Operation Manual – MPLSH3C S9500 Series Routing Switches Chapter 4 MPLS L2VPN4-2z Reliability and privacy of user routes. As no user routing information is imported,there is no need for MPLS L2VPN to obtain and process the information, ensuringthe privacy of user routes.Figure 4-2 illustrates the structure of an MPLS L2VPN packet.Tunnel label VC labelLabel Stack L2 PDUTunnel label VC labelLabel Stack L2 PDUTunnel label VC labelLabel Stack L2 PDUTunnel label VC labelLabel Stack L2 PDUFigure 4-2 Structure of an MPLS L2VPN packetThe fields in an MPLS L2VPN packet are described as follows:Tunnel label (the outer label) is an MPLS label or a GRE label. It is used to transmit apacket from one PE to anther.VC label (the inner label) is a lower layer label used to identify the links between PEsand CEs. Packets of MPLS L2VPNs implemented through circuit cross connect (CCC)do not contain this label.Data in MPLS L2VPN can be encapsulated as Ethernet or VLAN packets at the datalink layer. At present, data of different nodes in a VPN must be encapsulated as thesame type of packets.4.1.2 Packet ForwardingIn an MPLS L2VPN, CE, PE, and P operate in the same way as those in a BGP/MPLSVPN. That is, they also forward packets in an MPLS network transparently by usinglabel stacks. To forward packets in an MPLS L2VPN, tunnels must be established firstbetween PEs (this can be achieved by either manual configuration or signalingprotocols). When the interfaces connecting PEs and CEs are up, PEs insert VC labelsfor packets sent by CE, and then mark them with tunnel labels. On receiving thesepackets, the remote PEs strip off the tunnel labels and send the packets to thecorresponding CEs according to their VC labels.Table 4-3 illustrates changes of the label stack of a packet during the forwarding.
