Packet Forwarding in an Mpls Vpn Network

As explained in the previous section, the packets cannot be forwarded as pure IP packets between sites. The P routers cannot forward them because they do not have the VRF information from each site. MPLS can solve this problem by labeling the packets. The P routers must then have only the correct forwarding information for the label to forward the packets. The most common way is to configure Label Distribution Protocol (LDP) between all P and PE routers so that all IP traffic is label-switched between them. You can also use RSVP with extensions for traffic engineering (TE) when implementing MPLS TE, but LDP is the most common for MPLS VPN. The IP packets are then label-forwarded with one label from ingress PE router to egress PE router. A P router never has to perform a lookup of the destination IP address. This is the way the packets are switched between the ingress PE and egress PE router. This label is called the IGP label, because it is the label that is bound to an IPv4 prefix in the global routing table of the P and PE router, and the IGP of the service provider network advertises it.

How does the egress PE router know which VRF the packet belongs to? This information is not in the IP header, and it cannot be derived from the IGP label, because this is used solely to forward the packet through the service provider network. The solution is to add another label in the MPLS label stack. This label indicates which VRF the packet belongs to. Therefore, all customer packets are forwarded with two labels: the IGP label as the top label and the VPN label as the bottom label. The VPN label must be put on by the ingress PE router to indicate to the egress PE router which VRF the packet belongs to. How does the egress PE router signal to the ingress PE router which label to use for a VRF prefix? Because MP-BGP is already used to advertise the vpnv4 prefix, it also signals the VPN label (also referred to as the BGP label) that is associated with the vpnv4 prefix.

NOTE Actually, the concept of having one VPN label indicating the VRF that the packet belongs to is not quite correct. This might be true in some cases, but most of the time it is not. A VPN label usually indicates the next hop that the packet should be forwarded onto on the egress PE router. Therefore, most of the time, its purpose is to indicate the correct CE router as the next hop of the packet.

To recap, the VRF-to-VRF traffic has two labels in the MPLS VPN network. The top label is the IGP label and is distributed by LDP or RSVP for TE between all P and PE routers hop by hop. The bottom label is the VPN label that is advertised by MP-iBGP from PE to PE. P routers use the IGP label to forward the packet to the correct egress PE router. The egress PE router uses the VPN label to forward the IP packet to the correct CE router.

Figure 7-9 shows the packet forwarding in an MPLS VPN network. The packet enters the PE router on the VRF interface as an IPv4 packet. It is forwarded throughout the MPLS VPN network with two labels. P routers forward the packet by looking at the top label. The top label is swapped at each P router. The labels are stripped off at the egress PE router and the packet is forwarded as an IPv4 packet onto the VRF interface toward the CE router. The correct CE router is found by looking at the VPN label.

Figure 7-9 Packet Forwarding in an MPLS VPN Network

Figure 7-9 Packet Forwarding in an MPLS VPN Network

The later section "Packet Forwarding" has a more detailed example of this packet forwarding through the MPLS VPN network. First, however, you must learn more details on the role of BGP in the MPLS VPN network.

BGP version 4 (BGP-4) has been around for many years and is the standard protocol for interdomain routing. BGP is the protocol that makes the Internet work so well today. The service providers that make up the Internet run BGP between each other. They peer with other service providers through eBGP and run iBGP in their own networks. BGP is a routing protocol that is well suited to carry hundred of thousands of routes and has a proven record to back this up. BGP is also a routing protocol that allows flexible and extended policies to be implemented. That is why it is such a good candidate to carry MPLS VPN routes. As mentioned before, the combination of the RD with the IPv4 prefix makes up the vpnv4 prefix. It is this vpnv4 prefix that iBGP needs to carry between the PE routers.

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