Err-disable in Switch 2960 [closed]

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I have a problem.

I have a Cisco 2960, this switch conect 23 Macs. The problem is that a pc puts the port in err-disable.

When I put the command “show interfaces status err-disabled” the output is this:

Port      Name               Status       Reason          Err-disabled Vlans
Fa0/23                       err-disabled loopback
Fa0/24                       err-disabled loopback

First it happened with the 23, I changed the machine to 24 and after several hours the same thing.

In Cisco blog: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/69980-errdisable-recovery.html#anc15

They only mention this:

Loopback error

A loopback error occurs when the keepalive packet is
looped back to the port that sent the keepalive. The switch sends
keepalives out all the interfaces by default. A device can loop the
packets back to the source interface, which usually occurs because
there is a logical loop in the network that the spanning tree has not
blocked. The source interface receives the keepalive packet that it
sent out, and the switch disables the interface (errdisable). This
message occurs because the keepalive packet is looped back to the port
that sent the keepalive:

 %PM-4-ERR_DISABLE: loopback error detected on Gi4/1, putting Gi4/1 in err-disable state

Keepalives are sent on all
interfaces by default in Cisco IOS Software Release 12.1EA-based
software. In Cisco IOS Software Release 12.2SE-based software and
later, keepalives are not sent by default on fiber and uplink
interfaces. For more information, refer to Cisco bug ID CSCea46385
(registered customers only) .

The suggested workaround is to disable
keepalives and upgrade to Cisco IOS Software Release 12.2SE or later.

Can someone help me? Please.

Answer

The NIC of your machine resend the STP-keepalive to his connected port. So the switch close this port.

Maybe, the loop can come from two interfaces installed on your machine which can be plugged on your switch. The interfaces can be virtualized interfaces of a virtualization software and misconfigured.

Or the interface of your machine can be configured in a specific mode which resend all his received traffic. It would be interesting to see your traffic with tcpdump or wireshark when you plug your NIC on the switch.

Attribution
Source : Link , Question Author : Joshua HG , Answer Author : Sorcha

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