VPWS
The PE devices provide a logical interconnect such that a pair of CE
devices appear to be connected by a single logical Layer-2 circuit.
PE devices act as Layer-2 circuit switches. Layer-2 circuits are
then mapped onto tunnels in the SP network. These tunnels can
either be specific to a particular VPWS, or shared among several
services. VPWS applies for all services including Ethernet, ATM,
Frame Relay etc.
Each PE device is responsible for allocating customer Layer-2 frames
to the appropriate VPWS and for proper forwarding to the intended
destinations.
8.2.1 Encapsulation
A L2VPN solution SHOULD utilize the encapsulation techniques defined
by PWE3 ([PWE3_ARCH]), and SHOULD not impose any new requirements on
these techniques.
+-----+ +-----+
+ CE1 +--+ +---| CE2 |
+-----+ | ........................ | +-----+
L2VPN A | +----+ +----+ | L2VPN A
+--| PE |--- Service ---| PE |--+
+----+ Provider +----+
/ . Backbone . \ - /\-_
+-----+ / . | . \ / \ / \ +-----+
+ CE4 +--+ . | . +--\ Access \----| CE5 |
+-----+ . +----+ . | Network | +-----+
L2VPN B .........| PE |......... \ / L2VPN B
+----+ ^ -------
PWE3
A pseudo-wire (PW) is a connection between two provider edge (PE)
devices which connects two attachment circuits (ACs). An AC can be a
Frame Relay DLCI, an ATM VPI/VCI, an Ethernet port, a VLAN, a HDLC
link, a PPP connection on a physical interface, a PPP session from an
L2TP tunnel, an MPLS LSP, etc.
|<------- Pseudo Wire ------>|
| |
| |<-- PSN Tunnel -->| |
V V V V
+----+ +----+
+-----+ | PE1|==================| PE2| +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | |==================| | +-----+
^ | +----+ +----+ ^
| | Provider Edge 1 Provider Edge 2 |
| | |
| Attachment Circuit |
| |
|<-------------- Emulated Service ---------------->|
Customer Customer
Edge 1 Edge 2
Figure 1: PWE3 Reference Model
During the setup of a PW, the two PEs will be configured or will
automatically exchange information about the service to be emulated
so that later they know how to process packets coming from the other
end. After a PW is set up between two PEs, frames received by one PE
from an AC are encapsulated and sent over the PW to the remote PE,
where native frames are re-constructed and forwarded to the other CE.
