Detailed analysis and solutions to frequently encountered network failures!

Fault 1: The network cannot communicate when the switch is first powered on.

1. Fault phenomenon:

The switch cannot connect to other networks when it is first turned on and needs to wait for a while. In addition, the speed of accessing other computers will only become faster after using it for a period of time. If you do not use the network for a period of time, the speed will slow down again when you access it again.

2. Failure analysis:

Since this switch is a network-managed switch, in order to avoid the existence of topological loops in the network, which will lead to network paralysis, the network-managed switches enable the spanning tree protocol by default. In this way, even if there is a loop in the network, only one path will be retained and other links will be automatically cut off. Therefore, when the switch is powered on, each port needs to enter the listening, learning, and forwarding states in sequence. This process takes about 3 to 5 minutes.

If you need to start the switch quickly, you can start "PortFast" on the port directly connected to the computer, causing the port to immediately and permanently transition to the forwarding state. This way, the device can immediately connect to the network and avoid the transition of the port from the listening and learning state to the forwarding state. And the necessary waiting time.

3. Troubleshooting:

If you need to forward data quickly after the switch is powered on, you can disable the extended tree protocol or set the port to PortFast mode. However, it should be noted that although these two methods omit the port detection process, once a topological loop occurs between network devices, network communication will be paralyzed.

Fault 2: The 5-port switch can only use 4 ports

1. Fault phenomenon:

There are 4 computers in the office, but there is only one information socket, so a 5-port (one of which is an UpLink port) switch is configured. It was originally thought that 4 computers were connected to 4 interfaces, and 1 UpLink port was used to connect to the LAN. However, after connecting to the network, port 1 adjacent to the UpLink port could not be used normally.

2. Failure analysis:

The UpLink port cannot be regarded as a separate port because it is actually the same port as the adjacent port, but the applicable connection objects are different. With the UpLink port, a hub device can be connected to a regular port on another hub device using a straight-through cable, eliminating the need for a crossover cable.

  The chips of switches and hubs are usually ×4, so the ports of hub equipment are mostly 4 ports, 8 ports, 16 ports, 24 ports, etc. If they are made into 5 ports, 3 modules will be wasted, thus increasing the cost.

3. Troubleshooting:

Replace the 4-port switch with an 8-port switch to solve the problem.

Fault 3: The "COL" indicator light is always on or flashes continuously, and communication cannot be achieved.

1. Fault phenomenon:

Computers in the LAN access the server through the hub, but one day it is discovered that all client computers cannot connect to the server, and pinging between clients is intermittent. Check the hub and find that the "COL" indicator light is always on or flashes continuously.

2. Failure analysis:

  The "COL" indicator light is used to indicate collision and conflict conditions in the network. The "COL" light keeps flashing, indicating that conflicts occur; the "COL" light that stays on indicates that a large number of conflicts occur. The cause of a large number of conflicts may be a hub failure or a network card failure. Under normal circumstances, the possibility of network card failure is relatively small, so the focus is on troubleshooting the hub.

3. Troubleshooting:

Replace the hub and the network returns to normal.

Fault 4: After upgrading to Gigabit network, the server connection is intermittent.

1. Fault phenomenon:

The original server used a 10/100Mbit/s network card and everything was running normally. However, after installing a 1000Mbit/s network card and connecting it to the 1000Base-T port of the central switch, the connection between the server and the network was intermittent, the connection was extremely unstable, and normal network services could not be provided. Use a network cable tester to test the network and find that there is no problem with the connectivity of the twisted pair link.

2. Failure analysis:

  The connection was normal at 100Mbit/s, but the failure occurred only when it was upgraded to 1000Mbit/s. It seems that the cause of this failure may be a Category 5e cabling problem. Although Category 5e systems theoretically support a transmission rate of 1000Mbit/s, if the quality of twisted pairs, patch panels, network cables and other network equipment is not very good, or there are problems with the termination process, 1000Mbit still cannot be achieved. /s bandwidth.

Since 1000Base-T needs to use all 4 pairs of twisted pairs, the effective transmission rate of each pair is 250Mbit/s, and full-duplex transmission is completed, so the signal attenuation, echo, and There are higher requirements for electrical performance such as return loss, crosstalk and anti-electromagnetic interference. If the performance of twisted pairs or other accessories is not good, severe crosstalk will occur between the line pairs, resulting in communication failure.

3. Troubleshooting:

Considering that the performance of the Category 5 cabling system may not meet the requirements of the Gigabit network system, the fault was solved after replacing it with Category 6 cabling products.

Fault 5: Although the Link light keeps flashing, the network speed is extremely slow

1.Fault phenomenon:

The server's Internet speed was very slow. At first, it was very slow to open the web page. Later, I couldn't even open the web page, and Ping the website couldn't resolve the address. At first I thought it was a DNS setting or server failure, but these are running normally. Tried pinging other computers and found that the packet loss rate was very high. At this time, the Link indicator light of the switch keeps flashing, and data is exchanged very frequently, indicating that the computer is constantly sending and receiving data packets. After turning off the switch and then turning it on again, the fault was alleviated, but the fault reappeared after a period of time.

