MPLS VPN Data Plane

• To support the forwarding of packets, 
• ingress PEs need appropriate FIB entries, 
• Ps & PEs needing appropriate LFIB entries
• The outer label identifies the segments of the LSP between between the ingress PE & the egress PE,
• but it doesn't identify how the egress PE should forward the packet.
• The inner label identifies the egress PE's forwarding details, in particular the outgoing interface for the unlabeled packet.
• Building the Inner (VPN) label:
• The inner label called VPN label
• VPN label must be allocated for each route added to each customer VRF.
• More specifically, a CE will advertise routes to the PE, 
• PE stores these routes in the corresponding customer's VRF
• In order to prepare to forward packets to those customer subnets, 
• the PE needs to allocate a new local label 
• That local label contain the prefix & the route's next-hop ip address & outgoing interface  & stores this information in LFIB.
• Steps in LSRs fill the FIB & LFIB when using MPLS VPNs
• An unlabeled packet arrives on an interface assigned to VRF, 
• which will cause ingress PE to use VRF's FB to make a forwarding decision.
• At ingress PEs VRF, FIB shows the outgoing interface for destination ip &
• Add a label stack with 2 labes
• an inner label(having original destination IP address)
• an outer label
• Then ingress PE forwards the packet to next Ps
• P uses the LFIB entry for incoming  label (outer label), swap this label.