How to check if the network cable plugged into Xiaomi router is only 100Mbps? The Gigabit port was reduced to 100 Mbit. I found the crystal head from a line in the background.

Last month, I helped a colleague check out his home's newly installed network. It has Gigabit broadband and a Xiaomi AX3000T. I put my phone against the router and measured that the wireless speed reaches 680 Mbit. It looks pretty good. But the desktop computer in his study was plugged in with a network cable. When tested, the speed was only about 94 Mbit/s, which was very stable and not unstable at all. His first reaction was that the broadband was cheated by the operator, and his second reaction was that the router was broken. I said don't worry, it's too "neat" to only run 94 MB when plugging in the cable. It's probably a matter of network port negotiation. Finally, it was found that the crystal head of the wire in the wall only pressed four cores. This article will talk about the fact that the network cable is only 100Mbps, from the backend of the router to the crystal head.

Understand the number first: Why is it 100 megabytes?

Plug in the network cable and measure the speed. The result is around 94 MB and the highest is no more than 95 MB. This number itself is a clue. 100Mbps, deducting the protocol overhead, can actually run 90 to 95 Mbps, which is so neat. If there is really a problem on the broadband side, the numbers will fluctuate, with peaks dropping at night and rising during the day. The negotiated pressure was 100 Mbit/s, and it was stuck there. Whenever I measured it, it was always 94. After reading this number from my colleague’s house, I immediately said that there is no need to check the broadband.

Here we need to distinguish two accounts first: The broadband is coming in at 1000 Mbit/s, and the network port negotiated is running at 100 Mbit/s. The speed measurement software shows the smaller of the two. WiFi can run 680 MB, which means that there is no problem from the router to the mobile phone, and there is probably no problem from the broadband side, but the wired section is suppressed. So the order of investigation is very clear.: Check the negotiation speed first, and then check who suppressed it. Network port negotiation is automatically negotiated between the router and the network card. If the negotiation fails, the network will be collectively downgraded to 100Mbps. This is an insurance left in the old agreement, and it is not broken.

How to see it on Xiaomi router: 31.1 That line of small print on the homepage

The computer is connected to the Xiaomi router and the browser clicks 192.168.31.1 , log in and enter the home page, which is the status overview. I have previously gone through the background menu one by one and written an article Backstage panorama , in addition to the number of online devices and real-time uplink and downlink on the home page, click on the plugged-in device in the device list. Some firmware versions will display whether its connection method is wired or wireless. Regarding the wired negotiation rate, Xiaomi's web backend is rather stingy. Different firmware versions have different locations and availability. The new version of my AX3000T's firmware does not directly negotiate the rate for the LAN port.

So what I actually do is do the opposite: Look at the computer. In Windows, right-click the network icon on the taskbar, open "Network and Internet Settings", click Ethernet, and there is a line "Link Speed ​​(Receive/Transmit)" on the status page. If it displays 1000/1000 (Mbps), it means that the Gigabit negotiation is successful. If it displays 100/100 (Mbps), it means it has been compressed to 100 Mbps. This line of text is easier to find than the router background, and it only takes ten seconds. The old path of Win11 is Control Panel, Network and Sharing Center, Change Adapter Settings, double-click Ethernet, and the status box that pops up also has the "Speed" column. If 100/100 is found, the root cause of the problem is locked on the link.: Either the line or the network cards at both ends, it has nothing to do with the operator.

By the way, I also need to check the WAN port line. If the optical modem to the router is pressed to 100 Mbit/s, the entire house's upstream and downstream connections will suffer, and the wireless network will not run fast. ； The LAN port is blocked, only plugging in devices. My colleague's mobile phone can run 680 MB, which means that the WAN port is innocent, and the problem is only in the study area. If you can’t tell the difference between the WAN port and the LAN port, look at This article Let me add that I have been in the wrong hole before.

The real murderer: The network cable looks like it has eight cores, but the crystal head only presses four cores.

This is the final answer from my colleague's family, and it is also the most common of this problem. Gigabit requires all eight cores, while 100M only uses four cores, so as long as two cores of the crystal head are connected virtually or are not pressed in at all, the negotiation will immediately drop back to 100M, and you can still access the Internet without any warning. This is the dark place: The four-core cable does not affect "communication", only "speed".

