Solve the problem that the switch cannot be pinged

Bkjia.com exclusive Article] a switch is a very important network device in the LAN. Its working status is closely related to the access status of the client system. However, in the actual working process, the status of the switch is easily affected by external interference, so various network faults may occur in the LAN. To ensure stable network operation, we must properly manage and maintain the switches at ordinary times to avoid switch faults. This is not the case. During the maintenance of the LAN, I encountered a fault where the floor switch could not be pinged due to improper physical connection. The Troubleshooting of this type of network fault has caused me a lot of trouble. As this fault is relatively typical and Its Troubleshooting ideas can be used for reference, I will share it with you.

Crime scene

My building contains several units. In order to ensure that each unit can access the Internet independently and require that their Internet access status be not affected by other units, the author selects a route switch as the core switch of the building network, and sets different virtual working subnets for each unit on the switch. As each unit is distributed on different floors, the number of units on each floor is also different. Some floors have two or three units, and some have five or six units, all unit subnets on different floors are connected to the LAN of the building through switches on the corresponding floor, and the Internet is accessed through the hardware firewall in the building network.

To improve network management efficiency, network administrators usually manage and maintain vswitches through remote connection, when I scan and diagnose the working status of each switch port of the LAN core switch, I find that a switch port is down. View the network management file and find a layer-2 switch on the fourth floor connected to the port. When you remotely log on to the switch on the floor, you find that you are not able to log on successfully. When you use the ping command to test the IP address of the switch, the returned result is "Request time out". When the author wondered why no one reported a fault, the telephone bell arrived as scheduled, and the users on the fourth floor began to report a network fault one after another. According to the above fault phenomenon, the author estimates that it may be an accident in the working status of the floor switch, so he ran to the faulty Switch site, cut off the power of the device, and then re-connected the power supply after a period of time, restart the vswitch. After the start operation is complete, the ping command is used to test the IP address of the vswitch. At this time, the returned results are normal and the remote logon operation can be performed smoothly. However, half an hour later, The Faulty Switch experienced the same fault and returned an abnormal test result during the ping command test, after repeated start tests, it was found that the Faulty Switch could not be pinged normally.

In-depth troubleshooting

Since the problem cannot be solved after repeated restart, the cause of the fault is estimated to be complicated. Considering this fault, it is often encountered in the network management process, as a result, the author conducts in-depth troubleshooting based on the following ideas:

1. Considering that only one floor switch on the fourth floor of the entire building has this phenomenon, the author initially determined that it may be caused by the switch's own problems, in order to ensure accurate identification of the cause of the fault, the author intends to use a normal working switch to replace the faulty switch to see if the fault still exists. At the same time, connect the suspected Faulty Switch to an independent network environment. After half an hour of testing and observation, I can see that the faulty switch connected to the independent network environment works normally and can ping its IP address in this network environment, after the new switch is connected to the building network, it cannot be pinged normally. According to these phenomena, I believe that the switch on the fourth floor has almost no possibility of problems.

2. After troubleshooting the status of the faulty switch, the author reexamines the network structure and status of the entire building network. Users on other floors of the building can access the Internet normally, but some users on the fourth floor cannot access the Internet. After reading the networking information on the fourth floor, I can see that there are five units on the fourth floor, at that time, the network administrator set up two floor switches on the fourth floor and connected them through cascade. At the same time, the two switches were divided into five virtual working subnets, this ensures that each organization can work independently in its own virtual work subnet. Since the corresponding port on the core switch has been down, all units on the fourth floor cannot access the Internet. Why is the fault reported only by some users? When I arrived at work time, I immediately contacted several other organizations that did not report a network fault and got a reply saying they had just discovered that the network access was abnormal and were preparing to ask the building network administrator for help, in this case, all units on the fourth floor cannot access the Internet normally. The cause of the failure should be in the Virtual Work subnets of these units.

