9 classic cases, online teaching how to troubleshoot network failures

Network failure is the most common and difficult problem to solve, especially for those who often deal with computers and switches. Network failure is inevitable.

[[331082]]

Most network troubleshooting ideas and case handling methods can solve most of the daily network problems.

01Troubleshooting ideas

Locating fault range

① Network-wide network failure: The source of the failure can be located at the exit or core area;

②Small-scale network failure: The source of the failure can be located in the corresponding device or link closest to the source of the failure;

③ Single-point network failure: The fault source can be located at the fault source itself. Troubleshooting

① The overall idea is "link" à "configuration".

②First confirm whether there are any human changes in the network or related equipment;

③ Secondly, check whether the physical link and equipment are normal;

④Finally, check the relevant properties or configurations of the network device.

02Common troubleshooting methods

1. Network equipment

① Check the status lights, including the power indicator, status light, and alarm light;

② Sense the temperature of the device and check whether the temperature is too high.

2. Physical link

① Check the link indicator light;

② Plug and unplug the link; (Note: It is not recommended to plug and unplug the optical fiber link multiple times, and the multi-mode optical fiber can be seen by the naked eye)

③Change the port.

3. Terminal host

The following common commands can be used in Windows system

Test whether the network is unobstructed.

Traceroute

4. Segment positioning

From the user PC to the access switch

From access switches to aggregation switches

From aggregation layer switches to core switches

From core switches to firewalls

From firewall to router

From router to egress gateway

03Use of network equipment and check of indicator lights

1. Network usage environment

①Temperature requirement: 15℃~30℃

②Relative humidity requirement: 40%~65%

③ Cleanliness requirement: ≤ 1.3×105 (dust particle diameter ≥ 5μm)

④ Anti-interference requirements: Stay away from high-power, high-frequency, and high-current equipment

⑤AC power supply requirements: 100-240V~/50-60Hz

⑥ Maintain good ventilation between equipment to prevent heat flashing

2. Check the indicator light

04Common Fault Case Collection

Failure Case-1: PC cannot obtain IP address

Troubleshooting steps: ① Check whether the DHCP server is normal and whether related services are running.

② Ping the DHCP server from the host and core switch respectively.

③After setting a static IP address on the intranet, check whether you can ping the gateway.

Based on the above results, check whether there is any fault from Link à Setup.

Troubleshooting Case 2: The PC can obtain an IP address but cannot access the Internet

Troubleshooting steps:

On the host that cannot access the Internet, run cmd—>ipconfig /all to check the IP address information obtained.

If the following figure appears, it means that the "003 default route" setting of the corresponding domain of the DHCP server is incorrect.

Failure Case-3: PC obtains wrong IP address

Troubleshooting steps:

Check whether anyone in the intranet uses Windows 2003 Server as the operating system and has enabled the DHCP service, or uses a wireless broadband router, which causes confusion in address acquisition.

Solution:

You can enable DHCP Snooping on the access switch to only allow DHCP offer messages to be obtained from the trusted uplink port, but not from the downlink port.

Failure Case-4: PC cannot access the website via domain name

Troubleshooting steps:

DNS is not set up or is set up incorrectly.

Troubleshooting Case 5: ARP Spoofing Attack

Troubleshooting steps:

On the host that cannot access the Internet, run cmdàipconfig /all to check the IP address information.

Related command: showarp | include xxxx.xxxx.xxxx

Fault Case-6: Configuration Error—Interface VLAN Division Error

This mode usually appears in an environment where a static IP address is set.

Fault phenomenon: The host cannot ping the gateway.

Troubleshooting steps: cmdàipconfig /all to check the host IP address.

Check the VLAN settings of the corresponding ports on the switch.

Related commands: show run show int status

Troubleshooting Case 7: Network Loop

Phenomenon: The switch port indicators flash synchronously, the PC pings the external network and loses packets, and the Internet access is very slow

Troubleshooting method: Find the network cable or port that causes the loop by closing the port;

Solution: Solve this problem by enabling spanning tree. For the second method, solve it by enabling spanning-tree bpduguard enable on the connected PC port.

Fault Case 8: Only some network segments in the school can access the Internet, and others are not allowed

Phenomenon: Only some network segments within the campus network can access the Internet, while other network segments are not allowed

Troubleshooting method:

1. Are there any restrictions in the egress router list?

2. Check whether the router's return route is normal;

Solution:

1. Modify NAT ACL;

2. Add a return route;

Failure Case-9: The intranet server cannot be accessed from the public network

Phenomenon: The school's WEB server cannot be accessed through the public network address outside the school;

Troubleshooting method: Check whether the egress router has port mapping;

Solution: Add port mapping to solve