Looks like no one’s replied in a while. To start the conversation again, simply ask a new question.

Level 1

25 points

1Gbps ethernet speed changed to 100Mbps

I have 1Gbps fiber internet. It's been working great. After months and months, I finally updated MacOS from Sierra to High Sierra.

Nothing changed with the hardware. Nobody bumped the desk, the iMac, the network switch, or anything. Upon finishing the update, the network will not connect faster than 100Mbps.

The system is a Late 2013 27-inch iMac. It's has a perfectly functional 1Gbps ethernet for years. I update the OS and boom, the network is stuck at 100Mbps.

Interesting thing to note, even Windows 10 running in BootCamp failed to connect faster than 100Mbps... but only AFTER updating to macOS High Sierra. It never failed to connect at 1Gbps before the macOS update.

I tried resetting SMC.

I tried resetting NVRAM.

I booted into and out of Safe Mode.

I completely wiped the system clean and installed a fresh macOS install.

Nothing worked.

I even plugged a USB 3.0 1Gbps ethernet adapter into the system, and it connects at 100Mbps.

I have a Windows 10 PC right next to this, and it works at 1Gbps no problems. The USB ethernet connects at 1Gbps no problem. I swapped the ethernet cords just in-case. The iMac still connects at 100Mbps and the Windows still connects at 1Gbps both internally and using USB.

If I wasn't already bald, I'd be pulling my hair out! Can someone please help me?

Thanks.

iMac, macOS High Sierra (10.13.6), null

Posted on Aug 21, 2018 11:04 PM

Best reply

Marco Klobas

Level 6

8,255 points

Posted on Aug 22, 2018 12:14 AM

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

View in context

25 replies

Feb 26, 2019 8:40 AM in response to alexcmchan

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.

Feb 26, 2019 9:03 AM in response to JMBIELSKI

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.

Aug 22, 2018 2:30 AM in response to Fraxie

(ignore)

rccharles

Level 6

12,957 points

Aug 22, 2018 8:39 PM in response to Fraxie

pita. You could try loading whatever macOS you had before and see what you get.

Aug 23, 2018 10:38 PM in response to Fraxie

Glad to know the Thunderbolt adapter works and that eventually you have full Ethernet speed back. 🙂

rccharles

Level 6

12,957 points

Aug 24, 2018 9:10 AM in response to Fraxie

Thanks for reporting back.

Very strange.

Aug 28, 2018 3:17 AM in response to Fraxie

I'm having the exact same issue. AT&T says it's my Mac. Apple says it's ISP. This thread is helpful. Hopefully, there's a solution soon.

vokey

Level 1

4 points

Dec 10, 2018 11:31 AM in response to VikingOSX

I had this same problem....

then realized that WiFi was on at the same time.....it was overriding my GB Ethernet connection

I turned out WiFi....and problem solved

BDAqua

Level 10

239,213 points

Jan 4, 2019 6:06 PM in response to Cirigliano

Nice tip. :)

Jan 20, 2019 9:38 PM in response to kriskoeller

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.

1Gbps ethernet speed changed to 100Mbps