The verification method I used is a local trick.: Take apart the wall panel and see if all eight of the stub wire cores behind the crystal head are pushed in. On the panel in my colleague's study, the crystal head is beautifully made and the sheath is complete. However, when I took it apart, I saw that the 45 and 78 sets of wire cores were half short and did not touch the blade at all. At that time, the maintenance technician was very quick, and he left immediately after pressing and testing. It was easy to test the four-core wire, but he didn't care about the measurement speed. I have to press the crystal head myself. I have written about the line sequence and techniques. An article specifically about pressing crystal heads , I also crushed six of them the first time, and one type of T568B is enough. Another way to check the broken core of the wire inside the wall is The line measuring instrument article , the scene where only four of the eight lights are on is exactly the same.

There is also a hidden version: The wire sheath is printed with CAT5E or even CAT6, and the inside is copper-clad aluminum or fake wires with insufficient wire diameter. It seems that it can be passed through at short distances, but it cannot be negotiated over long distances. I cut several wires and compared them. The cross-section colors of genuine oxygen-free copper and copper-clad aluminum can be distinguished at a glance. The article on network cable category After unfolding it, look for Category 6 oxygen-free copper when buying wire. One meter, one piece, five to two pieces is enough.

The real murderer two: The network card on the other end of the device itself is 100 Mbit/s.

The second common cause is on the computer side. The onboard network cards of old notebooks and desktops are of 100M specification. It is very common for machines before 2015. No matter how good your cable is or how new your router is, the link rate ceiling is 100M. The method of checking is still the "link speed" above, but this time I have to go to the device manager to check the network card model.: Press the Windows key plus Realtek's Gigabit network cards usually have GbE in their names, while the 100M network cards only have Fast Ethernet, and they are not deceptive.

Desktop computers are easy to handle. Just plug in a Gigabit PCIe network card worth dozens of dollars to the 100M motherboard and you're done. If your laptop doesn’t have a port, a USB3.0 to Gigabit Ethernet card can be used for around 30 yuan, but please note that it must be plugged into the USB3.0 port (the blue one), and the USB2.0 port will still only output 100Mbps. I have seen two people fall into this trap. Another situation is that the network card driver is too old and cannot be negotiated. Update the driver in the device manager and replug the network cable. Occasionally, it will return to Gigabit. It is free to try this first.

The third murderer: The port on the router itself is only 100 Mbit/s.

The third method is to suspect the router port itself if you haven't found it after checking the computer after checking the cable. Xiaomi's old entry-level models do have full 100M network ports. For example, Xiaomi Router 4A has a 100M version and a Gigabit version. The names are exactly the same, but the network port specifications are ten times different. My third uncle's one runs 90M and it is the problem. Here are three tips for identifying the version. The 4A article Wrote in. Be especially careful when buying second-hand ones. In the same way, there is a difference between a optical modem and a 100M port. If you want to know the specifications of the machine you have, look at the packaging or the model parameter list. All LAN ports from the AX3000T generation onwards start with Gigabit, and the BE series also has 2.5G ports.

There is another one that is less likely but real.: If a certain LAN port is oxidized or has shrapnel fatigue, it will return to Gigabit when plugged into another port. Replace the port on the back of the router with another plug and troubleshoot in ten seconds. The Redmi AC2100 I have been using for four years has this problem with port 4. Changing to port 3 has immediate results. If the port is broken, there is no need to repair it. There are usually three or four Gigabit ports, just use them interchangeably.

How to review after repairing: Three-step confirmation to return to Gigabit

After my colleague pressed the crystal head hard, he took three steps to review. The first step is to consult: The "link speed" in the computer's Ethernet status changes to 1000/1000. This is the hardest indicator. Only by negotiating the higher speed can you be qualified. Second step speed test: The plug-in speed test returned from 94Mbps to around 905Mbps, which is consistent with the wireless face-to-face test. The third step of stability: If you run the download continuously for ten minutes and the speed does not drop back to 100 Mbit/s, it is truly stable. This thing sometimes goes off during negotiation. This is the symptom of the half-virtual connection of the wire core. Just after negotiating Gigabit, it falls back again when the line is touched, so don't skip the third step.

Finally, give an order: Before checking this, check the "link speed" of the computer. If it is 1000, don't mess with the line. The problem lies in the speed measurement method or other aspects. ； If it is 100, just line up the cable, network card, and router port one by one. After checking these three items, you will definitely have the answer. Don't reset the router as soon as you start it. The reset has nothing to do with the negotiated speed, and it will be a waste of settings. After the wired section is clear, it is still slow. Then it is the turn to check the wireless and broadband sides. The complete order of troubleshooting is as follows The five levels , it saves time to follow the levels.