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.

closed as off-topic by Jenny D, Tom O'Connor, mdpc, yagmoth555, Dave M Sep 17 '17 at 12:21

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  • 2
    Help you what? Fix the switch? Fix the computer? Get coffee? If you move the connection from the machine in question to a different switch port and the problem follows the machine then I'd say it's a machine problem. Have you tried moving the machine to any other ports to see if the problem follows? – joeqwerty Aug 25 '17 at 12:35
  • @joeqwerty Yeah, and the problem persists. I will send the machine to warranty. You think is the nic of the machine? – Joshua HG Aug 25 '17 at 13:31
  • Probably. It could be a software bug or malware but I'd suspect a hardware problem at first blush. – joeqwerty Aug 25 '17 at 13:39
  • Upgrade the switch to the latest IOS version for it. If that doesn't work, raise a ticket with Cisco TAC using your Smartnet contract. You do have a smartnet contract, right? – Tom O'Connor Sep 11 '17 at 15:01
  • The problem was on the iMac NIC. Thanks anyway. – Joshua HG Sep 11 '17 at 15:43

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.

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