Improve network efficiency and quickly resolve switch faults

In order to improve the LAN operation and maintenance efficiency, we need to accumulate various switch troubleshooting experience at ordinary times so that the switch can be quickly rectified in case of a fault, this article has contributed several vswitch troubleshooting skills to my friends. I hope these content will help you!

1. Solve the trouble of resetting VLAN

When managing and maintaining a LAN, if the cascade port connecting to a common switch changes, the VLAN previously configured in the switch system may not work normally. In this case, do we have to re-divide and set VLANs? If this is the case, the network maintenance workload is obviously huge. In fact, after changing the cascade ports of a common switch, we only need to enter the backend management interface of the switch, modify the working mode of the cascade port so that all VLAN access can be passed. In this way, you can avoid resetting VLAN operations. Now we will take the LAN of a certain unit as an example to introduce the specific configuration steps of the switch to you:

Assume that there are 6 VLANs in the LAN of the Organization, where S1 switches are located in subnet A and S2 switches are located in subnet B. Recently, the organization has purchased several workstations, now we need to move the S1 switch to the subnet B. Previously, the S1 switch was directly connected to the core switch of the local area network with an optical fiber cable on port 24. To avoid VLAN division in the switch system, we can change the port operating mode of the S1 and S2 switches. For example, we can first check the port settings of the S1 switch. During this check, we can remotely log on to the background management interface of the switch using the telnet command, run the "display interfaces" string command to check the specific configurations of each port of the vswitch. From the results returned by the preceding command, we can see that port 26 of the S1 switch that maintains a cascade relationship with the S2 switch is in the "interface ethernet0/26, port access vlan 2" status ", in this status, we can easily understand that the S1 switch only belongs to VLAN2. That is to say, the switch only allows the workstation from VLAN2 to pass through, and the workstation in other VLANs cannot pass through. When the S1 switch changes its placement position, it will certainly be located in the new VLAN. To make all workstations in the new VLAN accessible, we need to change the port 26 working mode of the S1 switch to "trunk" here ", in this way, the S1 switch does not need to re-divide and set VLAN, so that all workstations in the new VLAN can pass through.

Many users may wonder why the S1 switch can communicate with the lan network normally before? The original S1 switch was previously connected to the core switch of the Unit through an optical fiber cable. The working mode of the optical fiber connection port has been set to "trunk". When the placement position of the S1 switch changes, because the optical fiber cable is not used to connect to the switch, the corresponding Optical Fiber Connection port does not work.

When modifying the operating mode of port 26 of the S1 switch, we can remotely log on to the backend management interface of the switch, and execute the string command "system" in the command line of the interface ", switch the operating status of the S1 switch to the system configuration status, and then run the "interface ethernet 0/26" command to enter the port configuration status of the S1 switch No. 26th, then, enter the string command "port link-type trunk" in this status and click the Enter key. Then, the operating mode of port 26 on the S1 switch is changed to the "trunk" type; to allow all VLANs in the LAN to access the S1 switch through this port, we also need to execute the string command "port trunk permit VLAN all ", this allows you to specify port 26 to allow workstation access from all VLANs. In the same way, we can modify the cascade port mode of the S2 switch to ensure that all workstations in the LAN can access the S2 switch.

2. Solve host failure Ping failure

When managing and maintaining networks, we often perform Ping command tests on the IP addresses of a host in the LAN on the vswitch, during the test, if the IP address of the target host cannot be pinged, how can we solve the problem? When we confirm that the target host has been powered on and the system is operating normally, we can perform the following troubleshooting operations in the switch:

First, use the telnet command to log on to the background management interface of the target switch, and execute the string command "display interfaces" in the command line of the interface ", check whether the IP address of the port connecting the target host and the local switch is in the same CIDR block from the pop-up interface, or check whether the working mode of the specified connection port of the local switch is "trunk". If these parameters are not set correctly, we must modify them in time.

Second, execute the string command "display arp" and carefully check whether the content of the ARP table for Local Switch management and maintenance is set correctly from the pop-up result interface. Once an incorrect record or entry is found, it must be modified in time.

Then, check the virtual subnet in which the communication port of the local switch connects to the target host. Find the corresponding virtual subnet and check whether the VLAN communication interface is correctly configured for the virtual subnet, if it is already configured, check whether the IP address of the VLAN communication interface is in the same working subnet as the IP address of the target host. If the configuration is incorrect, must be modified in time.

If the preceding configuration parameters are normal and the Local Switch cannot Ping the target host address in the LAN, enable the ARP debugging switch in the local switch system, in order to check in detail whether the local switch can correctly send ARP packets and receive ARP packets, if the local switch can only send ARP packets but cannot receive ARP packets from outside, the cause of the failure is probably the physical link layer of Ethernet. At this time, we need to focus on the physical link layer check.

3. Resolve the failure to forward IP Packets

A layer-2 switch in the LAN is not configured with dynamic routing protocol. During management and maintenance, we found that the interface link layer protocol status of the vswitch and the physical status of the interface are all displayed as UP, but the vswitch cannot normally forward IP packets from the LAN workstation. So how can we solve this fault quickly and effectively?

If the Protocol Status of the local switch interface link layer and the physical status of the interface are all displayed as UP, and the switch cannot forward IP datagram packets normally, most of the requests are due to the Protocol specified by the local switch, and the route parameters are not correctly set, or the static route settings of the local switch are not effective. In this case, we can remotely log on to the background management interface of the target switch using the telnet command, and enter the command line status, and enter the string command "display ip routing-table protocol static ", click the Enter key to check whether the local switch has correctly configured the static route. If no static route is configured, You need to reconfigure it in time;

After confirming that the preceding configuration is correct, run the string command "display ip routing-table" to check whether the local static route settings take effect, if it does not take effect, you need to re-enable and set static routes so that the IP packets cannot be forwarded.

4. Solve serious data packet loss faults

Recently, the offices in the organization reported to the network administrator that some of their workstations access the server very slowly and can upload or download text files to or from the server, if you want to transmit a large volume of multimedia information to the server, the speed will be very slow, and sometimes it will be unable to connect. Therefore, the network administrator uses the ping command to test the connection status of the server on one of the faulty workstations. The test results show that some data packets sometimes suffer from high latency, in addition, packet loss occurs during network transmission. Because the network topology of the room is that all workstations are connected to a 10 m hub through a common twisted pair, the 10 m hub is cascade to the core switch of the LAN, all communication ports of the core switch are in the 10 M/M adaptive working state, and the server is also directly connected to the switch.

After learning about these conditions, the network administrator first tries to connect the hub line to the faulty workstation directly, and then runs the ping command on the server to test. The test result shows that no packet delay occurs, no data packet loss occurs, and the test results are normal. Then the network administrator runs the ping command on the old computer where the 10 m Nic is installed. The test result is normal, the faulty computer happens to be a new workstation with a m Nic device installed. The Network Administrator has repeatedly analyzed this phenomenon. Is there a problem that the NIC transmission speed of the workstation matches the transmission speed of the switch? With this in mind, the network administrator forced the transmission speed of the M Nic device to 10 M in the faulty computers, and then conducted access tests. The results showed that the fault was gone, obviously, the above fault is indeed caused by speed mismatch. In the future, when we encounter the same fault, we may wish to carefully check whether the transmission speed of the faulty workstation and the switch matches. If not, you only need to modify the transmission speed of the NIC device in the faulty workstation to ensure that the NIC device and the switch maintain a matching working speed.

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