MSTP represents a modification of RSTP that allows the grouping of different VLANs into multiple MSTIs. To enable different devices to participate in MSTIs, they must be consistently configured. A collection of interconnected devices that have the same MST configuration (region-name, revision, and VLAN-to-instance assignment) comprises an MST region.
There is no limit to the number of regions in the network, but every region can support a maximum of 16 MSTIs. Instance 0 is a special instance for a region, known as the Internal Spanning Tree (IST) instance. All other instances are numbered from 1 to 4094. IST is the only spanning-tree instance that sends and receives BPDUs (typically, BPDUs are untagged). All other spanning-tree instance information is included in MSTP records (M-records), which are encapsulated within MSTP BPDUs. This means that a single BPDU carries information for multiple MSTIs, which reduces overhead of the protocol.
Any MSTI is local to an MSTP region and completely independent from an MSTI in other MST regions. Two redundantly connected MST regions use only a single path for all traffic flows (no load balancing between MST regions or between MST and SST region).
Traditional Layer 2 switches running MSTP protocol assign all VLANs to the IST instance per default. The operator may then ‟re-assign” individual VLANs to a specified MSTI by configuring per VLAN assignment. This means that an SR-series PE can be considered as a part of the same MST region only if the VLAN assignment to IST and MSTIs is identical to the one of Layer 2 switches in the access network.