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Community Member

Spanning Tree - "self looped" cure

Hi Folks,

We have several sites that have Stacked 3750s using two Multi Chassis Etherchannel links to connect to the wide area which is provided on World Wide Packets Metro Ethernet equipment. We have two VLANs; one for the link back to the main core WAN site and another to connect the sites that are geographically close to the 3750 sites. The etherchannel on the VLAN to the core works perfectly while the other is blocked by spanning tree with the tag "self looped". We cannot see why one would loop while the other doesn't. Can anyone shed any light, please?

Many Thanks


Cisco Employee

Re: Spanning Tree - "self looped" cure


I have seen these problems happen when configuring Etherchannels mostly without the management protocol (PAgP or LACP) when one switch had already its ports put into Etherchannel while the other was not yet configured for Etherchannel.

If it is possible, I suggest double-checking the Port-channel configuration on both switches: make sure that the members ports are all identically configured and that all relevant configuration appears also on the Port-channel virtual interface. If you do not use a management protocol I suggest you implement the LACP or PAgP (I slightly prefer the LACP).

After this check (please be really thorough!), shutdown all member interfaces including the Port-channel interface itself on both switches. Make sure that all member ports including the Port-channel interface indeed contain the "shutdown" command on both switches. Then, one switch after another, turn on the Port-channel interface using the "no shutdown" command. After the Port-channel is brought up again on both switches, check if it is still reported as looped.

The best practices when setting up Etherchannels are:

1.) Shutdown all physical switchports on both switches before assigning them to a Port-channel interface.

2.) Do not ever activate a port channel against another switch whose ports have not yet been assigned to a port channel.

3.) The recommended order of operations is: First, shutdown the switchports to be bundled on both switches. Second, assign the ports into Port-channel groups. Third, turn on these ports, one switch at a time.

4.) Whenever possible, use a management protocol such as LACP or PAgP.

Please let us know if this worked for you.

Best regards,


Community Member

Re: Spanning Tree - "self looped" cure

Hi Peter,

Thanks for that - we do suspect that our provider hasn't configured their kit correctly though they insist that they have, hence the question. The only evidence against them we have is that we can recreate the self looped state in a lab by configuring two ports in a channel-group then connecting them directly with a crossover cable.

I'll push them for some assistance and let you know the outcome.



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