Let’s assume that you have an IPv4 core backbone on which you did enable MPLS. Your customers are connected to your PE routers and they need to have IPv6 reachability between their sites. To have things more clear, please have a look to the following network topology:
IPv6 transport over IPv4 MPLS

As you can see in the above image, customers have the IPv6 addresses 2001::1/128 and 2001::2/128. Actually in this topology, “my customers” are two Loopback addresses (Lo200) configured under PE routers, to minimize the used resources (routers).

We have a typical MPLS (LDP) construction with following characteristics:
– OSPF is used for IGP
– each router has a L0 interface advertised into IGP
– L0 of each router is configured as MPLS LDP router-id
– PE routers have an additional L100 interface advertised into IGP
– iBGP on the PE routers is configured using L100 interface as neighbour peer points

You can find below some output of iBGP and MPLS configuration:



At this point we have a functional IPv4 MPLS. Now, let’s use the MPLS to transport IPv6 networks. First, on the PE routers we have to enable IPv6 routing support and CEF:

Next, I’ll have the L200 interface configured:

Now let’s make the magic and configure the PE routers so that the IPv6 networks are reachable. We will make this by using the 6PE method. This solution include that the Provider edge routers (PE) are IPv6 capable and it enables IPv6 sites to communicate with each other over an MPLS IPv4 core network using MPLS label switched paths (LSPs). To make it clear the Provider routers (P) inside MPLS have not idea about any IPv6 addressing or routing.

The 6PE routers are configured to be dual-stack using IPv4 and IPv6, and use the IPv4 mapped IPv6 address for IPv6 prefix reachability exchange. The 6PE uses multiprotocol internal BGP (MP-iBGP) to distribute IPv6 routes over the MPLS IPv4 core network and to attach an MPLS label to each route.

Let’s enable this. The BGP configuration looks like this:

I did enable the BGP neighbour under IPv6 address family using the IPv4 addresses. The IPv4 addresses are already know in MPLS using the IPv4 IGP protocol (OSPF). Very important here is the send-label option as it enables binding and advertisement of aggregate labels when advertising IPv6 prefixes in BGP.

Let’s check on the PE routers if we receive properly the IPv6 addresses:

And the last test:

If you have questions or there is something which is not clear please let me know by using the Comments section below.

Cisco: IPv6 over IPv4 MPLS (6PE)

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