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[wildweaselmi]

Many businesses have the need of finding what port the user is on or wants active or moved to a certain VLAN.  All that is required is a loopback plug like the one shown here:
 rj-45_loopback.gif   4.01K   1 downloads


error condition results from the existence of a loopback on the port. The switch sends keepalive packets out all interfaces. If a keepalive packet is received on the same interface it was sent from, then a loop exists that has not been blocked by Spanning-Tree Protocol. If this occurs, these messages are generated (these specific messages are from a Cisco Catalyst 2940).

Quote

%ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/3.
%PM-4-ERR_DISABLE: loopback error detected on Fa0/3, putting Fa0/3 in err-disable state

To resolve the problem, you must investigate the network beyond the port that is complaining and discover what sort of logical loop might exist that was undetectable by STP. Often, less capable devices (inexpensive unmanaged switches or hubs) are the culprit.

You can check the status of errdisabled ports before and after to validate

Quote

#show errdisable recovery
ErrDisable Reason    Timer Status
-----------------    --------------
udld             Enabled
bpduguard            Enabled
security-violatio    Enabled
channel-misconfig    Enabled
pagp-flap            Enabled
dtp-flap         Enabled
link-flap            Enabled
l2ptguard            Enabled
psecure-violation    Enabled
gbic-invalid     Enabled
dhcp-rate-limit      Enabled
mac-limit            Enabled
unicast-flood        Enabled
arp-inspection   Enabled

Timer interval: 300 seconds

Interfaces that will be enabled at the next timeout:

Interface      Errdisable reason      Time left(sec)
---------    ---------------------    --------------
  Fa0/3                bpduguard          273