In Figure 76, a service provider is offering IPv6 VPN service over an MPLS-enabled IPv4 backbone. The base MPLS tunnels are established in the IPv4 core network with either of the MPLS signaling protocols (LDP or RSVP). The ingress PE router pushes the LSP tunnel label directly onto the label stack of the labeled IPv6 VPN packet. The topmost label imposed corresponds to the LSP that runs from the ingress PE router to the egress PE router. The BGP next-hop field identifies the egress PE router, and therefore the topmost label to be pushed on the stack. The bottom label is the label bound to the IPv6 VPN prefix by means of BGP.
The CE devices can attach to the VRFs on the PE routers using both an IPv6 link and an IPv4 link. In Figure 76, the CE devices attach to the VRFs over an IPv4 link, and use MP-BGP to connect to the VRFs on the PE routers. This arrangement enables the PE routers to learn IPv6 routes from MP-BGP running over TCPv4 from the CE devices. You can also configure IPv6 static routes in the VRFs on the PE routers to reach the networks through the CE IPv6 link. Alternatively, you can configure the static routes with any routing protocol that supports IPv6, such as OSPFv3.
Figure 76: IPv6 VPN Services over IPv4 MPLS
The PE routers use an MP-BGP session over TCPv4 to advertise the IPv6 routes from the CE devices to the remote PE routers. The IPv6 routes are advertised as labeled VPNv6 routes with a BGP next hop set to the base tunnel endpoint destination address. The next hop is formatted as an IPv4-mapped IPv6 address.
For IPv6 VPN services over an IPv4 backbone, the BGP next hop in the MP_REACH_NLRI attribute contains a VPN-IPV6 address with the RD set to zero and with the 16-byte IPv6 address encoded as an IPv4-mapped IPv6 address that contains the IPv4 address of the advertising PE router. This IPv4 address must be routable in the service provider’s backbone.