carrier supporting carrier is a special kind of layer3 mpls vpn
when the customer isp requests a labeled layer3 vpn transport from the
backbone isp. from the backbone isp's point of view, it's just a simple layer3 mpls
vpn where they must use the per-prefix label allocation scheme for the
vrf. the customer isp then could choose how to peer with the backbone carrier.
it could extend it's internal isis or ospf with ldp
and some constrains regarding the routing protocol features, or may choose bgp
labeled unicast afi, and redistributing between it and it's igp.
the final result in any case will be and end to end labeled
path through the backbone isp, who will have full visibility into the customer
isp's igp routes and outer labels.