MSTP Instance Calculator
Map VLANs onto MST instances and see how much spanning-tree state MSTP saves against per-VLAN spanning tree.
About MSTP Instance Calculator
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
Worked example
With the default values below, MSTP Instance Calculator returns MSTP Logical Interfaces per Switch of 192. Change any input above and every figure here is recalculated in your browser.
Inputs used
- VLANs in the regionaccepts 1–4,094 VLANs
- 400 VLANs
- MST instancesaccepts 1–64 instances
- 8 instances
- Trunk ports per switchaccepts 1–512 ports
- 24 ports
- Switches in the regionaccepts 1–500 switches
- 12 switches
Results
- 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
- 1Values used
- 2MSTP Instance
- 3MSTP Logical Interfaces per Switch= 192
- 4PVST+ Logical Interfaces per Switch= 9,600
- 5VLANs per Instance= 50.0
- 6State Saved vs PVST+= 98.00
- 7Region-Wide MSTP Instances= 96
- 8Unused MST Instances= 56
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.