3com 8807 User Guide

Figure 153   Hierarchical VPLS network architecture

As shown in Figure 153, the network topology of the VPLS network is hierarchical, 
and the access range of the network is expansible. The core devices (NPEs) in the 
core network require high performance because VPN traffic concentrates there, 
while the edge devices (UPEs) require lower performance because they are mainly 
used for VPN service access. In addition, you can back up the links between NPEs 
and UPEs to make the network more robust. The access networks between UPEs 
and NPEs can be either a MPLS edge network connected by LSP, or a simple 
Ethernet network for VLAN-VPN user access.

As shown in the following figure, the whole VPLS network is just like a huge 
switch. For each VPN, it sets up PWs between the sites of the VPN on MPLS 
tunnels and transparently transmits user’s layer 2 packets from one site to another 
through these PWs. In this network, PEs forward packets, learn source MAC 
addresses, create MAC forwarding entries, and map the MAC addresses to 
corresponding ACs and PWs. While, the P devices (provider routers, that is, core 
switches in the backbone network), only implement MPLS forwarding according 
to MPLS labels without considering layer 2 user data encapsulated in MPLS 
packets.