6 Network Faults

Good afternoon, my network engineer friends.

Network devices often encounter various faults, which not only lead to degraded network performance but also may cause business interruptions, resulting in unnecessary losses.

Network faults occur frequently, and troubleshooting them can be challenging. However, being adept at reviewing and organizing can significantly improve work efficiency.

Today, let’s talk about common network faults and troubleshooting strategies that network engineers encounter in their daily work. The six faults mentioned next are undoubtedly familiar to everyone. If you can resolve them, congratulations, you are one step closer to becoming a technical expert.

1 Network Unreachable After Switch Power-On

When the switch is powered on, devices in the network cannot communicate normally.

01 Possible Causes

• Unstable power supply or insufficient voltage
• Hardware failure of the switch
• Network configuration errors
• Incorrect port connection or physical layer faults

02 Troubleshooting Steps

• Check Power Supply: Ensure the switch power cord is correctly connected and the power adapter is functioning properly.
• Observe Indicator Lights: Check the status of the indicator lights on the switch to confirm if the power light is on normally.
• Inspect Hardware: Check if the switch ports are damaged, and ensure the cables are correctly connected and undamaged.
• Review Configuration: Log in to the switch management interface to check if VLAN configuration, port mode (such as access or trunk), etc., are correctly set.
• Test Connectivity: Use the ping command to test the connectivity between switch ports and other devices.

03 Solutions

• If the power supply is unstable or the voltage is insufficient, consider replacing the power adapter or using a voltage regulator.
• If hardware failure is detected, contact the supplier for repair or replacement.
• For configuration errors, reconfigure VLAN, port mode, etc., according to the network design.
• If a port is damaged, replace the affected port or the entire switch.

2 5-Port Switch Only Allows 4 Ports

In an office with 4 computers, but only one information outlet, a 5-port (one port is an UpLink port) switch was configured.

Originally, it was thought that 4 computers would connect to 4 ports, and 1 UpLink port would connect to the LAN. However, after connecting to the network, the port adjacent to the UpLink port (port 1) cannot be used normally.

01 Fault Analysis

The UpLink port should not be considered as a separate port because it is essentially the same port as the adjacent one, just for different connection purposes. With the UpLink port, hub devices can use straight-through cables to connect to another hub device’s ordinary port, eliminating the need for crossover cables.

Switches and hubs usually have ×4 chips, so their ports are mostly 4-port, 8-port, 16-port, 24-port, etc. If a 5-port is made, 3 modules would be wasted, increasing costs.

02 Fault Resolution

Replace the 4-port switch with an 8-port switch to resolve the issue.

3 “COL” Indicator Light Constantly On or Flashing but Unable to Communicate

The “COL” (Collision) indicator light on the network device (usually a switch or router) is constantly on or flashing, indicating a line conflict, causing abnormal network communication.

01 Possible Causes

• Incorrect connection configuration between network devices
• Using half-duplex mode instead of full-duplex mode
• Broadcast storm or excessive traffic in the network
• Hardware failure or cable quality issues

02 Troubleshooting Steps

• Check Connections: Ensure all connections between network devices are correct, including cable types and connection methods.
• Check Port Settings: Log in to the device management interface to check if the port working mode (half-duplex or full-duplex) matches.
• Monitor Network Traffic: Use network monitoring tools to analyze network traffic and check for broadcast storms or other abnormal traffic.
• Test Cables: Use a cable tester to check the quality of the cables, ensuring no physical damage.
• Restart Devices: Try restarting the devices to clear potential temporary faults.

03 Solutions

• Adjust device port settings to full-duplex mode.
• If a broadcast storm is detected, isolate the affected devices or network segments.
• Replace cables with quality issues.
• Update device firmware to fix potential software issues.
• In severe cases, replace faulty hardware.

4 Server Connection Intermittent After Upgrading to Gigabit Network

After upgrading the network to gigabit speed, the connection between the server and clients becomes unstable, showing intermittent connectivity.

01 Possible Causes

• Incompatible network devices
• Network configuration errors
• Hardware failure
• Cable quality issues

02 Troubleshooting Steps

• Check Device Compatibility: Ensure all network devices (such as switches, routers, servers, etc.) support gigabit speed.
• Check Port Settings: Log in to the device management interface to check if the port working mode (auto-negotiation or fixed speed) is correctly set.
• Test Cables: Use a gigabit tester to check if the cables support gigabit speed, especially CAT5e or higher-grade cables.
• Monitor Network Traffic: Use network monitoring tools to analyze network traffic and check for traffic bottlenecks.
• Update Firmware: Ensure all network devices have the latest firmware versions and update them as needed.

03 Solutions

• Upgrade incompatible devices to models that support gigabit speed.
• Adjust device port settings to auto-negotiation mode or manually set to gigabit speed.
• Replace cables with quality issues.
• Add necessary buffers between devices or adjust QoS (Quality of Service) settings to optimize traffic control.

5 Link Light Flashing but Network Speed is Extremely Slow

The Link light on the network device (such as a switch or router) shows a normal connection, but the actual network transmission speed is very slow.

01 Possible Causes

• Network congestion
• Cable or connector issues
• Device configuration errors
• Hardware failure

02 Troubleshooting Steps

• Check Network Load: Use network monitoring tools to check if the network is under high load.
• Test Cables: Use a cable tester to check the quality of the cables, ensuring no physical damage.
• Check Device Configuration: Log in to the device management interface to check if port settings, VLAN configuration, etc., are correctly set.
• Restart Devices: Try restarting the devices to clear potential temporary faults.
• Inspect Hardware: Check the hardware status of the devices, including fans, power supplies, etc., to ensure they are functioning properly.

03 Solutions

• Optimize network traffic to reduce congestion, such as increasing bandwidth, adjusting QoS strategies, etc.
• Replace cables or connectors with quality issues.
• Reconfigure device ports and VLAN settings according to the network design.
• If hardware failure is detected, consider replacing faulty components or the entire device.

6 Hub and Router Cannot Share Internet

In a small network environment, using a hub and router combination, multiple computers cannot share internet access.

01 Possible Causes

• Configuration errors
• Hardware compatibility issues
• Incorrect network settings

02 Troubleshooting Steps

• Check Network Topology: Ensure the connection method between the hub and router is correct.
• Check IP Address Configuration: Ensure all devices have correct IP address configurations, especially if the router’s DHCP service is properly enabled.
• Check Routing Settings: Log in to the router management interface to check if static or dynamic routing protocols are correctly configured.
• Test Physical Connections: Use the ping command to test the connectivity from devices connected to the hub to the router.
• Update Firmware: Ensure all devices have the latest firmware versions and update them as needed.

03 Solutions

• Reconfigure IP addresses and subnet masks to ensure all devices are in the same network segment.
• Enable the router’s DHCP service to automatically assign IP addresses to connected devices.
• Correctly configure static routes or enable dynamic routing protocols according to the network topology diagram.
• If hardware is incompatible, consider replacing it with devices that support the required functions.