Common LAN hardware fault diagnosis)

During the establishment and use of the LAN, users often encounter network failures due to hardware equipment faults. This article uses the specific fault examples and Q & A methods, describes the causes and troubleshooting methods for faults of hardware devices (transport media, NICS, hubs, and switches) in the LAN.

I. Common Faults and troubleshooting of transmission media

1. Network Communication fails due to changes in line resistance

[Question]: A computer terminal that can communicate normally cannot connect to a vswitch. When the network line Tester is used to test the line, the network line Tester signals show that the connection is normal; data communication can be restored only when the end is connected to another network switch for signal amplification.

[Answer]: It is preliminarily determined that the fault is caused by a large line resistance. Use the nettool network multimeter to perform access measurement on the computer access port, when it is found that it is connected to the switch, the signal level is lower than the normal communication level value. When you use a multimeter to measure each pair of cables, it is found that the impedance of the cables is large. Open the wall packing box and check the junction of the cabling module. After the inspection, it is found that the card blades on the network module are severely oxidized due to moisture on the wall, and the cable contact point is severely oxidized. After the network module is changed, the fault is rectified.

2. "network storm" caused by network cable breakdown"

[Question]: The Ethernet in the network center is divided into two network segments: 192.168.1.x and 192.168.2.x (CIDR Block 1 and CIDR Block 2 ). One production machine and one development machine are used to process the information of each branch, and two routers are used to remotely communicate with each branch. There are also some computers for open and monitoring. CIDR Block 2 has many computers for client development and debugging. The development machine and a computer are connected to two network segments at the same time. One day, the network administrator finds that the data transmission of each branch is abnormal, and the data of the branch that uses the TCP/IP protocol to connect to the center is abnormal. After a while, I found that all data communication between these branches was not available. What is the problem?

[Answer]: If a fault occurs, the communication problem is caused. In CIDR Block 1 and CIDR Block 2, the connection between the computer and the production machine experiences intermittent interruption. The connection is directly established with the production machine (without Ethernet) the connection between the main console and the host is normal.

After careful inspection, it is found that there is a section in the middle of the twisted pair. After a new twisted pair is replaced, the fault disappears and returns to normal. When the network cable is damaged, the signal cannot be transmitted normally and an error signal is generated, and the network segment is filled with an error signal. As a result, the normal signal cannot be transmitted smoothly, resulting in intermittent interruption of network communication.

In Ethernet, especially the shared Ethernet connected by a hub, any point in the Network may cause exceptions or even paralysis of the network.

3. Line interruption due to physical damage to the line

[Question]: The computer successfully pinged the address of the Local Machine. Different external addresses are pinged. The network line is measured using a wire measuring instrument, and some of the main core wires used for transmission are found to be disconnected.

[Answer]: You can first use a network cable tester to test the connection between the two ends of the twisted pair wires. If necessary, you can make the two ends of the twisted pair wires out of the cable frame or the crystal head for direct measurement and diagnosis to prevent misdiagnosis due to connection problems, the fault point can be manually searched along the network route after diagnosis. If a dedicated network tester is available, you can directly find the distance between the breakpoint and the measurement point to locate the fault point more accurately. The twisted pair wires are usually used to disconnect a line. After one-to-one matching of Copper Core wound connection, weld and seal the outer skin, you can also unlock all the core lines of the breakpoint, press them into the crystal head respectively, and then use the docking module for direct connection. If you cannot find the power failure or welding failure, you can set the priority of the intact core line to 1, 2, 3, and 6 at both ends to ensure that no more than 4 core wires are disconnected, to ensure effective signal transmission. When conditions permit, the new twisted pair wires can also be re-deployed, but the project is large and the process is cumbersome.

Ii. Nic common faults and troubleshooting

In the enterprise LAN, when the network card is connected to the Internet, the first mark is used. If the network card is set to conflict or hardware failure occurs, the computer will prompt to refuse to log on.

(I) for abnormal faults, the system is forced to enter the pure DOS state when the system is started, and hwinfo (a hardware testing software) is used) view the interrupt and I/O address assigned by the system. The parameters are usually accurate.

