A contracted connection above 100 Mbps may appear “stuck” for completely different reasons. The operator's plan is one thing, the result of an Internet test is another and the speed negotiated by the Ethernet cable is a third measure. Diagnosis becomes much faster when you start by separating these three numbers.
If the system itself shows that the Ethernet link is at 100 Mbps, there is no point in starting by changing the Speedtest server or discussing the speed of the plan. This number describes the local negotiation between the computer's network card and the port on the equipment to which it is connected, such as a router or switch. TP-Link makes exactly this distinction in its documentation: link speed is not Internet speed.
The objective, therefore, is to discover on which link the negotiation falls from gigabit to Fast Ethernet. Cable, connector, wall socket, switch, LAN port, USB adapter, dock, network card and driver configuration can take part along the way. Change one variable at a time and see if the link goes from 100 Mbps to 1.0 Gbps.
First, find out what speed is limited
The contracted plan informs how much the operator promises to make available during the connection, within the service conditions. Speedtest measures the traffic that managed to traverse the entire path at that moment, including the local network, the router, the ISP connection and the test server. The Ethernet link speed shows the rate negotiated only in the physical section between the computer and the equipment immediately connected to it.
On Windows, a simple way to see this rate is to press Windows + R, type ncpa.cpl, open Ethernet Interface Status and look at the Speed field. TP-Link documentation, updated in July 2026, uses this same path and clarifies that the value displayed is the speed of the wired link. If 100.0 Mbps appears, investigation should begin on the Ethernet path. If 1.0 Gbps appears, the specific bottleneck of this local negotiation is not limited to
100 Mbps.
This difference avoids a common mistake. A computer can negotiate 1.0 Gbps with the router and still get a Speedtest well below that because of a slower WAN port, plan, ISP, router processing, or other test conditions. The opposite is also decisive: if the interface negotiates only 100 Mbps, this local link already establishes a ceiling before the traffic even reaches the Internet.
Why a gigabit device can only negotiate 100
Mbps The effective speed of a wired path depends on the components present in it. TP-Link points to the type of Ethernet port on the router, the cable and the computer adapter as direct factors. A port that only supports Fast Ethernet, for example, will not switch to gigabit because the cable has been changed. Likewise, a gigabit adapter cannot negotiate 1 Gbps if it is connected to a 100 Mbps port.
The cable can also create a misleading situation. Gigabit Ethernet over copper uses all four pairs of twisted pair cable. Therefore, a cable, connector or termination that has lost adequate continuity in part of the conductors may no longer support 1 Gbps and still maintain some type of connection at a lower speed. Intel notes in its troubleshooting material that failed cables can continue to operate at 100 Mbps, which is why a known and proven working cable is one of the most useful tests.
For 1000BASE-T in common use, prefer a Cat 5e or higher cable in good condition. Cisco specifies Cat 5e or better for 1000BASE-T connections on its copper modules. The category label, however, does not alone prove that the specific patch cord, its connector, or the in-wall installation are intact.
The most efficient diagnosis is to change one variable at a time
The value of cross-testing is in answering a specific question. If you change cable, port, computer and router at the same time and the problem disappears, you just know that something has changed. If you change a single link and trading goes up to 1.0 Gbps, you gain much stronger evidence about the source of the bottleneck.
Just change the cable and the link goes up to 1.0 Gbps
This result points to the removed cable or its termination. Use a short cable known to trade gigabit over another connection. If possible, test directly between your computer and a proven gigabit port. Do not use as a “test cable” another cable whose origin is also uncertain, because two problematic cables can produce the same result and hide the cause.
Just change the LAN port and the link will go up to 1.0 Gbps
Here the cable and the computer remain the same, so the change in result focuses suspicion on the previous port or configuration of that equipment. Intel recommends testing different ports on your router, switch, or access point, and preferably using a port that has already been proven at 1 Gbps with another machine.
Connect directly and gigabit comes back
If the computer was passing through a switch, dock, wall socket, keystone, patch panel or other segment and the direct connection starts to negotiate 1.0 Gbps, the defect is in one of the
components that have been bypassed. Intel suggests a direct connection with a single known cable to separate the computer and the final port from all intermediate cabling.
After that, replace the elements one by one. First the switch, then the wall section, then the next patch cord, for example. When trading returns to 100 Mbps, the last link inserted becomes the main suspect.
Another computer does 1.0 Gbps on the same cable and on the same port
This is a strong test against the hypothesis that the problem is elsewhere in the network. If the second computer negotiates gigabit using the exact same cable and port, focus the investigation on the first computer: RJ45 connector, network adapter, dock or converter, driver, and advanced interface properties.
The problematic computer remains at 100 Mbps even on a proven path
When a machine continues at 100 Mbps even when connected to a cable and port that deliver 1.0 Gbps to other equipment, suspicion on the computer side becomes stronger. Intel recommends testing the adapter on another system or network when possible, using the equipment manufacturer's driver on OEM adapters, and checking for compatibility and recent hardware or software changes.
Check the specs before blaming the cable
Before purchasing any part, confirm that each port in the path actually supports gigabit. Some older routers advertise Wi-Fi speeds well in excess of 100 Mbps but have 100 Mbps Ethernet ports. TP-Link emphasizes that the advertised rate for the wireless network does not transform a Fast Ethernet port into a gigabit port.
Perform the same check on switches, extenders with Ethernet ports, USB to RJ45 adapters, docks and equipment provided by the operator. Look in the manual or technical sheet for terms such as 10/100/1000 Mbps, Gigabit Ethernet or 1000BASE-T. If a single link in the path is only 10/100, the 100 Mbps limit stops being a defect and becomes a characteristic of that hardware.
