MPLS - Connecting IPv6 sites over an IPv4 backbone

Interconnecting remote IPv6 sites over an IPv4 MPLS backbone is a common scenario, especially in situations where service providers or enterprises are transitioning to IPv6 but still have significant IPv4 infrastructure. One of the popular methods to accomplish this is using 6PE (IPv6 Provider Edge Router over MPLS) and "6VPE" (IPv6 VPN over MPLS). Here's a brief overview:

6PE (IPv6 Provider Edge Router over MPLS):

With 6PE:

  1. Dual Stack on Edge Routers: The Provider Edge (PE) routers are made dual-stack, meaning they support both IPv4 and IPv6.
  2. MPLS Backbone: The core MPLS network can remain IPv4-only, so there's no need to upgrade the Provider (P) routers to support IPv6.
  3. MP-BGP Extensions: The PE routers use MP-BGP extensions to advertise IPv6 prefixes to other PE routers across the IPv4 MPLS backbone. The IPv6 prefix is carried as an NLRI with an IPv4-mapped next hop.
  4. Label Switching: When an IPv6 packet arrives at a PE router, the router pushes an MPLS label (based on the IPv6 destination) onto the packet and forwards it through the IPv4 MPLS core. The MPLS label guides the packet to the correct egress PE router, where the label is removed, and the original IPv6 packet is forwarded to its destination.

6VPE (IPv6 VPN over MPLS):

6VPE is an extension of 6PE that adds VPN capabilities, allowing different IPv6 sites to communicate privately over a shared MPLS backbone.

  1. VRF (Virtual Routing and Forwarding) Instances: Each IPv6 VPN customer has a dedicated VRF on the PE routers. This ensures segregation and privacy of each customer's traffic.
  2. MP-BGP for VPNv6: Just like 6PE, MP-BGP is used. However, here the PE routers advertise VPNv6 prefixes. The IPv6 prefix, along with a Route Distinguisher, forms a VPNv6 NLRI.
  3. Route Targets: Route Targets are used to control which VRFs on which PE routers should receive a particular VPNv6 route.
  4. MPLS Labeling: The process is similar to 6PE, but here two labels are used. The top label directs the packet to the egress PE router, and the bottom label ensures the packet is forwarded to the correct VRF/VPN.

In both cases, the core MPLS network remains unchanged and doesn't need to be aware of the IPv6 prefixes, thanks to the label switching nature of MPLS. This provides a seamless way to transport IPv6 over an existing IPv4 MPLS backbone.