In configuration shown in the following figure, STP is activated on the MTU and two PEs to resolve a potential loop.
To remove such a loop from the topology, Spanning Tree Protocol (STP) can be run over the SDPs (links) which form the loop such that one of the SDPs is blocked. Running STP in every VPLS in this topology is not efficient as the node includes functionality which can associate a number of VPLSes to a single STP instance running over the redundant SDPs. Node redundancy is therefore achieved by running STP in one VPLS. Therefore, this applies the conclusions of this STP to the other VPLS services.
The VPLS instance running STP is referred to as the management VPLS or mVPLS. In the case of a failure of the active node, STP on the management VPLS in the standby node changes the link states from disabled to active. The standby node then broadcasts a MAC flush LDP control message in each of the protected VPLS instances, so that the address of the newly active node can be relearned by all PEs in the VPLS. It is possible to configure two management VPLS services, where both VPLS services have different active spokes (this is achieved by changing the path-cost in STP). By associating different user VPLSes with the two management VPLS services, load balancing across the spokes can be achieved.
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.
This is done by using ‟MAC-flush” messages defined by RFC 4762, Virtual Private LAN Services Using LDP Signaling. In the case where STP acts as a loop resolution mechanism, every Topology Change Notification (TCN) received in a context of 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 specific STP instance.