VPLS provides scaling and operational advantages. A hierarchical configuration eliminates the need for a full mesh of VCs between participating devices. Hierarchy is achieved by enhancing the base VPLS core mesh of VCs with access VCs (spoke) to form two tiers. Spoke-SDPs are generally created between Layer 2 switches and placed at the Multi-Tenant Unit (MTU). The PE routers are placed at the service provider's Point of Presence (POP). Signaling and replication overhead on all devices is considerably reduced.
A spoke-SDP is treated like the equivalent of a traditional bridge port where flooded traffic received on the spoke-SDP is replicated on all other ‟ports” (other spoke and mesh SDPs or SAPs) and not transmitted on the port it was received (unless a split horizon group was defined on the spoke-SDP; see section Configuring VPLS spoke-SDPs with split horizon).
A spoke-SDP connects a VPLS service between two sites and, in its simplest form, could be a single tunnel LSP. A set of ingress and egress VC labels are exchanged for each VPLS service instance to be transported over this LSP. The PE routers at each end treat this as a virtual spoke connection for the VPLS service in the same way as the PE-MTU connections. This architecture minimizes the signaling overhead and avoids a full mesh of VCs and LSPs between the two metro networks.
A mesh SDP bound to a service is logically treated like a single bridge ‟port” for flooded traffic where flooded traffic received on any mesh SDP on the service is replicated to other ‟ports” (spoke-SDPs and SAPs) and not transmitted on any mesh SDPs.
A VC-ID can be specified with the SDP-ID. The VC-ID is used instead of a label to identify a virtual circuit. The VC-ID is significant between peer SRs on the same hierarchical level. The value of a VC-ID is conceptually independent from the value of the label or any other datalink specific information of the VC.
Figure 1 shows an example of a distributed VPLS service configuration of spoke and mesh SDPs (unidirectional tunnels) between routers and MTUs.