Windows is setting a 1GBit connection down to 100mbit

My current network should support wired 1Gbit connection. It used to be 1Gbit on my PC, but some time ago Windows started to see it as 100mbit. After trying several things, I am 100% it is an OS issue.

Things I've tried so far:

• fiddled with device settings, set up speed to 1Gbit full-duplex, without downgrading connection speed, disabled Gigabit Lite - didn't help
• bought new Cat6 cable, to rule out bad cable - didn't help
• tried upgrading NIC drivers - didn't help
• disabled hibernation in Windows - didn't help
• bought an USB3.0 NIC, to rule out faulty card - didn't help
• set up another PC, connected both old and new cables - second PC detects as 1Gbit on both cables
• tried using Linux Live distro on main PC - detects as 1Gbit on both cables and NICs

This suggests it is a Windows installation issue, but I'd very much like to avoid reinstalling Windows. Is there anything else I could try out?

• on-board NIC is Realtek PCIe GBE Family Controller
• driver is 10.10.714.2016, unsure what it was before updating
• second PC has driver version 9.1.410.2015. I'm unsure if reverting to that version could help, tried doing so but failed - Realtek drivers are pain to install
• main PC winver is 1909 (build 18363.592), auto-update enabled and OS is up to date
• second PC winver is 1903 (build 18362.592), auto-update enabled

3 Answers

Late and I think the issue is the cabling. Perhaps because I'm old enough to remember when it was a huge deal getting a CAT 5. Non e because they weren't available quite yet. Sorry for reminiscing, but I was surprised that nobody mentioned it. Maybe it's so we'll known it's a given, or maybe a long lost internet fact from the days of yore either way, CAT 5 cable tops out at 100Mbps. It looks very similar to CAT 5e but it can handle 1Gbps. Look closely at the insulation the type should be printed, dot matrix-style.

Reviewing & Confirming what hasn't solved the problem yet

You may want to check the settings on your router as I've heard that even if you set the adapter settings to full duplex, if the router is set to automatic, it may sometimes cause undesirable effects.

Otherwise, I've noted you've mentioned fiddling with these settings (as well as as mentioned in the comments) just to confirm in case others coming to this page miss what to try for themselves:

• Speed & Duplex ("set up speed to 1Gbit full-duplex, without downgrading connection speed") Since you've set it on your adapter to 1.0 Gbps Full Duplex, you may want to try changing it back to Auto Negotiation if after doing the other Settings listed still doesn't help and in case your router is set to automatic.
• Gigabit Lite ("disabled Gigabit Lite") - Leave disabled
• WOL & Shutdown Link Speed ("...is Not Speed Down") This shouldn't effect anything as it's a Wake on Lan setting for when your computer is off or in standby, set however you want.
• Windows Power Plan is High Efficiency. - You may want to try setting it to High performance instead or open advanced and change the specific network adapter adapter setting within your preferred power plan
• And you confirmed that Auto Disable Gigabit, Green Ethernet, Energy Efficient Ethernet are all set to disabled as SQLTemp already mentioned.

Suggested Solutions

Now here are some other steps you can try:

• The advanced setting for Jumbo Frame set to disabled.
• In the NIC Properties windows under the Power Management tab, ensure that "Allow the computer to turn off this device to save power" is unchecked. You may also want to go one step further and check device manager, organize it by connection, and ensure none of the devices your NIC is connected to has the power management checkbox active as well (but this is going extra)

If not, try...

Opening an elevated command prompt and run the following command:

netsh int tcp set global autotuninglevel=disabled

if that doesn't help you can re-enable it with

netsh int tcp set global autotuninglevel=normal

In case it may be related to Windows Auto Tuning

If not, try...

Instead of updating your drivers, try rolling back to an older driver. Some users on have reported encountering the same issue following the 1903 or 1909 Windows update.

If not, try...

Configuring the duplex setting using power shell instead in case there may be other policies in place that override the settings you set using the GUI...

Open a Powershell window and run the following command

Get-NetAdapter | SELECT name, LinkSpeed, fullduplex | ft -autosize

Take note of the adapter name as shown (although you mentioned "Realtek PCIe GBE Family Controller" it might be just called "Ethernet" when running the powershell command) and under LinkSpeed, it probably wont say 1 Gbps if the error persists but may show something like 100 Mbps instead and if under the fullduplex column it shows True, then you can try running this command after:

Set-NetAdapterAdvancedProperty $adaptername -DisplayName "Link Speed & Duplex" -DisplayValue "1000 Mbps Full Duplex"

Whereas you need to replace $adaptername with the name of the adapter as noted earlier.

If none of these help, the only thing left I can think of is to see if you can uninstall the update (though this applies more for other people since this post is so long ago Windows won't let you do this) or the specific update for your NIC if one is listed under installed updates from Windows.

I am not sure this is your whole issue, but if you change the setting "speed & Duplex" from AUTO on the NIC then you also have to change it on the switch port....otherwise when the connection is first made to the switch and the switch attempts to auto-negotiate the connection speed it will not get a response and then the switch will downgrade to the lowest setting (100 or 10 mb) to make sure it has a connection of some kind so if you were to look at an error log in the switch you would have a speed/duplex mismatch error.....we used to run into this a lot setting up VOIP phone networks. Cabling isn't an issue if the connectors are done correctly because I have seen CAT3 do 100mb connections....