The key attribute with VPLS and VPWS is the concept of an Ethernet virtual circuit (EVC). VPLS and VPWS constructs may be summarized as port based and VLAN based. The port-based example is point-to-point for deployment of an Ethernet wire service (EWS) which is deployed with a private line replacement via a router or a bridge. A point-to-point Ethernet service may interconnect with ATM and Frame Relay. Another port-based example is multipoint-to-multipoint for an Ethernet multipoint service (EMS), where each location acts as if it is connecting to a switch.
Ethernet Relay Service (ERS) is an example of a VLAN-based construct where one could have multiple EVCs per port. This service may be used as a Frame Relay replacement, where an EVC is similar to a Frame Relay PVC in function. Ethernet Relay Multipoint Services (ERMS) is another example of a VLAN-based construct for multiple EVCs per port. Finally, at Layer 1, there is the concept of Ethernet Private Line (EPL), such as over SONET/SDH. Figures 7-2 and 7-3 summarize these service examples for Ethernet-based services. Together, these service types comprise the Metro Ethernet Architecture.
The main components of the Metro Ethernet Architecture are as follows:
Access user PE or access U-PE, which is where the admission control and security policy enforcements occur.
Aggregation-PE or PE_Agg, which aggregates traffic and enforces congestion management mechanisms.
Edge Network PE or N-PE, where services such as VPLS and VPWS are fundamentally deployed.
Provider node or P-node for packet forwarding and implementing traffic engineering.
Figure 7-4 illustrates the Metro Ethernet Architecture model and functional roles.