We have just subscribed to a new L2 service through a local provider, this is meant ot provide our office with connections to two other branches via L2 connections to each.  At our end we provide an 802.1q trunk and at the two ends they provide a device that removes the tags.

I believe they double-tag, the add 1001 (or whatever our customer tag is) and 160/150 as outside and inside vlan tags to our traffic.  I have a 3560 on our end that I have an interface configured as a trunk with 150 and 160 (actually with all permitted at the moment).  The new vlans trunk ok, I can connect devices to other swtiches, move their ports onto the new vlan and connect through but if I take something to the other end and connect it then I cant see it.

Config for our end is as follows

interface GigabitEthernet0/1
description MetroFlute Link
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,100-200,1002-1005
switchport mode trunk
switchport nonegotiate
media-type rj45
speed 1000
duplex full
end

The service provider isn't very technical; they havent even been able to tell me what devices are on teh other end.  What debug commands can I use on my switch so that I can try and identify whats happening with the traffic on this link?  He tells me they ahve it configured identically to toher customers and he sees no reason why it shouldnt work.  It is a non-cisco device on the other side.

Thanks