(Ii) The compatibility of the limited board is poor. Although it is a plug-and-play (PNP) device, it may not necessarily work well with the motherboard or operating system. When it is not working properly, to avoid interruptions and I/O address conflicts, you can change the pnp OS installed option in Bois.

(3) When the network card works normally and the network cable is normal, you can only see the local machine. If you cannot find other computers, you may wish to change the network card interruption settings.

1. A 10 m/Mbps adaptive Nic cannot be used in a Mbps Corporate Network

[Question]: A 10 m/Mbps adaptive Nic is used in the company's mbps bandwidth. However, no matter how configured, the NIC cannot be used, but it is no problem to use the NIC in other networks.

[Answer]: The network speed may not match the network speed. You can manually set the NIC to 100 Mbps. If not, manually set 10 Mbps. The reason for this is that the Adaptive Mode of the NIC and the switch is not compatible, and the NIC speed cannot be automatically identified. Therefore, the 10 Mbps Nic cannot run properly. Therefore, when purchasing or installing the NIC, it is best to first confirm the port rate of the switch connected to the NIC.

2. After the wake-up function is enabled, the computer Often crashes.

[Question]: After installing a NIC with the wake-up function, connect the self-made Wol line. The computer is often black or cannot be started. If you unplug the wol line, this problem will no longer occur.

[Answer]: Generally, NICS with the wake-up function must use their own cables. If other network adapters or self-made Wol cables are used, the computer may be black or cannot be started.

3. Electromagnetic Interference causes abnormal network operation

[Q]: Recently, I found that the company's computers cannot be pinged, but there are no problems with the PC NIC AND motherboard. Sometimes it disappears after a slight adjustment of the machine location, what's going on?

[Answer]: There should be strong electromagnetic interference near the computer. Please remove high-power electrical appliances and wireless communication devices near the computer. Of course, this interference source may also come from inside the computer. Check whether the NIC inside the chassis is too close to that installed on other expansion cards. The interference between the display card and the independent sound card is very strong. It is best to avoid installation close to the NIC.

4. Nic abnormal due to power failure

[Question]: After the NIC is installed on a PC, the computer not only has problems with the LAN, but also slows down the local network. I have previously ruled out external interference, internal video card, sound card and motherboard problems, and other factors. What should I do?

[Answer]: this is because the computer power failure causes the NIC to work abnormally. If the power supply fails, the discharge interference signal may affect the output port of the NIC. When it enters the network, it will occupy a large number of re-transmission frames and invalid frames, seriously interfering with the operation of the entire network system. Solution: repair or replace the new power supply.

Iii. Common hub faults and troubleshooting

For the most common and commonly used star topology, a hub is the heart of a Network. Once a problem occurs, the entire network cannot work. Therefore, its quality is very important to the entire network.

1. slow network transmission rate

[Q]: The LAN of a small enterprise is 10 Mbps Ethernet connected by a hub. Why does the system prompt that only kb ~ Kb/s transmission rate, sometimes even less?

[Answer]: this is because the speed metering method is different. A time bit and a byte bit. Generally, there are 8 bits in one byte. The measurement units of many download tool software are byte/s, so the difference between the two is eight times.

In addition to the metering method, the main reasons why the network cannot reach the nominal transfer rate include:

① Restrictions on the hub. The hub has 10 Mbps shared bandwidth. If all ports are in the communication status, each port can receive a transmission rate of about 0.625 MB per second. If you want to get a transfer rate close to the theoretical bandwidth, you must use a switch as the set line device.

②. Nic causes. If the quality of the network adapter is poor and errors occur frequently in the data packet sent, the data packet may be resold frequently, or the received data packet may have many errors, which may also reduce the speed of copying files.

3. network cable reasons. If the network cable is too long, the signal attenuation is severe, or although the network cable is near, but the network cable quality is not good, it cannot reach the theoretical speed. In addition, when the network is busy, it cannot reach the ideal speed.

2. Mbps hub cascade, resulting in network communication failure

[Question]: The company has set up a LAN that can accommodate 60 computers for computer training. In consideration of multimedia transmission requirements, a 10 Mbps/Mbps adaptive switch is selected as the set line device. During LAN formation, computers cannot communicate with each other. No error occurred while connecting to vswitch.

