Switch port automatically disabled because of loop back

[danny dang]

Hi,

One of the ports on my Cisco 3560 switch has been disabled because of loop back. I used SmartPort from Cisco Device Manager to set the port as Cisco Phone + Desktop.

In Cisco Network Assistant, the Event Notification says:

Description: Fa0/14: 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.

Can someone show me how to correct this error.

Thank you.

[Richard Burts]

I had a situation that sounds similar in which switch ports were going into error disabled because of loopback. I found that on a different switch in the network that there was a connection that was actually creating a loop. But spanning tree was not detecting it because the switch on one side called it vlan 5 and the switch on the other side of the connection called it vlan 6. So spanning tree was not detecting the loop. It resulted in my switch port seeing its own BPDU coming back to it. When I broke the real loop then my switch port became just fine. Perhaps you have a situation somewhat like that.

HTH

Rick

[danny dang]

Thanks Richard for the response. The weird thing is there is nothing plugged in into this port.

[Richard Burts]

If there is nothing physically connected to Fa0/14 and it is showing as disabled then that is indeed pretty weird. Can you post the output of show interface, do a shut and a no shut, and the output of another show interface command?

HTH

Rick

[danny dang]

Here they are:

After shut and no shut command:

Show running-config int fastethernet0/14:

In the running configuration of the port, the switchport port-security lines and the spanning-tree bpduguard line are created automatically when the port is disabled. Other ports dont have these lines.

I tried to use No command....to reset the port, then use Network Assistant to configure the port again as Cisco IPphone + Desktop port. When I chech the running-config on the port, those switchport port-security lines and spanning-tree bpduguard line show up again.

Thank you.

[danny dang]

sorry, this is the show running-config int fa0/14: