The method of network connectivity fault elimination

Failure phenomenon

When the computer is unable to connect to the Internet, find the user's IP address from the document, try to ping several, and find all the connections timed out. Then, Ping the Library building's central switch is normal.

The electronic reading Room uses Cisco Catalyst 2950t-24-smi as a hub device: and through a twisted pair of cables with the central switch of the library building Cisco Catalyst 3550-48-emi connection. Since all users in the room are unable to connect. Then the first suspicion may be cascading cable problems or cascading end-to-end problems. So, first to the Library Network Management Center in the cabinet to view the Cascade Port LED indicator, did not find obvious anomalies. To the electronic reading room. Checked several computers, found not only unable to access the local area network and the Internet, not even between the ping, and can not find the other way.

Diagnostics process

Such a large number of computer network cards can not be damaged at the same time, so the initial judgment of the fault may be on the switch, Cascade Cable and switch port. Therefore, the first use of twisted pair tester to detect the connectivity of the network cable, no problem found. The cascade cable is plugged into another port on the Catalyst 3550 switch and then plugged into another switch, and the failure is still unresolved. Then look at the catalyst2950t switch lights, usually plugged into the port, the LED is bright, it seems that there is no problem. Barina can only use the method of replacement used to repair the computer, replacing Catalyst 2950T with another alternate switch. However, a few minutes later, the computer was unable to access the Internet, and communication between them was broken. It seems that the problem is not on the Catalyst 2950T switch! Since it is not the reason for the switch, what causes the computer in the network segment to lose contact with each other within a few minutes? The reason can only be one, that is broadcast storm, by the network card damage caused by the broadcast storm!

Turn off the power of the catalyst 2950T and then, before each computer, use Ping127.0.0.1 to test all the computers in the Internet room. When you find a computer with a network card failure, unplug the network cable it is connected to, turn on the switch again, and the network is finally back to normal. The next thing of course is to replace a new network card for the computer, which solves the problem.

Excluding experience

Why are other computers in the library building not affected? Since all are connected to the same catalyst 3550-emi, then all the computers in the library building should have connectivity problems. The switch as a two-tier device, all connected to the computer is in the same broadcast domain: should be subject to the factory broadcast storm is, very reasonable, but why is there such a problem? In order to improve the communication efficiency, effectively avoid the students to the school some important parts of the attack, protect sensitive data, We have divided a large number of VLANs, almost every student room is a VLAN. Because the communication between VLANs must be based on a third-tier device, there is simply no way to broadcast between VLANs, so the broadcast storm will be limited to one room (VALN). Therefore, in the local area network, the proper partition VLAN, not only helps to improve the network communication efficiency and network security, but also can effectively improve the stability of the network, so that one or several equipment damage, do not affect the entire network of normal communication.

In addition, Cisco switches provide a variety of hint lights, when a network failure, as long as the careful observation of the LED can be roughly judged the cause of the failure. Only then negligence of this point, so, in the troubleshooting to take some detours.

Connectivity failures are usually manifested in the following situations:

* The computer cannot log on to the server.
* Computers can see only themselves in their network places, but not other computers, making it impossible to use shared resources and shared printers on other computers.
* The computer cannot connect to the Internet over the local area network.
* The computer cannot browse the Web server in the local area network or send and receive e-mail.
* Some computers in the network are running at a very slow speed.

Connectivity failures may occur for the following reasons:

* The network card is installed at the end, or it is installed properly or conflicts with other devices.
* NIC hardware failure.
* Network protocol at the end of the installation, or settings are not correct.
* Network cable, jumper or information outlet failure.
*ups fault.
* Switch power is turned on, switch hardware failure, or switch port hardware failure.
*vlan Setup issues.