So rule out possibilities on router and cable. The issue is coming from my imac.

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.

Nice tip. :)

I've noticed the same issue on my late 2015 iMac possibly since installing Mojave. Did you ever resolve this and get back to full speed? My hardware has been stuck at 100baseT.

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?