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Identify what port on the switch you are on


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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:


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).

%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

#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

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