I'm really struggling here. My new setup is stuck at a painfully slow 100 Mb/s for Ethernet and barely breaks 120 Mb/s on Wi-Fi. Meanwhile, all my other devices are getting speeds of 400-600+ on Wi-Fi and 650-700+ on Ethernet using the same cables and modem port. I've spent over 6 hours troubleshooting, and I'm running out of ideas. I've swapped multiple Cat-6 cables, plugged directly into the modem, power-cycled everything, updated BIOS, cleared the CMOS, performed fresh installs of Windows 11 and Pop!_OS, and tried all sorts of Realtek drivers. I even disabled settings like Green Ethernet and tried to force a 1 Gbps connection, but nothing has improved. My build features an AMD Ryzen 9 9900X3D, MSI MAG X870E Tomahawk WiFi, and other top-tier components, yet the Realtek NIC won't negotiate beyond 100 Mb/s. Any tips or suggestions before I consider returning this rig? Is there a known issue with the RTL8125B, or could it be a faulty port?
5 Answers
Make sure your NIC settings are optimized. In Device Manager, check under properties that your NIC is set to "Auto Negotiation" for best speeds. Also, check if there's a setting for the 5 GHz band on your Wi-Fi adapter. A faulty pin in the Ethernet port could also be the culprit, so ensure everything looks good there.
It sounds like your NIC might be faulty if it won't negotiate a gigabit connection while other devices work fine with the same setup. Instead of going through the hassle of an RMA, you might just want to grab a PCIe NIC. They're pretty inexpensive and could solve your problem easily without more headaches.
Just a quick note: when I use a USB dongle, my speeds jump back up to what they should be. If nothing else works, you might want to try one temporarily to see if it's an issue with the motherboard.
Have you thought about updating your chipset drivers? Since this is an MSI board, just double-check that you haven't installed the Killer network software, which can sometimes cause conflicts.
I've had a similar issue with my old gaming setup! It was linking at gigabit sometimes but mostly stuck at 100 Mb/s. I tried swapping cables and switches but eventually opted for a 2.5 Gbps PCIe card. Problem solved! Full speed now. Maybe give that a shot.
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