Allied Telesis AT-x210-9GT - Manual (Page 59)

AT-x210-9GT, AT-x210-16GT, and AT-x210-24GT Switches Installation Guide
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Problem 5: A port’s LINK/ACT LED is blinking.
Solutions: The link between the port and the network device is intermittent.
Try the following:
„ Connect another network device with a different cable to the port. If
the Link LED remains steady on, then the problem is with the
original cable or the network device.
„ If the problem is with an SFP transceiver, check that the
transceiver is fully inserted in the slot.