What is a reason for 6PE to use two MPLS labels in the data plane instead of one?
A.
6PE allows penultimate hop popping and has a requirement that all P routers do not have to be
IPv6 aware.
B.
6PE does not allow penultimate hop popping.
C.
It allows MPLS traffic engineering to work in a 6PE network.
D.
It allows 6PE to work in an MPLS network where 6VPE is also deployed.
“n order for the IPv6 transport to be transparent to all but 6PE routers, it is necessary to impose a hierarchy of labels at the 6PE ingress router. The top label provides connectivity inside the IPv4 MPLS core network: LDP (Label Distribution Protocol), TDP (Tag Distribution Protocol) or RSVP (Resource Reservation Protocol) distributes it. The next label is used at each 6PE egress router for IPv6 forwarding: it is distributed by MP-BGP (Multi-protocol BGP) in the “IPv6+label” address family”
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_data_sheet09186a008052edd3.html
penultimate hop popping : remove one layer of the MPLS label, it is important for the none-dual stack network to transit the IPV6 traffic in MPLS
PHP is one thing, but the receiving PE router has to know what protocol is being carried. Without additional label IPv4 packet would be expected. In an ordinary frame the type field tells us that it is IPv6. If the packet is carried by MPLS we do not have type field (or the type field tells us the protocol is MPLS). The additional label lets us recognize that it is IPv6.