2.Fault analysis

  Judging from the fault phenomenon, this is a broadcast storm within the network. Broadcast storms can occur for many reasons, such as worms, switch port failures, network card failures, link redundancy without spanning tree protocol enabled, network cable sequence errors or interference, etc. Checking the switch indicator lights when a network failure occurs is a very convenient way to visually check network connectivity and network traffic.

3. Troubleshooting:

Judging from the current situation, worms are the main cause of network paralysis. Update the system patches for the server in a timely manner, install the network version of the virus scanning software, and upgrade the virus database for the server in a timely manner. After the anti-virus client program is installed on the server, the fault is resolved.

Fault 6: Server resource sharing failure

1. Unable to assign access rights to users

Fault phenomenon:

The entire network uses Windows domain, and the client is Windows2000 Professional. The server's IP is set to 192.168.0.1, the DNS is 127.0.0.1, and the router's internal IP address is 192.168.0.1. All clients automatically obtain IP addresses and belong to the DomainUser group. When setting up shared files on the server, although permissions can be specified, access is not possible.

Failure analysis:

  In Windows domains, NTFS permissions and share permissions are used to set access permissions for shared folders. However, NTFS permissions are higher than shared folder permissions, which means you must first set NTFS permissions for the folder you want to share, and then set shared folder permissions for it. If there is a conflict between the two, then the NTFS permissions will take precedence.

Troubleshooting:

Specify NTFS permissions for the user first, and then specify shared folder permissions. For example, if you need to create a shared folder TESTA for user A so that the shared folder can be fully controlled by user A and accessed by any other user, you must first set the access permissions of TESTA and specify the "full control" permission for user A. And set "read-only" permissions for Everyone. Similarly, you should also set the permissions of shared folders in this way.

2. Shared folders cannot be displayed in "My Network Places"

Fault phenomenon:

Some folders have been shared, but cannot be viewed in "Network Neighborhood", but some shared files on the same computer can be seen.

Failure analysis:

Since some shared folders can be seen, it means that the computer's network configuration and connection are basically normal. And this is not actually a fault, but a configuration type that belongs to the shared attribute. In Windows systems, there are two main types of shared files, one is for system calls; the other is for other users to access. Shared files for system calls do not appear in "Network Neighborhood", but can be displayed using commands such as "Net View"; shared files for other users to access can be seen in "Network Neighborhood".

So how to configure invisible shared folders? Just add a dollar sign "$" after the shared folder name. For example, in the Windows Server2003 system, the folder automatically created for each user is such a shared folder. Each user can only see his own user folder, but cannot see other people's user folders. There are also some disks. In Windows Server 2003, these disks are shared after installation, but their shared file names have a "$" symbol after them, so client users cannot see them.

Troubleshooting:

Delete the "$" symbol after the shared file name, and the shared files that cannot be displayed can appear in "My Network Places".

Fault 7: The hub and router cannot share the Internet

Fault phenomenon:

Multiple computers use broadband routers and hubs, and use the hub expansion ports to network and share the Internet. After the connection is completed, the three machines directly connected to the LAN port of the broadband router can access the Internet, but the computer connected through the hub cannot access the Internet. No matter a cross cable or a parallel cable is used between the router and the hub, and the hub is connected to the router LAN port The connected light does not light up. In addition, the computer on the hub cannot ping the router or other computers. What is the reason?

Failure analysis:

1) The hub itself is faulty

The fault phenomenon is that the computers on the hub cannot ping each other, let alone the router. This failure affects only all computers connected to the hub.

2) Cascading faults

  For example, the cascade jumper between the router and the hub uses an incorrect line sequence, or the jumper connectivity fails, or an incorrect cascade port is used. The fault phenomenon is that computers on the hub can ping each other, but cannot ping the router. However, Internet access from computers connected directly to the router's LAN port will not be affected.

3) Broadband router failure

If it is a LAN port failure, the result will be similar to a cascading failure: if it is a routing failure, the result will be that no computer in the network can access the Internet, regardless of whether it is connected to the LAN port of the router or connected to the router.

Troubleshooting:

Judging from the fault phenomenon, the computer connected to the hub can neither ping the router nor other computers. It is initially concluded that there should be a connection failure between the computer and the hub. At this time, you can try to replace a network cable first. If the fault still cannot be eliminated, you can replace the hub to solve the problem.

Fault 8: IP address conflict

Fault phenomenon:

Recently, my computer often encounters the following situation, prompting "The system has detected an address conflict between the IP address xxx.xxx.xxx.xxx and the network hardware address 00 05 3B 0C 12 B7. The network operation of this system may be suddenly interrupted." Then it went offline for about a minute and then the network connection was restored. What is the reason for this and how to solve it?

Failure analysis:

This system prompt is a typical IP address conflict, that is, the IP address used by this computer is exactly the same as the IP address of another computer on the same network, resulting in communication failure. The MAC address of the network card that conflicts with this computer is "00 05 3B 0C 12 B7". Usually, IP address conflicts are caused by improper allocation of IP addresses by network administrators or by other users setting IP addresses indiscriminately.

Troubleshooting:

Since the MAC address of the network card is unique, you can ask the network administrator to use the MAC address to find the computer that conflicts with you and modify the IP address. Use the "IPCONFIG /ALL" command to view the computer's IP address and MAC address. Finally, use the "ARP -S IP address network card physical address" command to bind this legal IP address to your network card MAC address.