MPLS L3VPN Packet Forwarding

For basic MPLS L3VPN applications in a single AS, VPN packets are forwarded with two layers of labels:

• Layer 1 labels: Outer labels, used for label switching inside the backbone. They indicate LSPs from the local PEs to the remote PEs. Based on layer 1 labels, VPN packets can be label switched along the LSPs to the remote PEs.

• Layer 2 labels: Inner labels, used for forwarding packets from the remote PEs to the CEs. An inner label indicates to which site, or more precisely, to which CE the packet should be sent. A PE finds the interface for forwarding a packet according to the inner label.

                                                            VPN packet forwarding

1) Site 1 sends an IP packet with the destination address of 1.1.1.2. CE 1 transmits the packet to PE 1.

2) PE 1 searches VPN instance entries based on the inbound interface and destination address of the packet. Once finding a matching entry, PE 1 labels the packet with both inner and outer labels and forwards the packet out.

3) The MPLS backbone transmits the packet to PE 2 by outer label. Note that the outer label is removed from the packet at the penultimate hop.

4) PE 2 searches VPN instance entries according to the inner label and destination address of the packet to determine the outbound interface and then forwards the packet out the interface to CE 2.

5) CE 2 transmits the packet to the destination by IP forwarding.