MPLS 6PE uses two labels in the data plane
When implementing MPLS in an IPv6 environment, 6PE ιs used to run IPv6 over an IPv4-only MPLS core where we use dual stack PE routers. In such a scenario, even though there is no VRF in use, 6PE still uses a second label.
This is because the top label is the transport label, which is assigned hop-by-hop by LDP or by MPLS traffic engineering (TE).
When 6PE was released, a main requirement was that none of the MPLS core routers (the P routers) had to be IPv6-aware. That requirement drove the need for two labels in the data plane.
- MPLS Penultimate hop popping functionality
- Load balancing
More information can be found in the links below: