MSTP Instance Calculator
Map VLANs onto MST instances and see how much spanning-tree state MSTP saves against per-VLAN spanning tree.
Inputs
MSTP Logical Interfaces per Switch
192
PVST+ Logical Interfaces per Switch
9,600
VLANs per Instance
50.0
State Saved vs PVST+
98.00%
Region-Wide MSTP Instances
96
Unused MST Instances
56
Step by step
Values used
VLANs in the region = 400 VLANs; MST instances = 8 instances; Trunk ports per switch = 24 ports; Switches in the region = 12 switches
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 Logical Interfaces per Switch
= 192
PVST+ Logical Interfaces per Switch
= 9,600
VLANs per Instance
= 50.0
State Saved vs PVST+
= 98.00
Region-Wide MSTP Instances
= 96
Unused MST Instances
= 56
How it works
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.
Formula
MSTP Instance
MSTP state = min(VLANs, instances) × trunk ports, against PVST+ state of VLANs × trunk ports; the saving is the ratio of the two.
- instances
- MST instances, 0–64 including the IST (instance 0)
- logical interfaces
- Spanning-tree state counted once per instance per trunk port
- VLANs per instance
- VLANs ÷ instances — the blast radius of one topology change
Frequently Asked Questions
How is MSTP Instance calculated?
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.
Why does MSTP Instance matter?
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.
What values do I need to enter?
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.
How many MST instances should I run?
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.