The Multiple Spanning Tree Protocol (MSTP) extends the concept of IEEE 802.1w RSTP by allowing grouping and associating VLANs to Multiple Spanning Tree Instances (MSTI). Each MSTI can have its own topology, which provides architecture enabling load balancing by providing multiple forwarding paths. At the same time, the number of STP instances running in the network is significantly reduced as compared to Per VLAN STP (PVST) mode of operation. Network fault tolerance is also improved because a failure in one instance (forwarding path) does not affect other instances.
The Nokia implementation of M-VPLS is used to group different VPLS instances under a single RSTP instance. Introducing MSTP into the M-VPLS allows interoperating with traditional Layer 2 switches in an access network and provides an effective solution for dual homing of many business Layer 2 VPNs into a provider network.