Gigabit card only connecting at 100mbps? Here’s the fix.

Last Updated on May 9, 2023 by Dave Farquhar

I finished a modernization project where I replaced all of my 100-megabit gear with gigabit-capable gear, including my cabling and router and access points. But after I replaced my last 100-megabit switch, I found we had two Windows desktops refusing to speed up. Here’s how to fix a gigabit card only connecting at 100mbps.

First, if you know you’re not connecting at gigabit, you probably already know how to do this. But if not, here’s how to check your network speed in Windows 10. Then here’s how to fix it. After all, you want to enjoy the advantages of a gigabit LAN if you have the hardware.

Make sure your driver isn’t hard coded for 100mbps

Gigabit card only connecting at 100mbps - check your driver
Check your driver to make sure you haven’t hard-coded a slower speed. Use auto negotiate whenever possible. This may not be the most common cause of a gigabit card only connecting at 100mbps, but it’s easy to check.

I only say to do this first because it’s usually easier to check than the cables. Open your network properties and check to make sure you didn’t hard-code a slower speed.

The easiest way to do this in Windows is to right-click your network icon in your system tray and click either Network and Internet Settings, then click Network and Sharing Center, then click change adapter settings. Right-click on your wired network card, click Properties, then click Configure. One of the options will be labeled Speed and Duplex. This is faster than going through Device Manager.

Generally you should set your link speed to auto-negotiate. Some drivers may give you the choice to set it to gigabit. If yours does, try hard-coding it to gigabit to see if it works. But frequently if it won’t auto-negotiate gigabit, it won’t sync up if you try to force gigabit either. The card may negotiate down to 100 megabit or fail to connect entirely and indicate the link is down.

Some drivers won’t allow you to hard-code gigabit connection speed, as it violates the IEEE standard. In my experience, Broadcom drivers do not, although other brands, including Intel, usually do.

Make sure your cables are good and rated for Gigabit

Gigabit card only connecting at 100mbps - check the cable
Frequently the cheapest and easiest fix for a gigabit card only connecting at 100mbps is just to replace the network cable.

Cables should normally be the first thing you check. I make it second since it’s usually easier to check the driver. I’ve had network cables not only stop connecting at a gigabit, but sometimes stop connecting at all, for no apparent reason. Also remember only CAT5e cable or faster (which includes CAT6 and CAT7) are qualified for gigabit Ethernet. Some cables aren’t labeled very well.

First, just unplug the Ethernet cable from each end and plug it back in, and watch the connection lights. A loose connection won’t sync up well. If the cable has a hard time catching, try alternately pushing it in or pulling it out slightly and see if that gives you a more secure connection.

When in doubt, swap the Ethernet cable for a known-good one if you’re only getting 100mbps on CAT5e or better. I’ve also found that some cards are pickier about cables than others. If you do this a lot, a network cable tester can save you a lot of time. But frequently it’s cheaper just to pick up a couple of spare CAT6 cables on Ebay.

Try different ports

While you’re checking cables, you might as well try different ports if you’re getting 100mbps instead of 1gbps. It’s possible for one switch port to go bad. It’s not something I see frequently, but I’ve seen it happen.

If your switch is giving you trouble, a used Dell Powerconnect 2816 switch is a good buy. You should be able to get one for around $40. They’re reliable, and efficient. I get less lag and far fewer dropped connections since I upgraded to one.

Try a known-good computer

My laptop has a gigabit port on it, and so do my sons’ laptops. So I tried my laptop and one of my sons’ laptops to try to eliminate all other possibilities.  If they couldn’t connect at gigabit speeds with a known-good cable, it’s either a bad port on the switch, or a bad cable run in the wall.

While you’re testing, try every switch port. It only takes a few seconds to verify all of them are good.

Change drivers

This rarely works, but most network chipsets give you three choices of drivers: one from Microsoft, one from the company who sold you the card, and one from the company who made the chip on the card. You can try different drivers and see if one of those will sync up at gigabit.

The last time I saw this work was in 2008 or 2009, and clearly it was due to a bug in a network driver that one of the alternative drivers fixed. Most network drivers are pretty mature by now, so I wouldn’t expect this to do much for you this decade, but it’s usually easier than changing hardware–which is the next thing on the list.

Swap out the network adapter for a better Gigabit adapter

Usually if a network adapter won’t sync up at gigabit, it’s because of a busted pin in the Ethernet port or because the card sustained damage, possibly due to a power surge. Realtek-based cards are really cheap these days. Be sure to get one that fits in whatever slot you have available: PCI vs PCI-e, full height vs. low profile. But an appropriate Realtek card ought to cost well under $10.

Gigabit card only connecting at 100mbps - replace the card
If you have a gigabit card only connecting at 100mbps, the last resort is to replace the card with another one. A quality secondhand Intel card like this one often costs less than $15.

If you want a bit more quality, look for a used Broadcom or Intel-based NIC. Again, match up PCI/PCIe, full height/low profile according to your system’s needs, and look for one that’s fully tested. You can find as-is used NICs really cheap, but there’s no guarantee it works at gigabit if the seller didn’t fully test it. Also beware of PCI-X cards. They’ll work in a regular PCI slot if they fit, but they won’t always fit. PCI is cheaper than PCIe, but PCIe is the future. If you have an available PCIe slot, that’s the better option.

Broadcom and Intel cards are less demanding on the CPU, and tend to be made of higher-quality components since they usually go into corporate environments. I’ve talked about the advantages of an Intel NIC over a Realtek. Under Windows, the difference between Broadcom and Intel isn’t great, but if you also run other operating systems, Intel cards have better driver support.

I’ve fixed a lot of weird problems over the years by replacing Realtek and other low-end cards with Intel or Broadcom gigabit cards.

Don’t mistake local LAN speed for Internet speed

If you have gigabit cards and you’re getting speeds of 100 megabits on Internet speed tests, remember that your local LAN speed and your Internet connection speed can be two different things. Even if you get gigabit Internet, only expect to get 80-90 percent of the speed.

If your Internet speed tops out at 100 megabits and you’re paying for faster than 100 megabits, make sure your router has gigabit Ethernet ports on it.

Gigabit card only connecting at 100mbps: My verdict

In my case, one of the cards started working on the second or third cable I tried. Oddly, the cable the Intel card in that computer didn’t like worked fine with the Intel cards in my laptops. Like I said, some cards are picky.

The other computer required some surgery. I had a spare Intel PCI gigabit card on the shelf. I swapped it into the system that fit. It only connected at 10 megabits, no matter what I did. That might have been why it was on the shelf. So I bought a $7 Broadcom PCIe card. It arrived in five days. When I plugged it in, it connected at gigabit. Problem solved. And it’s worked great ever since.

Back in early December, a transformer in our neighborhood took a lightning strike in a rare winter thunderstorm. Some of the networking equipment didn’t come back. Apparently the network card in that system sustained some damage too.

Fortunately the days when network cards cost $50 or more are long gone. I got a replacement card that was better than the original for about the price of a fast-food lunch.

It took a little doing, but that’s how I solved my problem of a gigabit card only connecting at 100mbps.

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One thought on “Gigabit card only connecting at 100mbps? Here’s the fix.

  • January 17, 2018 at 10:09 am

    My network speed was half after upgrading to Insider build 17063 on a Dell E6400 with SSD and SSHD.
    Desktop Asus Z170 Premium no problems.
    Laptop with old harddisk containing Insider build 16237 gave no problems, normal speed! had to upgrade to build 17063 and same configuration also went down to half of my speed. Awaiting build =>17074 when that comes available

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