Why is my gigabit Ethernet link limited to 100 Mbps?
Asked by
MellowPine47
My internet plan provides 300 Mbps, but my PC's wired connection is stuck at 100 Mbps. The PC uses the integrated gigabit LAN on an ASRock Fatal1ty Z77 Performance motherboard, connected to a gigabit-capable router with a 15-meter Cat 5e cable. I've updated and reinstalled the network drivers, but Windows shows the connection as 100.0/100.0 Mbps. In the adapter's Speed & Duplex settings, 1.0 Gbps is not listed at all; 100 Mbps Full Duplex is the highest option. Could the cable be missing conductors or have a damaged wire, or is this more likely a driver, network adapter, or port issue?
Answers
Make sure the installed driver matches the exact LAN chipset on the motherboard. Download it from the chipset manufacturer if possible rather than relying only on an old motherboard support package. Uninstall the current network adapter and driver, reboot, and install the downloaded driver. Afterward, leave Speed & Duplex set to Auto Negotiation instead of forcing a speed.
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A bad cable usually causes gigabit autonegotiation to fall back to 100 Mbps, but the 1.0 Gbps option would normally still appear in the adapter settings. Since that option is completely missing, the driver or the detected network adapter is the more likely problem. Try removing the adapter and its driver from Device Manager, then install the correct driver for the actual onboard chipset. Testing with another Ethernet adapter would also help separate a driver problem from a faulty onboard port.
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Check whether you’re testing the router’s LAN connection rather than its internet WAN port. Some routers have gigabit LAN ports but only a 100 Mbps WAN port. However, because Windows itself reports a 100 Mbps link and does not offer 1.0 Gbps, the immediate issue is between the PC’s adapter, cable, and router port. Try another router port and cable, then test with a separate PCIe or USB gigabit adapter if possible.
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A 15-meter Cat 5e cable should support gigabit without a problem; gigabit Ethernet is designed to run much farther than that. Still, a cable with only four connected wires, a damaged conductor, or a bad connector will negotiate at 100 Mbps, so swapping in a known-good Cat 5e or Cat 6 cable is an easy test. All eight contacts should be wired correctly at both ends.
SilverLynx58: Cat 5e is normally fine for gigabit at that distance. Cat 6 is reasonable as a replacement, but there’s no need to pay extra for Cat 7.
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MellowPine47: I previously used USB internet sharing and was able to reach roughly 120–180 Mbps, although it was inconsistent, so the PC can receive more than 100 Mbps.