3Com S7906E Configuration Manual page 1628

Figure 1-4 H-VPLS QinQ access
As shown in Figure 1-4, MTU is a standard bridging device and QinQ is enabled on its interfaces
connected with CEs.
Data forwarding in H-VPLS QinQ access mode is as follows:
Upon receiving a packet from a CE, MTU labels the packet with a VLAN tag as the multiplex
distinguishing flag, and transparently sends the packet to PE 1 through the QinQ tunnel.
When receiving the packet, PE 1 determines which VSI the packet belongs to by the VLAN tag and,
based on the destination MAC address of the packet, tags the packet with the multiplex
distinguishing flag (MPLS label) for the PW. Then, it forwards the packet.
Upon receiving the packet from the PW, PE 1 determines to which VSI the packet belongs by the
multiplex distinguishing flag (MPLS label) and, based on the destination MAC address of the
packet, labels the packet with the VLAN tag. Then, it forwards the packet through the QinQ tunnel
to MTU, which in turn forwards the packet to the CE.
For packets to be exchanged between CE 1 and CE 2, MTU can forward them directly without PE 1
because it holds the bridging function by itself. For the first data packet with an unknown destination
MAC address or a broadcast packet, MTU broadcasts the packet to CE 2 through the bridging function
and, at the same time, forwards it through the QinQ tunnel to PE 1, which replicates the packet and
sends a copy to each peer CE.
3) PW switchover
The network design with a single PW between a UPE and an NPE has a distinct drawback: once the
PW experiences a failure, all VPNs connected to the aggregate device will lose connectivity. The
H-VPLS in LSP access mode provides redundant links for PW backup. Normally, only the main PW link
is used. When the main link fails, the backup link proceeds to provide VPN services, as shown in
1-5.
1-7
Figure