[Answer]: first, take all the lines of the switch and test it with a network cable tester. If no problem is found, it means that each line is connected. After inserting the line, try again and ping the computer. The connection is still unavailable. Note that the vswitch has an uplink port. I tried to connect the uplink port of one vswitch to the normal port of the other vswitch after the test. After testing, it is found that the local area network uses a 100base-tx hub with a similar serial number as a switch. After debugging, the fault is rectified immediately.

A hub shares a bandwidth. That is to say, all computers in the network share a bandwidth of 1000 Mbps. A vswitch shares a dedicated bandwidth. That is to say, communication between any two vswitches in the network is Mbps. In addition, in a shared network, only two computers can communicate at the same time. In a switched network, all computers can communicate at the same time.

3. The connection to the hub is improper, leading to abnormal Internet and LAN connection on the client

[Question]: in the LAN, clients connect to vswitches through hubs and access the Internet through routers. Recently, users have repeatedly reported that the Internet and LAN are not normal. In severe cases, it is difficult to ping the routers in the LAN on a single machine.

[Answer]: According to the fault analysis, the possible causes of the above faults are as follows:

①. The router crashes. In many cases, you can restart the vro to resume normal operation.

② Switch failure. Some switches may suddenly crash, and the related networks are paralyzed. Restart the computer to restore the normal state.

③ The above problems can also be caused by broadcast storms in the LAN, which is generally caused by network connection errors.
After inspection, an error occurred in a Hub Connection. The cascade ports of the hub are two. You need to manually connect one of them to the computer based on the port of the Switched Device. However, both ports are actually Ethernet ports. The difference is that the positions of the input and output cables have changed. Second, check whether the connection router cable is damaged and whether it is infected with computer viruses.

4. Low hub transmission rate due to electromagnetic interference

[Question]: If a 500 series hub is set to 10 Mbps, the network speed will be restored to the desired status of the 10 Mbps hub. However, when the port is set to adaptive or Mbps, the transmission rate will be very low. Is that the case?

[Answer]: in this case, electromagnetic interference occurs. When the main device is very close to the power cable or Power Switch Cabinet, the shielding effect of the power switch cabinet is not good. The solution is to shield electromagnetic interference.

Iv. Common vswitch faults and troubleshooting

A vswitch has the same functions as a hub in the network. Its failure affects the operation of the entire network system.

1. The network becomes slow after the switches are stacked together

[Question]: When 500 series switches are stacked together, the network will become very slow. When the switch stack is split, the network will become faster.

[Answer]: the root cause of the problem is a permission stack matrix module, stack interface module, or stack table. The solution is to change the stack module.

2. Fast Ethernet channel failure

[Question]: to implement the FEC (Fast Ethernet channel) function, the two pairs of ports of the two switches are connected with two lines at the same time, but it is found that each switch is always a normal port, another red light is displayed.

[Answer]: in general, this situation is normal for the switch. Because the two switches are connected by two ports, the switch will automatically disconnect one of them if it is regarded as a loop, turn off the corresponding port (the port with a red light. The solution is to open the Spanning Tree to let the switch know that the port is a FEC function and logically a port.

3. There is little difference in the switch speed when the hub is changed to a vswitch.

[Question]: When a LAN is set up to serve as an enterprise multimedia meeting room, it has 36 PCs and a server. In the past, three hubs were used to connect to each other, and the speed of broadcasting meetings was slow, even a network storm occurs. Now, replace the three hubs with three switches. I thought it could improve the network speed, but I didn't expect it to be almost the same as before.

[Answer]: The difference between a vswitch and a hub is the unicast transmission efficiency. When multiple ports communicate at the same time, the vswitch is superior to the hub. In addition, the switch is in full duplex mode, while the hub is in half duplex mode. Therefore, the former is more suitable for information exchange transmission than the latter. For broadcast transmission, the performance of the hub and switch is almost identical, that is, the speed and efficiency of the hub and switch with the same bandwidth are basically the same.