Also confirm which port is being used. Equipment with multiple interfaces can mix ports of different capabilities, and a connection built years ago may have gone through a 100 Mbps switch that was forgotten behind the desk or in a rack.
Test the cable without relying solely on appearance
A cable can look perfect on the outside and still have a problem with the plug, an internal pair or the termination made in the wall. The most reliable strategy for home diagnostics is to temporarily replace the entire length with a short cable, known to work at gigabit, connecting the computer directly to a proven port.
If this resolves, do not immediately conclude that the visible patch cord was the only culprit. In a wall outlet installation, the actual “cable” includes the patch cord from the computer, the outlet, the built-in run, the termination on the other side, and another patch cord to the switch or router. Intel's own cabling guide reminds us that intermediate components, such as wall plates and termination blocks, need to maintain class and link integrity.
If the connection fluctuates between 100 Mbps and 1.0 Gbps when the plug is moved or replaced, treat this as a sign that it is worth inspecting the cable and connectors. Reseat until you hear the lock click, look for bent pins or dirt and repeat the test with another cable before moving on to software changes.
Auto Negotiation: do not force 1 Gbps as the first “fix”
On Intel adapters, the official guidance for Gigabit Ethernet is to keep Speed & Duplex in Auto Negotiation. The company also notes that some drivers don't even display a separate 1.0 Gbps Full Duplex option, because auto-negotiation is the expected behavior. Therefore, the absence of this option does not prove a defect in the board.
Forcing 1 Gbps does not fix a cable with bad pairs, a 100 Mbps port, or an adapter unable to sustain the rate. If the link only works at 100 Mbps and loses connection when a higher speed is imposed, that's all the more reason to revert to physical isolation rather than consider forced tuning as a solution.
On cards from other manufacturers, the names of advanced properties may vary. The safest procedure is to maintain the default configuration or the one recommended by the manufacturer and only change Speed & Duplex when there is a specific orientation for that adapter and for the equipment on the other end.
When the driver and network card enter the investigation
Driver is a valid hypothesis, but should come in after the physical path has been tested. If another computer does 1.0 Gbps on the same cable and port, and the problem machine remains at 100 Mbps, update or reinstall the driver from the computer, motherboard, or adapter manufacturer, as applicable. Intel recommends OEM drivers for adapters integrated into OEM equipment.
It is also worth noting whether the problem started after changing the system, updating the driver, installing a dock or changing hardware. This doesn't prove software causation, but it helps you decide which variable to test first. If the adapter is USB, test another appropriate USB port and, if possible, connect the same adapter to another computer.
If the RJ45 connector on the machine itself is damaged, a known gigabit USB or PCIe adapter can serve as a test replacement. If the alternate adapter negotiates 1.0 Gbps on the same cable and port, the evidence against the original interface increases.
How to interpret the results without confusing link with Internet
The system shows 100 Mbps and the Speedtest is below 100 Mbps
The first problem to solve is Ethernet negotiation. As long as the local link remains at 100 Mbps, the Internet test goes through a section that is already limited by that order of magnitude. Correct the link until the system shows 1.0 Gbps before evaluating whether there is still another bottleneck in Internet access.
The system shows 1.0 Gbps, but the Speedtest remains close to 100
Mbps In this scenario, the computer and the immediately connected port have already negotiated gigabit. The search must proceed to the rest of the path: WAN port, other switch or converter, router capacity, operator equipment, contracted plan, provider network status and conditions
from the test itself. There is no point in repeatedly changing the computer cable just because the Speedtest is low, if the local link remains stable at 1.0 Gbps.
One device does gigabit and another doesn't, using the same network point
The difference between the devices focuses the investigation on the adapter, driver, connector or accessories of the equipment that is at 100 Mbps. This is a much more informative result than testing each device on different cables and ports.
All equipment connected to the same device is set to 100
Mbps Check the specification of the ports of this device and the link that powers it. A Fast Ethernet switch or 100 Mbps WAN port can limit all traffic passing through it. If the LAN ports are supposed to be gigabit, test with a short cable and a known computer to separate hardware limitation from defect or configuration.
What to avoid during diagnosis
Avoid changing router, cable, network card and settings at the same time. This can resolve the symptom without revealing the cause and still leads to unnecessary purchases. Also avoid using just the Speedtest as proof that the card “is at 100”, because the Internet test alone does not inform the negotiated Ethernet speed.
Another bad shortcut is to assume that “Cat 6” printed on the cable ends the investigation. Category and specification matter, but a correctly rated cable may be damaged, poorly crimped, or connected to a problematic termination. The litmus test is the behavior of a known cable on a known port.
Finally, formatting the system, completely resetting the router and aggressive changes to advanced properties should be postponed. In a typical case of a link trading 100 Mbps rather than 1 Gbps, physical and cross-testing often produces much clearer evidence about where the limit lies.
A practical sequence to reach the bottleneck
Start by looking at the link speed on the system. If it's at 1.0 Gbps, stop treating the local interface as if it's stuck at 100 Mbps and investigate the rest of the Internet path. If it is 100 Mbps, confirm that the card and port support gigabit.
Then just replace the cable with a known short cable. If it remains at 100, just change the LAN port. If it still continues, eliminate switches, wall sockets, docks and other intermediaries with a direct connection. Then put another computer on the same cable and port. Each change must be followed by a new reading of the link speed.
If the problem is with the cable, correct the cable or its termination. If it comes with the door, investigate or replace the equipment in that door. If it disappears when removing a switch or socket, rebuild the path until you find the faulty segment. If restricted to a single computer on a proven gigabit network, review Auto Negotiation, driver, adapter, and connector.
This order reduces the diagnosis to observable evidence. Instead of buying parts “to see if it improves”, you turn the 100 Mbps number into a clue: first identify where the negotiation changes, then correct only the responsible link.
