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Switch Issues

An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode
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
Can't configure "switchport trunk encapsulation dot1q"
Cisco: err-disable: psecure-violation error detected
Cisco: "Operational Mode: down" instead of "static access"
Cisco switch port shows err-disabled
Cisco switch still runs old version after upgrade
Cisco Switch: Unexpected BPDU received. Port has been Disabled
CNA message: "The embedded TFTP server cannot start".
CNA shows 2960 switch as Unsupported Device
Detected a Loop back. Port has been Disabled
Get access denied when login Cisco using putty
It doesn't show the new VLAN
Line protocol is down (err-disabled)
Line protocol is down (notconnect)
Network devices keep losing connection
Network goes down while configure Cisco wireless
New switch can't ping public IP address
New switch can't ping a VLAN
Operation mode down and err_disable: psecure-violation was detected
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
Trunk mode shows desirable with screenshot
Unexpected BPDU received. Port has been Disabled
Unrecognized command: srr-queue bandwidth share 10 10 60 20
Upgrading Cisco SG switch gets "illegal format"
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




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