QFX5100 : QinQ with routed/IRB interfaces?

I'm trying to setup a QinQ interface where the S-TAG will remain the same popped/pushed but the inner C-TAG can be a VLAN from 1000-2000.

That bit's fine but each of the C-TAG VLAN's need to have an associated IRB/SVI interface on the same QFX for routing IP traffic.

When I setup the QinQ interface the eassociated IRB interface stays in a down/down state.

I'm guessing as the C-Tag is an inner VLAN tag the Juniper doesn't see it on an interface so leaves the routed interface in a down state?

If I setup a spare port as an access port in that VLAN it comes up fine.

I may be missing something really obvious or maybe it's not actually possible and it has to traverse the QFX5100?

Thanks