Within this model there is more complexity due to the fact that the MPLS forwarding is extended end to end of the customers network. The ASBR routers that connect between the two remote sites utilise an LSP for transit rather than a unicast MP-BGP next hop described with the option A solution. The requirement for the LSP establishment is not fulfilled using an IGP due to the limitation of domain wide LSP and Opaque LSAs.
The solution used for the transit LSP labels is to use an additional address family within MP-BGP. The SAFI 4 address family is used to carry the transit labels within a BGP NLRI and is documented in RFC3107 (Carrying Label Information in BGP4). The NLRI does not contain RTs or RDs like other address families as it’s simply being used as a transit label signalling protocol.
Within the option C solution three labels are added to the transported frame. The inner label is the VRF label being carried as a service, the next label is the label used between the ASBRs within the two customer locations; this is the label that is carried within the SAFI 4 NLRI. The top label is the transit label for the carriers core and also is used from customer CE to carrier PE router (this uses MPLS in this solution unlike the previous solution).