Level 1

26 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

25 replies

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

13,037 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

13,037 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.

BDAqua

Level 10

250,253 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.

Fraxie Author

Level 1

26 points

Aug 23, 2018 10:21 PM in response to Marco Klobas

Thank you again for all your replies and suggestions.

After trying a thunderbolt gigabit ethernet adapter, I have full 1000Mbps speeds again. I can only assume there was some kind of firmware mess-up with the OS update. At least that won't affect 3rd party hardware.

I can't be sure if any of the suggestions helped the situation, but I learned a few things, and at least I don't have to worry with it now.

Thanks again.

Cheers!

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

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1Gbps ethernet speed changed to 100Mbps