Understanding MST Terminologies

The MST protocol was invented by Cisco to address the issue of spanning-tree instances growing with VLANs, while there were few to no changes to the Topology.

Cisco switches typically run on Rapid-PVST+ or PVST+ which creates a spanning-tree instance for every VLAN created, this puts a high demand on the CPU as the VLANs keep growing.

MST? separated spanning-tree instances and VLANs and allowed you to map multiple VLANs to the same instance, thereby reducing the spanning-tree instances created and in turn reducing CPU usage while still maintaining the load-balancing ability of PVST.

Here a common MST terminologies you need to know to understand how it operates;

• Common Spanning-Tree (CST) – default for MST backward compatibility for all other spanning-tree protocols. ?It also allows connectivity amongst different MST regions (through their ISTs).
• Internal Spanning-Tree (IST) – For all switches in an MST Region (Same name, revision number, and VLAN to instance mapping). It is the default spanning-tree for the region. IST 0 on a Cisco switch. Any VLAN not assigned to an MSTI is by default in this IST instance, and all VLANs in a region are part of the IST instance irrespective of their instances. It also handles BPDUs Hello timers, max-age, and all other information related to the topology.
• Multiple Spanning-Tree Instance (MSTI) –These are instances created and Mapped to VLANs, A region can contain multiple MSTIs, a VLAN can only be assigned to one MSTI, and a switch can only be a part of a region. MSTIs only serve and are relevant within the originating region.
• Common and Internal Spanning-Tree (CIST) – It connects all ISTs in each region and the CST connecting those regions as well as other STP modes.

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