Happens enough to have documentation written up about it here
. When troubleshooting unexpected link flapping, it is important to know: who initiated the link flap and the actual reason for the link being down. The common cause of link flap is a Layer 1 issue such as a bad cable, duplex mismatch, or bad Gigabit Interface Converter (GBIC) card.
The bit rate exceeds the threshold and puts the port into an error disabled state.
Disable and then enable the port.
A hardware failure or intermittent hardware error causes a packet drop in the switch. A software error causes a packet drop. A control frame is erroneously sent to the device.
Determine the reason for the link flap as indicated by the MAC driver.
Link flapping on the upstream switch.
Determine the reason for the link flap on that switch.