In the configuration shown in Figure 1, STP is activated on the MTU and two PEs to resolve a potential loop. STP only needs to run in a single VPLS instance, and the results of the STP calculations are applied to all VPLSs on the link.
In this configuration, the scope of the STP domain is limited to MTU and PEs, while any topology change needs to be propagated in the whole VPLS domain including mesh SDPs. This is done by using so-called MAC-flush messages defined by RFC 4762. In the case of STP as an loop resolution mechanism, every TCN received in the context of an STP instance is translated into an LDP-MAC address withdrawal message (also referred to as a MAC-flush message) requesting to clear all FDB entries except the ones learned from the originating PE. Such messages are sent to all PE peers connected through SDPs (mesh and spoke) in the context of VPLS services, which are managed by the specified STP instance.