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Switch Issues

Cisco Switch: Unexpected BPDU received. Port has been Disabled
Can ping but access a remote computer because of Cisco switch port configuration
Can't connect Cisco 2955 to 3560G switches
Can't connect two Cisco switches in fiber-optic network
Can't connect to two Fiber Converters via Cisco switches
Detected a Loop back. Port has been Disabled
Line protocol is down (err-disabled)
Line protocol is down (notconnect)
Network goes down while configure Cisco wireless
Port led is orange  in Cisco switch
This port has been disabled because Link has been found to be fluctuating between Up and Down states rapidly
Unexpected BPDU received. Port has been Disabled
VLAN doesn't work after replacing core switch and router

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Detected a Loop back. Port has been Disabled

One of our Cisco 3560 switch keeps showing this warning: Description: Gi0/15: Detected a Loop back. Port has been Disabled. Recommendation: Check Configuration of the switch, also make sure devices connected to the switch are not mirroring the traffic back to switch. Enable this port again.

However, there is no thing connect to the port Gi0/15 as shown the below. Why do you get this warning?

Case 1: This message occurs because the keepalive packet is looped back to the port that sent the keepalive. To fix it, disable keepalives and upgrade to Cisco IOS Software Release 12.2SE or later.

Case 2: one or some ports were configured trunk with portfast as shown below. removing portfast and bpduguard
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree portfast
spanning-tree bpduguard enable