Map VLANs onto MST instances and see how much spanning-tree state MSTP saves against per-VLAN spanning tree.
802.1s decouples VLAN count from topology count: many VLANs share one instance, so state grows with instances rather than VLANs. Balancing VLANs across instances also lets you load-share, with different instances rooted on different switches. Per-VLAN spanning tree is what drives access switches into their logical-interface limit, and MSTP is the standard fix — but only if the VLAN-to-instance map is identical on every switch, or the region silently splits.
MSTP Instance
MSTP state = min(VLANs, instances) × trunk ports, against PVST+ state of VLANs × trunk ports; the saving is the ratio of the two.
MSTP state = min(VLANs, instances) × trunk ports, against PVST+ state of VLANs × trunk ports; the saving is the ratio of the two. 802.1s decouples VLAN count from topology count: many VLANs share one instance, so state grows with instances rather than VLANs. Balancing VLANs across instances also lets you load-share, with different instances rooted on different switches.
Per-VLAN spanning tree is what drives access switches into their logical-interface limit, and MSTP is the standard fix — but only if the VLAN-to-instance map is identical on every switch, or the region silently splits.
This calculator takes 4 inputs: VLANs in the region, MST instances, Trunk ports per switch, Switches in the region. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Two to four is typical: enough to load-share across redundant uplinks without multiplying state. The instance count matters far more than how many VLANs each one carries, since the topology computation is per instance.