3. After locking the troubleshooting scope in the five units on the fourth floor, the author believes that, since the equipment of a switch on the fourth floor can be restarted, the network fault can be restored temporarily, only half an hour later will the same network fault occur again. In contrast to this special phenomenon, I suspect it may be a network broadcast storm, the switch is blocked for a certain period of time, and the switch port of the core switch is blocked. To facilitate fault analysis, I used professional network monitoring tools to analyze network transmission data packets on the cascade ports of vswitches on the fourth floor, they are all very large, almost more than 100 times the normal value, which indicates that the network on the fourth floor is blocked.

4. Are there network congestion caused by network viruses or network congestion caused by network loops? I plan to observe the status changes of the faulty switch cascade ports, especially the changes in the output broadcast packets. If the output broadcast packets keep increasing every second, in, we can prove that there is a network loop in the network on the fourth floor. Based on this analysis, the author uses the Console control line to directly connect to the faulty switch and log on to the system background as a system administrator, at the same time, I used the display command to view the changes in the output broadcast package of the vswitch cascade port, and checked the results every second. After repeated tests, I found that the output broadcast packet size of the faulty switch is constantly increasing, which indicates that there must be a network loop among the five units on the fourth floor.

5. I carefully checked the two switches on the fourth floor and found that the physical connection between them is normal. In addition, the switch ports of the two vswitches are directly connected to the Internet Plug-in on the wall of each room on the fourth floor. It is reasonable to say that as long as the switch is not freely used for cascade in each room, there should be no network loops. Now that the fourth-floor network has a network loop, it indicates that someone is using a vswitch to expand the Internet. We only need to find the extension switch and check its physical connection, the specific faulty node can be quickly found. Therefore, the author contacted the network administrator of each unit on the fourth floor and asked them to inspect each office room and report the room using a lower-level switch; it didn't take long for the results to be reported to the author. In fact, about 10 rooms were expanded using lower-level switches.

6. I know the network connection of these 10 rooms is most likely to have a network loop. Which room is it? Do I need to go to the site of each room in sequence to check their network connections? After careful consideration, I found the networking materials, found the exchange port numbers used in the 10 rooms one by one, and then inserted them directly into these exchange ports using network cables, in the View Mode of these ports, ping the IP address of the faulty switch in sequence. As a result, when the sixth switch port is pinged, the ping from this port fails; in order to determine whether the switch port is really faulty, I used the display command to view the status of the switch port in the view mode, I found that the size of the input and output data packets of the switch port is obviously abnormal, So I estimate that the switch port is definitely the cause of abnormal working status of the faulty switch. After checking the archives, I quickly found the corresponding Internet Room Based on the exchange port number. After arriving at the site, I found that there were only two Internet ports in the room, these two hubs are connected to several computers. what's even worse, there is a network line that connects them directly, in this way, a network loop is formed between the two hubs. The broadcast storm caused by the loop eventually blocks the cascade port of the faulty switch, as a result, the network on the fourth floor cannot access the Internet normally.

Troubleshooting

After removing the redundant network cable, I re-checked the status information of the switching port and found that the size of the input and output data packets has been restored to normal, when you view the status of the switch port corresponding to the core switch again, the "down" status is changed to "up, at this time, the author can successfully ping the faulty switch on the fourth floor, which indicates that the problem was caused by the illegal expansion of a user in a room on the fourth floor using the switch or hub.

Later, after further inquiry, I learned that their room had been cleaned up the night before, and all the network lines were pulled out. When the cleaning work was over, internet users do not know much about the connection, so they can plug in at will, resulting in a network loop.

Fault Summary

Through in-depth troubleshooting of this network fault, we can easily see that during network management and maintenance, we must have a comprehensive and clear understanding of the network structure of the entire network, at the same time, you must carefully consider the Internet access configuration of the switching port. When a network fault occurs, you must gradually narrow down the troubleshooting scope based on the fault phenomenon. Then, you can use professional tools to test the size of the online data packets and quickly locate the faulty node.

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