To interconnect 7210 SAS devices (PE devices) across the backbone, service tunnels (SDPs) are used. These service tunnels are shared among multiple VPLS instances. The Nokia implementation of the Spanning Tree Protocol (STP) incorporates some enhancements to make the operational characteristics of VPLS more effective. The implementation of STP on the router is modified to guarantee that service tunnels is not blocked in any circumstance without imposing artificial restrictions on the placement of the root bridge within the network. The modifications introduced are fully compliant with the 802.1D-2004 STP specification.
When running MSTP, spoke-SDPs cannot be configured. Also, ensure that all bridges connected by mesh SDPs are in the same region. If not, the mesh is prevented from becoming active (trap is generated).
To achieve this, all mesh SDPs are dynamically configured as either root ports or designated ports. The PE devices participating in each VPLS mesh determine (using the root path cost learned as part of the normal protocol exchange) which of the 7210 SAS devices is closest to the root of the network. This PE device is internally designated as the primary bridge for the VPLS mesh. As a result of this, all network ports on the primary bridges are assigned the designated port role and therefore remain in the forwarding state.
The second part of the solution ensures that the remaining PE devices participating in the STP instance see the SDP ports as a lower cost path to the root instead of a path that is external to the mesh. Internal to the PE nodes participating in the mesh, the SDPs are treated as zero cost paths toward the primary bridge. As a consequence, the path through the mesh are seen as lower cost than any alternative and the PE node designates the network port as the root port. This ensures that network ports always remain in forwarding state.
A combination of the preceding features ensure that network ports are never blocked and maintain interoperability with bridges external to the mesh that are running STP instances.