Cisco Catalyst 3560 Loopback Error Recovery

Recently we encountered an issue on Cisco Catalyst 3560 switch in which ports Fa0/10 and Fa0/22 has been disabled automatically by the IOS Software due to an error.

First thing to do is determine the cause of the error. This topic can be found on the Cisco website on this link.

IOS commands issued to determine the error.

In my case I always utilize putty in accessing Cisco IOS.

Login to Cisco IOS (telnet) using putty then enter the equivalent password for Cisco IOS

Run enable command then enter the equivalent password

Then run this command to determine the status of the port

Switch26# show interface Fa0/10 status
Port Name Status Vlan Duplex Speed Type
Fao/10 err-disabled 1 auto auto 10/100BaseTX


To determine the cause of the error run the following command
# show log

This command will dump a log on all the ports. Check specifically the logs on the port that encountered the issue.
In our case its interface Fa0/10

The log would show up something like this

%PM−4−ERR_DISABLE: loopback error detected on Fa0/10, putting Fao/10 in
err−disable state

So the error encountered was a loopback.

Cisco Catalyst 3560 Switch loopback issue

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:

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

Run the following command to disable keepalive on a particular port

Switch26#configure terminal
Switch26 (config) #interface Fa0/10
Switch26 (config) #no keepalive
Switch26 (config)end

Run this command to check the current configuration
Switch26#show running-config interface Fa0/10
Building configuration...

Current configuration : 48 bytes
!
interface FastEthernet0/10
no keepalive
end

Copy the running configuration to the startup configuration

Switch26#copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
[OK]
Switch26#

Now we need to enable the new settings to the port by resetting the port

Run the following command shutdown and no shutdown


Switch26#
Switch26#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch26(config)#interface Fa0/10
Switch26(config-if)#shutdown
Switch26(config-if)#end
Switch26#show running-config interface Fa0/10
Building configuration...

Current configuration : 58 bytes
!
interface FastEthernet0/10
no keepalive
shutdown
end

Switch26#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch26(config)#interface Fa0/10
Switch26(config-if)#no shutdown
Switch26(config-if)#end
Switch26#show running-config interface Fa0/10
Building configuration...

Current configuration : 48 bytes
!
interface FastEthernet0/10
no keepalive
end

Switch26#

That’s all folks . . .