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.