# Example: Transporting Packets Across an IP Backbone with MPLS

The process of data transport is shown in Figure 75. PE 1 has already received announcements from PE 2; an LSP has been established between PE 1 and PE 2.

Figure 75: Traffic Across the MPLS Backbone of a BGP/MPLS VPN

Host 1 constructs an IP packet with the address of Host 2 as the final destination, and sends the packet to router CE 1. CE 1 encapsulates the packet appropriately and forwards it to PE 1.

PE 1 receives the packet from CE 1. Based on the interface the packet came in on, PE 1 determines that it must use the forwarding table for VRF A to route the packet. PE 1 looks up the destination address of Host 2 in the forwarding table of VRF A and finds the following instructions:

- Push label 16; that is, prepend it to the data packet. This innermost label identifies the VRF on PE 2, where the final destination and interface lookup takes place. Label 16 was previously allocated by PE 2 and communicated to PE 1 by MP-BGP. VRF A shows this label as part of the NLRI for destination address Host 2.
- Push label 21 and forward the MPLS-encapsulated data packet to router P 1. Label 21 is prepended to label 16; the labels are stacked. Label 21 becomes the outermost label and is assigned to the first segment—PE 1–P 1—in the label-switched path from PE 1 to PE 2. The LSP was previously configured.

P 1 receives the data packet from PE 1 and pops label 21. P 1 looks up label 21 in its forwarding table and determines it must push label 19 on the stack, and forwards the data packet to P 2.

P 2 receives the data packet from P 1 and pops label 19. P 2 looks up label 19 in its forwarding table and determines it must push label 46 on the stack, and forwards the data packet to PE 2.

PE 2 receives the data packet from P 2, and looks up label 46. PE 2 determines it is the egress router of the LSP and must pop label 46. Then it proceeds to look up the next label, label 16, and determines that the packet goes to VRF A. Then the IP address is looked up in VRF A to determine the destination and outgoing interface for the packet. PE 2 forwards the packet to CE 6.

CE 6 receives the IP packet from PE 2 and looks up the destination address Host 2. Subsequent forwarding to Host 2 occurs by means of the IGP in the customer site.

The network structure shown in Figure 75 consists of two VPNs, A and B. VPN A comprises CE 1, CE 5, and CE 6. VPN B comprises CE 2, CE 3, and CE 4. CE 1 has data traffic destined for both CE 5 and CE 6. Because both of these destination sites are within the same VPN, PE 1 uses the same forwarding table, in VRF A, to do the lookups and MPLS encapsulation. The innermost label determines the destination VRF and is the same for all packets in that VPN, even if they are destined for different CE routers. CE 2 and CE 3 have traffic destined for CE 4. Because these all are in VPN B, PE 1 uses a different forwarding table, in VRF B, for looking up destinations for traffic originating with these sites. However, both VPNs use the same LSP, because both VPNs use the same ingress (PE 1) to and the same egress (PE 2) from the service provider core. Remember that the illustrated LSP carries data traffic only from PE 1 to PE 2. Traffic from PE 2 to PE 1 requires a different LSP.