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

Aug 22, 2018 10:05 PM in response to Fraxie

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.

Fraxie Author

Level 1

26 points

Aug 23, 2018 5:16 AM in response to Marco Klobas

Thanks again for the reply.

The firmware, file location (and name) and version are identical to your post.

Also, the output of the dmesg command shows the same AppleBCM5701.

While searching for answers to this, I did stumble upon this update problem, but the fix is not the same. Also the software patch that Apple hosted for that has since been removed.

As for the suggestion to install an older version of macOS, I cannot with my current work needs. I suppose I could just to wipe it out after the test, but I do need the latest version now.

I will be purchasing a thunderbolt ethernet adapter to see if that works.... it's a pain, but I need to try.

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!

VikingOSX

Level 10

123,054 points

Nov 1, 2018 9:48 AM in response to Fraxie

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.

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

(ignore)

rccharles

Level 6

13,027 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

13,027 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

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1Gbps ethernet speed changed to 100Mbps