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.
Nokia routers have implemented special features for improving the resilience of hierarchical VPLS instances, in both MTU and inter-metro applications.