The ingress 6PE router can push two or more MPLS labels to send the packets to the egress 6PE router. The top labels are associated with resolving the transport tunnels. The bottom label is advertised in MP-BGP by the remote 6PE router. Typically, the IPv6 explicit null (value 2) label is used, but any arbitrary value can be received when the remote 6PE router is not an SR OS router.
The egress 6PE router pops the top transport labels. When the IPv6 explicit null label is exposed, the egress 6PE router knows that an IPv6 packet is encapsulated. It pops the IPv6 explicit null label and performs an IPv6 route lookup to find the next hop for the IPv6 packet.