IPv4 address exhaustion forces carriers to put IPv6 network planning issues high on their agendas. For the BGP/MPLS IPv4 VPN services that have been widely deployed, supporting VPN dual-stack access is a practical and readily available solution to the transition from IPv4 to IPv6.
Originally, interfaces in a VPN support only a single type of protocol stack. After VPN dual-stack access is configured, both IPv4 and IPv6 address families can be configured in a VPN so that the interfaces bound to this VPN can support not only IPv4 VPN access but also IPv6 VPN access. This implementation greatly improves the feasibility of the transition from IPv4 to IPv6.
As shown in Figure 1, IPv4/IPv6 VPN dual-stack access allows VPN sites to support both IPv4 and IPv6 networks and to be connected to a PE through the same interface.