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L2Protocol-Tunnel causes loopback errors IE Lab 20 posted 11/25/2007
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Hi Experts,

This is from IE lab 20 Task 1.1 and 1.2.

Shortly after configuring the l2protocol tunnels on SW4, either SW1 or
SW2 goes err-disable on the non tunneling ports:

%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on FastEthernet0/20
%PM-4-ERR_DISABLE: loopback error detected on Fa0/20, putting Fa0/20
in err-disable state

The strange thing is that the ports that are going err-disable are not
even part of the tunnel configuration!
The only way I could stop this from happening was to remove vlan 100
and 200 (the access vlans used for tunneling) from the allowed vlan
list on SW4.

Any ideas on why this happens? I can send you my configs if needed.