A second application of hierarchical VPLS is using MTUs that are not MPLS-enabled that must have Ethernet links to the closest PE node. To protect against failure of the PE node, an MTU can be dual-homed and have two SAPs on two PE nodes.
There are several mechanisms that can be used to resolve a loop in an access circuit; however, from an operations perspective, they can be subdivided into two groups:
STP-based access, with or without M-VPLS.
Non-STP based access using mechanisms such as MC-LAG, MC-APS, MC-Ring.