The VPLS standard (RFC 4762, Virtual Private LAN Services Using LDP Signaling) includes provisions for hierarchical VPLS, using point-to-point spoke-SDPs. Two applications have been identified for spoke-SDPs:
to connect to Multi-Tenant Units (MTUs) to PEs in a metro area network
to interconnect the VPLS nodes of two networks
In both applications, the spoke-SDPs serve to improve the scalability of VPLS. While node redundancy is implicit in non-hierarchical VPLS services (using a full mesh of SDPs between PEs), node redundancy for spoke-SDPs needs to be provided separately. In VPLS services, only two spoke-SDPs are allowed in an endpoint.
The 7210 SAS routers have implemented special features for improving the resilience of hierarchical VPLS instances, in both MTU and inter-metro applications.