The optical module lights up but the network connection is lost
Use an optical power meter to test the receive power of the port and check whether the optical fiber is disconnected. If the optical module is installed on a GE port, run the display interfaceGigabitEthernet x/x/x command to view port information when the optical module is inserted, including the rate and wavelength. Understanding how to troubleshoot and prevent a failing optical module is vital for good network stability. The SFP on the 2960 has flashing link lights and shows as connected [line protocol is up, and "show interface gi0/1" command shows packets being sent but none received], but the interface on the 3750 shows as not connected.
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