MPLS-Enabled Applications: Emerging Developments and New Technologies

12.3. VPLS MECHANISM OVERVIEW

This section gives an overview of the VPLS mechanisms, using the service provider network shown in Figure 12.2 as a reference model. Shown in the diagram are the sites of two VPLS customers, X and Y. Customer X has sites attached to PE1, PE2 and PE3. Customer Y has sites attached to PE1, PE3 and PE4. From the point of view of each of the customers, the network appears to be a single LAN on to which that customer's, and only that customer's, CE devices are attached. That is to say, customer X belongs to one VPLS and customer Y belongs to another VPLS. This is illustrated in Figure 12.3, which shows the network from the point of view of customer Y. In Figure 12.2, each customer's device, whether a router or a switch, only requires a single Ethernet connection to the SP PE router (e.g. an untagged Ethernet interface or a VLAN), because the VPLS is a multipoint service, with the ingress PE taking responsibility of forwarding the frame according to its destination MAC address. For reasons of resilience, a CE can be attached to more than one PE. This is discussed in more detail in Section 12.5.2.2 of this chapter. Each site of customer X contains only a handful of PCs and the CE devices are all Ethernet switches. The sites of customer Y attached to PE3 and PE4 are offices containing a large number of PCs and the corresponding CEs are routers. Customer Y's site attached to PE1 is a small branch office and so a switch is used as a CE on that site. The repercussions ...