Operation of 6PE

In the 6PE solution, the PE routers are dual-stack, which means they run IPv4 and IPv6. The CE routers that are running IPv6 are connected to the PE router via a normal interface; the interface is not part of a VRF for IPv6 even though the same interface might be in a VRF for IPv4. The IPv6 routing distribution between the PE routers is done via MP-iBGP. At the same time, MP-iBGP distributes the label to be used for the specific IPv6 prefixes. This BGP label identifies or tags the IPv6 packet at the egress PE. The egress PE looks up this BGP label in the label forwarding information base (LFIB) and uses it to forward the IPv6 packet toward the egress CE. Look at Figure 9-5.

Figure 9-5 6PE Network

Customer One

IPv6 IGP, eBGP, or

IPv6 IGP, eBGP, or

' IPv4 IGP, eBGP, or Static

Customer One

MP-iBGP IPv6 + Label

MP-iBGP IPv6 + Label

IPv4 and IPv6 IGP eBGP, or Static

Full Mesh Of MP-iBGP

IPv4 and IPv6 IGP eBGP, or Static

te tti

w IPv6 IGP, eBGP, or Static

IPv6 IGP, eBGP, or Static

IPv6 IGP, eBGP, or Static

IPv6 IGP, eBGP, or Static

Customer Two

Customer Two

Figure 9-5 shows an MPLS network that is running 6PE. The PE routers have CE routers that are connected to them. Some of the CE routers are running IPv6, whereas others are running IPv4. The PE routers can even have a VRF interface and run MPLS VPN for IPv4 for some CE routers. The interfaces that are on the PE toward the CE routers that are running IPv6 are not in a VRF, though. The PE routers run a full mesh of MP-iBGP for IPv6. The iBGP sessions distribute the IPv6 prefixes and the associated MPLS label. This is known as IPv6 + label and is encoded according to RFC 3107. Look at Figure 9-6. It shows a network that has only two PEs running 6PE. The sydney PE router distributes the IPv6 prefix 2001:DB8:1:2::1/128 with a label of 22 to the london PE via MP-iBGP. All PE and P routers run an IGP and LDP. For packets to be forwarded to the CE router that is connected to the sydney PE, you must distribute the BGP next hop 10.200.254.4/32 with a label in the MPLS network.

Figure 9-6 6PE Routing and Label Distribution

6PE Network RIPng

Loopback 0 2001:0DB8:1:1::1/128

london-ce cust-one

MP-iBGP Update

2001:0DB8:1:2::1/128 Next-hop: 10.200.254.4 Label 22

RIPng

6PE Network RIPng

2001:0DB8:1:2::1/128 Next-hop: 10.200.254.4 Label 22

RIPng

Loopback 0 2001:0DB8:1:1::1/128

london-ce cust-one

Loopback 0 2001:0DB8:1:2::1/128

cust-one

LDP Binding

LDP Binding

LDP Binding

Loopback 0 2001:0DB8:1:2::1/128

cust-one

LDP Binding

10.200.254.4/32 Label 19

LDP Binding

10.200.254.4/32 Label 19

LDP Binding

10.200.254.4/32 Label Implicit-null

The packet forwarding is shown in Figure 9-7. You can see two labels on top of the packets: an IGP label as the top label and the BGP label as the bottom label. The IGP label is the LDP or RSVP (traffic engineering) label for the BGP next hop of the egress PE. This BGP next hop is encoded as an IPv4-mapped IPv6 address containing an IPv4 address of the egress PE router. As such, when the IPv6 packet arrives on the ingress PE and needs to be label-imposed, the IGP label used is the one associated with the IPv4 address derived from the IPv4-mapped IPv6 address. The BGP label is the label that the remote PE sent for the IPv6 prefix.

Figure 9-7 6PE Packet Forwarding

IPv6 Packet

Loopback 0 2001:0DB8:1:1::1/128

london-ce cust-one london

Label 19

Label 19

Label 22

Label 22

IPv6 Packet

Ii Ri new-york paris

Label 22

IPv6 Packet

Ii Ri new-york

IPv6 Packet

10.200.254.4 Loopback 0

2001:0DB8:1:2::1/128

%r sydney

1 i sydney-ce cust-one

One advantage of 6PE is that the P router does not need to run IPv6 or even be IPv6 capable. Therefore, the 6PE solution can be quickly deployed over an existing MPLS backbone. A second advantage is that the IPv6 packets are directly labeled without an extra header. In the case of an AToM solution, an extra Layer 2 header is transported; in the case of IPv6 over IPv4 tunnels, an extra IPv4 header is transported. Part of the reason for the deployment of the 6PE solution is that many service providers already have an MPLS backbone because of the popularity of MPLS VPN. The operation of 6PE is similar to the operation of MPLS VPN. As such, people who are familiar with MPLS VPN quickly understand 6PE. Following are some similarities to MPLS VPN for IPv4:

■ There needs to be an IGP for IPv6 or eBGP or static routing between the PE and CE.

■ The IPv6 packets are labeled with two labels.

Perhaps most important, the 6PE solution follows the peer-to-peer VPN model that was discussed in Chapter 1, "The Evolution of MPLS," and is used in the MPLS VPN for IPv4 solution. Each CE has only one routing peer across the MPLS network, namely toward the directly attached PE router.

Because this solution involves no VPNs, no VRF interfaces exist for IPv6 on the PE routers; as such, the PE and CE routers can use any routing protocol for IPv6 in the 6PE solution. The IPv6 routing protocol does not have to be VRF-aware.

The IPv6 packet has two labels when it is forwarded through the MPLS network. Strictly taken, the IPv6 packet can be forwarded through the MPLS network with only one label on top of it. Because of penultimate hop popping (PHP), the packet would then be forwarded unlabeled between the last P router and the egress PE router. This is possible only if the last P router is IPv6-capable. If only one MPLS label and PHP are in use, the penultimate hop router must be able to know what the MPLS payload is before sending the IPv6 packet toward the egress PE router. The penultimate router must know this to set the correct protocol type in the Layer 2 header when sending the frame. Updating the IPv6 header (updating the Hop Limit and Traffic Class fields) before forwarding the IPv6 packet to the egress PE router might also be a problem. To avoid the IPv6 capability in provider (P) routers, 6PE uses two MPLS labels for forwarding. To deploy 6PE, only the edge routers need to be IPv6 capable, which is a great advantage when deploying 6PE over existing MPLS backbones.

Micro Expression Master

Micro Expression Master

If You Could Read Everyone Life A Book You Can Have Better Career, Great Relationships And Become Successful. This Book Is One Of The Most Valuable Resources In The World When It Comes To Reading the smallest and tiniest body Language and know what people are thinking about.

Get My Free Ebook


Post a comment