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?