A second application of Hierarchical VPLS is in the use of Multi Tenant Units (MTU). MTUs are typically not MPLS-enabled, and therefore have Ethernet links to the closest PE node (see the following figure). To protect against failure of the PE node, an MTU could be dual-homed and therefore have two SAPs on two PE nodes. To resolve the potential loop, STP is activated on the MTU and the two PEs.
Like in the previous scenario, STP only needs to run in a single VPLS instance, and the results of the STP calculations are applied to all VPLSes on the link. Equally, the standby node will broadcast MAC flush LDP messages in the protected VPLS instances when it detects that the active node has failed.
Figure: H-VPLS with SAP redundancy
