Wired network faults caused by wireless networks and troubleshooting

The network of the school library has been in normal use, but recently it was found that the new center switch of the library and the core switch of the campus network often fail to connect.

Troubleshooting Process

I think there is a convergence layer switch in the library that actually reaches 4909) and the core switch Cisco 4006 S3 of the campus network) are connected through multi-mode optical fiber. Therefore, first consider restarting the two switches, however, I found that the Library's actual usage of 4909 will be disconnected after several seconds of connection with Cisco 4006 S3. Then I used the exclusion method to eliminate the problems of optical trunk, fusion splices at both ends of the optical fiber, Optical Fiber adapter, and optical fiber jumper. During the above test, I found that the two ends can be connected occasionally, but will soon be disconnected.

Is there a problem with the vswitch actually reaching 4909? After the vswitch is restored to factory settings and parameters are reset, the fault still persists. However, I found that there seems to be a problem with an optical jumper. If it is not connected to the network, and once it is connected, the Link port indicators of the GBIC ports of each module on the 4909 actually flash sharply, the network will be disconnected immediately. So the author located the problem on the device connected to this optical jumper or this optical jumper. After investigation, the optical fiber is connected to a gigabit module of a M switch. At this time, I found that two wireless APs were connected to this switch. After the two wireless APs were removed, the network immediately recovered. It seems that this fault was caused by two wireless APs.

Fault Cause

The original two Wireless AP modes should be Access Point, but some people set the mode of one Wireless AP to Access Point Client mode. In this way, two wireless APS connected to the same vswitch are connected to a wireless bridge in the air, forming a network loop, which not only causes the network failure of the actually connected M switch, it also caused a failure of the actually 4909 vswitch connected to it, resulting in a failure of the entire network.

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