MSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculated CST. Inside an MST region, multiple spanning trees, called MSTIs, are calculated. Among these MSTIs, MSTI0 is the IST.
Like STP, MSTP uses configuration BPDUs to calculate spanning trees. An important difference is that an MSTP BPDU carries the MSTP configuration of the bridge from which the BPDU is sent.
MSTI calculation
Within an MST region, MSTP generates different MSTIs for different VLANs based on the VLAN-to -instance mappings. For each spanning tree, MSTP performs a separate calculation process similar to spanning tree calculation in STP.
In MSTP, a VLAN frame is forwarded along the following paths:
Switches participating in MST instances must be constantly configured with the same MST configuration information. The collection of switches which have the same MST information form an MST region. This MST configuration determines the region to which each switch belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map. A region can have one member or multiple members with the same MST configuration. The number of MST regions in a network is not limited, but each region can support up to 16 spanning-tree instances. A VLAN can be assigned to only one spanning-tree instance at a time.
The Internal Spanning Tree (IST) runs in an MST region. Within each MST region MSTP maintains multiple spanning tree instances. Instance 0 is known as IST. All other instances are numbered from 1 to 15. The MST is the only spanning tree instance that sends and receives MST configuration messages—all other instance information is encapsulated in MST BPDUs. Thus, in MSTP there are two contexts of operation, one in the context of the entire topology called CIST (Common Internal Spanning Tree), and the other in the context of each individual spanning tree context, i.e. MSTI (Multiple Spanning Tree Instance).
All bridges in a single MST region have the same regional configuration. The communication among bridges inside the MST region is through the IST and the communication across bridges is taken care of by the CIST (Common Internal Spanning Tree) that spans across the entire topology irrespective of MST and SST regions. All bridge ports are part of all instances available in the system. Thus, when the MSTP protocol operates, the Port Role and Port State are calculated for the CIST (Common Spanning Tree Context) and separately for each instance. The operation of the protocol arrives at a single common / active topology consisting of all bridges in the topology, which is the CIST or the common internal spanning tree. The bridge with the best bridge identifier is chosen as the CIST Root. Apart from the CIST Root, the regions also calculate MSTI Regional Roots for each of the MSTI instances active inside the region and a CIST Regional root towards the CIST Root. The port on the CIST Regional Root towards the CIST Root is selected as a master port.