Stupid Router Trick - Double Tag that please.
Customer had a QinQ set up so they could assign their own vlans on a layer2 "vpn". The CPE at both site was just a normal 3750 which was doing a dot1q tunnel interface towards the customer, and a standard trunk going into the ISP Pop.
Topology looked like this
C1 (6500) -> CE1 (3750) (L2 Circuit) -> CORESwitches ->(L2 Circuit) CE2 -> C2 (6500)
Long story short - They killed one of the 3750's on site playing with power. It seems that customers get much more angry when THEY cause the outage. So with CEO screaming at us to get the connection back up and spare parts hours out - what do you do?
After asking what their uplink switch was and knowing the 6500 capabilities - I had the idea of adding the Provider Tag onto the customer switch directly. You would normally never do this but because this was the "Small ISP" where service usually sets you apart from the big guys - you do whatever you can to get them up.
First was to define the Service Provider vlan tag (say 100). Next was to configure another port on the 6500 to be the dot1q tunnel port -
interface gi1/1
switchport access vlan 100
switchport mode dot1q-tunnel
no cdp enable (This is very important)
And another uplink interface into the ISP - a normal trunk port with only vlan 100 defined
interface gi1/2
switchport trunk encapsulation dot1q
switchport mode trunk
switchport trunk allowed vlan 100
<other isp specific commands here>
You then hook a cable between the old uplink interface to CE1 into Gi1/1 and put the ISP into Gi1/2.
And that is how you hack a customer back online by self-double tagging. (some newer platforms can do this nativly)