Carrying IPv6 over an MPLS Backbone

Because of the huge success of MPLS VPN, most service provider networks are running MPLS in their network today. If the service provider has customers connected to his network who want to run IPv6 and the service provider needs to carry IPv6 across his network, the obvious solution is to have IPv6 running on his routers, too. However, this approach has two disadvantages. First, the service provider needs to enable a new protocol (IPv6) on all his routers. Because IPv4 and IPv6 are running on the router, the router is running a dual-stack. Second, the other customers— still running IPv4—are not going away or transitioning their network to IPv6 overnight. Therefore, IPv4 and IPv6 have to run in parallel for a long time into the future.

If the service provider also wants to run MPLS for IPv6, IPv6 needs LDP support, which is not implemented yet. However, because MPLS stands for Multiprotocol Label Switching, it can transport more than just IPv4 as a payload. In networks that are running MPLS today, the labeled packets might be IPv6 packets, without the need for the P routers to run IPv6. The solutions 6PE and 6VPE are based on this.

Another method to carry IPv6 over an MPLS backbone is Any Transport over MPLS (AToM). With this solution, the MPLS payload is actually a Layer 2 frame. On the edge LSRs, the frames are labeled and then transported across the MPLS backbone through a virtual circuit or pseudowire. The transported Layer 2 frames can be Ethernet, High-Level Data Link

MPLS VPN Network Using IPv6 over IPv4 Tunnels on the CE Routers 353

Control (HDLC), ATM, Frame Relay, and so on. This solution is described in Chapter 10, "Any Transport over MPLS." All three solutions have the advantage that the P routers in the MPLS backbone do not need to run IPv6 because the P routers switch only labeled packets. As such, these solutions are more popular than directly running IPv6 across the backbone. The AToM solution has two disadvantages compared to 6PE and 6VPE. The first is that the MPLS payload is made up of frames and not IPv6 packets. Therefore, an added Layer 2 header needs to be transported across the MPLS backbone. The second is that the pseudowires or virtual circuits are point-to-point in nature, whereas the 6PE and 6VPE solution are any-to-any.

One last method to carry IPv6 over an MPLS backbone is to use the MPLS VPN solution. In the case of MPLS VPN, IPv4 is carried inside VPNs over the MPLS backbone. To carry the IPv6 traffic over IPv4, the CE routers need tunnels between them. This means that the CE routers need to be dual-stack routers. These routers are the only routers that are running IPv6, because the PE routers see only IPv4 packets coming from the CE routers. In short, the advantage is that MPLS VPN is already deployed in many or most service provider networks, and the PE (and P) routers do not need to run IPv6. The disadvantage is that the CE routers need to have tunnels configured, and an extra IPv4 header adds overhead.

Continue reading here: Operation of 6PE

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