Figure: Dual homed 7210 SAS-D acting as MTU-s in two-tier hierarchy H-VPLS
In configuration shown in the preceding figure, STP is activated on the MTU and two PEs to resolve a potential loop.
In this configuration, the scope of STP domain is limited to MTU and PEs, while any topology change needs to be propagated in the whole VPLS domain. When TCN (topology change notification) is received with in the VPLS domain all MACs learned on SAPs are flushed except the SAP on which TCN was received.