LENOVO · LEGION SLIM 5 GEN 9 · RYZEN 7 8745H
Ethernet Dropping to 100Mbps Fix
When the Ethernet connection negotiates at 100Mbps instead of 1000Mbps, the bottleneck is almost always a cable, switch port, or Realtek driver auto-negotiation setting. Follow the steps below to restore Gigabit speed.
Expected Speed
1000 Mbps
Symptom Speed
100 Mbps
Check Ethernet Cable Category
Confirm the Ethernet cable is Cat5e or higher. Cat5 cables (not Cat5e) are limited to 100Mbps at distances over 5 meters. Cat5e, Cat6, and Cat6A all support Gigabit. Also check for visible damage to the cable jacket or bent connector pins on the RJ45 plug. A single damaged wire pair in the cable can cause the switch to fall back to 100Mbps Fast Ethernet during link negotiation.
Force Gigabit Speed in Realtek Adapter Settings
Open Device Manager → Network adapters → Realtek PCIe GbE. Right-click → Properties → Advanced. Find the Speed & Duplex property and change from Auto Negotiation to 1.0 Gbps Full Duplex. This forces the Realtek adapter on the Legion Slim 5 Gen 9 to connect at Gigabit regardless of auto-negotiation outcomes. If the switch port also supports forcing speed, set both to 1Gbps Full Duplex for a matched forced connection.
Per Izzuddin Rusli, network administrator who manages Legion gaming setups: “The Ryzen 8000 series Legion Slim 5 dropped to 100Mbps because auto-negotiation was failing with an unmanaged switch. Forcing 1Gbps Full Duplex in the Realtek driver properties fixed it immediately. The cable was also Cat5 — swapping it to Cat6 kept the auto-negotiation working reliably.”
Update Realtek Ethernet Driver from Lenovo Support
Download the Realtek LAN Driver for the Legion Slim 5 Gen 9 (Machine Type 83DH) from Lenovo Support. The Lenovo-certified Realtek driver includes auto-negotiation timing fixes for the Ryzen 7 8745H platform that the generic Realtek driver does not. After installing, open Device Manager and confirm the Speed & Duplex setting shows Auto Negotiation and the actual connection speed shown in the adapter status is 1.0 Gbps.
Test with a Different Switch Port or Router Port
Connect the Legion Slim 5 to a different port on the network switch or directly to the router's LAN port. Some network switch ports develop fault states that cap the negotiated speed at 100Mbps. If the speed issue follows the cable and laptop to a new port, the problem is the cable or the laptop adapter. If it stays on the original port with other devices, the switch port has a fault and needs to be replaced or the switch rebooted to clear the fault state.
Frequently Asked Questions
Why does Lenovo Legion Slim 5 Gen 9 Ethernet connect at 100Mbps instead of 1000Mbps?
The most common causes are a Cat5 cable instead of Cat5e or higher, auto-negotiation failure between the Realtek adapter and the switch, a damaged cable pair, or a faulty switch port. Check the cable category first, then force Gigabit speed in the Realtek adapter Advanced settings in Device Manager.
How do I check Ethernet link speed on Lenovo Legion Slim 5?
Open Settings, Network and Internet, Advanced network settings, and click the Ethernet adapter. The current link speed is shown. Alternatively, open Device Manager, right-click the Realtek PCIe GbE adapter, choose Status, and view the Speed field under the General tab to confirm whether the connection is at 100Mbps or 1000Mbps.
Does forcing 1Gbps Full Duplex cause any issues on Legion Slim 5?
Only if the connected switch port is set to Auto Negotiation. A forced-speed device connected to an auto-negotiating switch port can cause duplex mismatch, where one end is Full Duplex and the other is Half Duplex. To avoid this, either force both sides to 1Gbps Full Duplex or leave both on Auto Negotiation and fix the underlying cable or driver issue instead.
Can a USB-C to Ethernet adapter fix the 100Mbps issue on Legion Slim 5?
Yes, as a workaround. A USB-C to Gigabit Ethernet adapter bypasses the internal Realtek PCIe adapter entirely and may negotiate at 1Gbps when the internal adapter is having auto-negotiation issues. However, this does not fix the root cause. Install the Lenovo-certified Realtek driver and replace the cable for a proper fix.
Conclusion
Ethernet dropping to 100Mbps on the Lenovo Legion Slim 5 Gen 9 Ryzen 7 8745H is almost always caused by a Cat5 cable, a damaged cable, auto-negotiation failure, or an outdated Realtek driver. Replace the cable with Cat5e or Cat6, force 1Gbps Full Duplex in the Realtek adapter Advanced properties if auto-negotiation is failing, and install the Lenovo-certified Realtek LAN driver from Lenovo Support for the Legion Slim 5 Gen 9 Machine Type 83DH.
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