Within the context of VPLS services, a loop-free topology inside a fully meshed VPLS core is achieved by applying a split-horizon forwarding concept.The packets received from a mesh SDP are never forwarded to other mesh SDPs within the same service. The advantage of this approach is that no protocol is required to detect loops within the VPLS core network.
In applications such as DSL aggregation, it is useful to extend this split-horizon concept also to groups of SAPs or spoke-SDPs. This extension is referred to as a split horizon SAP group. Traffic arriving on a SAP or a spoke-SDP within a split horizon group is not forwarded to other SAPs and spoke SDPs configured in the same split horizon group, but is forwarded to other SAPs/spoke-SDPs, which are not part of the split horizon group.