636 CHAPTER 52: VPLS CONFIGURATION
Figure 154 L2VPN universal transmission components
The transmission components and their functions in a VPLS network are as
follows:
Attachment circuit
An attachment circuit (AC) is a virtual connection link between CE and PE. User’s
layer 2 and layer 3 data are transmitted to the peer site through AC without any
modification.
Pseudowire
A pseudowire (PW) is a bidirectional virtual connection between two VSIs in a VPN.
One PW contains a pair of unidirectional MPLS VCs (virtual circuits). It is
established by PW signaling protocol and carried on LSP. For a VPLS system, a PW
is just like a directly connected path between local and peer ACs, through which
user’s layer 2 data are transmitted transparently.
Forwarder
A forwarder is in fact a VPLS forwarding table, it chooses PWs to forward the
frames that PEs received from ACs.
Tunnel
A tunnel is a directly connected path between local PE and peer PEs, on which
data is transmitted transparently from one PE to another. A tunnel can carry
multiple PWs. In general, a tunnel is an MPLS tunnel.
Encapsulation
Standard PW encapsulation formats and technique are adopted when packets are
transmitted over PWs. VPLS packets carried on PWs have two encapsulation
modes: VLAN and Ethernet.
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