is an important issue that needs addressing. Hierarchical VPLS VPLS requires a full mesh in both the control and data planes; this can be difficult to scale. For BGP, the control plane scaling issue has long been addressed, through the use of route reflectors (RRs). RRs are extensively used in the context of Internet routing, as well as for several types of VPNs. To scale the data plane for multicast and broadcast traffic, there is work in progress to use point-to-multipoint LSPs as the underlying transport. For LDP, a method of subdividing a VPLS VPN into two or three tiered hierarchical networks was developed. Called hierarchical VPLS (HVPLS), it introduces a new type of MPLS device: the multi-tenant unit (MTU) switch. This switch aggregates multiple customers into a single PE, which in turn needs only one control and data plane connection into the mesh. This can significantly reduce the number of LDP sessions and LSPs, and thus unburden the core network, by concentrating customers in edge devices. HVPLS (LDP) may also be used to join two VPLS mesh structures together. Without using HVPLS, every node in each VPLS mesh must become meshed with all nodes in the other VPLS mesh. However, with HVPLS, the two meshes can essentially be joined together at certain locations. Techniques such as redundant pseudowires can provide resiliency in case of failures at the interconnection points. MAC addresses Since VPLS links multiple Ethernet broadcast domains together, it effectively creates a much larger broadcast domain. Since every PE must keep track of all MAC addresses and associated LSP routing information, this can potentially result in a large amount of memory being needed in every PE in the mesh. To counter this problem, sites may use a router as the CE device. This hides all MAC addresses on that site behind the CE's MAC address. PE devices may also be equipped with content-addressable memory (CAM), similar to high-end Ethernet switches. An alternative mechanism is using MAT (MAC Address Translation). However, at the time of writing this, there are no vendors providing MAT functionality. PE auto-discovery In a VPLS-based VPN with a large number of sites, manually configuring every participating PE does not scale well. If a new PE is taken into service, every existing PE needs to have its configuration adjusted to establish an LDP session with the new PE. Standardisation work is in progress to enable auto-discovery