IPv6 over IPv4 Network (6PE)

Before you start the hands-on part of this module, you should load the appropriate configuration and verify that the testbed is up and running by executing the corresponding robot file:

In order to get a better understanding, the lab setup is shown in the picture below.

If a network is based on IPv4 and wants to offer IPv6 internet service to customers, IPv6 Provider Edge (short 6PE) technology can be used. The basic idea is to implement IPv6 on edge router (aka PE routers) which are dual-stack in order to connect to the IPv6 network on the one side and to the MPLS core on the other side.

The core routers only run IPv4 and MPLS signaling protocols to setup LSPs, but are unaware of IPv6. Deploying 6PE only affects the PE routers, which simplifies maintenance and eliminates the need to upgrade the core routers.

Consider two PE routers, R1 and R4, that want to interconnect two isolated IPv6 sites. Because there is MPLS running in the network, we can assume there are LSPs already setup using any label distribution protocol (e.g., LDP), i.e., R1 has an entry for the loopback address of R4 in its ipv4 labeled-unicast routing table and vice versa.

The IPv6 routing information are exchanged between R1 and R4 via BGP. Standard BGP Network Layer Reachability Information (NLRI) only allows IPv4 prefixes and IPv4 next hops, but with multiprotocol extensions of BGP (MP-BGP), NLRI’s for other address families (AFI) and subsequent address families (SAFI) are supported. With 6PE, the AFI is set to 2 (IPv6) and the SAFI to 4 (labeled-unicast).

As next hop for the IPv6 prefix, the IPv4 loopback address of the BGP peer is used which is encoded as an IPv4-mapped IPv6 address in the BGP next hop field. In addition, an MPLS label is added to the IPv6 prefix to signal, that a IPv6 service is offered. In principle, this can be any label value but as R4 needs to perform an IPv6 lookup anyway, most implementations use the reserved label value 2 (IPv6 explicit NULL). The label value is encoded as part of the prefix.

When R1 receives a native IPv6 packet, R1 will inspect its IPv6 unicast routing table to find a match for the IPv6 destination address of the packet. As a result, it will find

After both of these labels are pushed to the IPv6 packet, the packet is forwarded to the next router along the MPLS LSP. The core routers will only forward the packet based on the outer transport label and the net-to-last router will perform penultimate hop popping.

Finally, R4 will receive a single-labeled packet, remove the label and perform a IPv6 routing table lookup.

Once we have successfully configured the BGP peers, there should be a couple of BGP sessions established.

From the following output we can see that R1 is receiving one BGP prefix from each peer, but not sending any prefixes itself.

Let’s have a more detailed look at the BGP session:

In order to understand how 6PE works, it is crucial to know that the 6PE router has two roles:

These two sides need to be connected. In RBFS, the keyword is the send-label option, that needs to be configured in the ipv6 unicast address family.

Now that BGP session is established and routes are exchanged, let’s have a look at the resulting routing tables:

Finally, we can verify that IPv6 packets are successfully sent and received across an IPv4-only core network:

Ups. Although we have received a valid route and installed it into the IPv6 unicast routing table, the ICMP Echo and Reply packet are not forwarded correctly. The reason can easily be seen by looking at the learned prefix in more detail:

In our scenario, the routers in the core network are completely unaware of IPv6 forwarding. Thus, we need to send the packet along an MPLS LSP towards R4. This is achieved by resolving the next hop address using the IPv4 labeled-unicast routing table.

This module outlined the basic concept of interconnecting IPv6 network over an IPv4-based MPLS network (also known as 6PE) and how to configure it on RBFS.

If you have completed the exercise, you can check the results by executing