VLAN Capacity Calculator
Find whether the VLAN table or the STP logical-interface limit caps how many VLANs a switch can really carry.
About VLAN Capacity Calculator
Switches publish a VLAN table size, but the number that usually bites first is logical interfaces: PVST+ builds a spanning tree for every VLAN on every trunk, so 320 VLANs on 24 trunks is 7680 instances. MSTP collapses that to instances × ports, which is why it scales. Exceeding the logical-interface limit does not fail cleanly — the switch stops running STP on new VLANs, and an unprotected VLAN is one cabling mistake away from a broadcast storm.
Formulas
VLAN Capacity
STP logical interfaces = instances × trunk ports, where PVST+ runs one instance per VLAN and MSTP runs one per instance; supportable VLANs is the lower of the VLAN table and that limit.
- 4094
- Ceiling imposed by the 12-bit 802.1Q VLAN ID
- logical interfaces
- Per-VLAN or per-instance spanning-tree state, counted once per trunk port
- instances
- VLANs under PVST+, MST instances under MSTP, 1 under plain RSTP
PVST+ VLAN ceiling
max VLANs under PVST+ = logical-interface limit ÷ trunk ports.
Worked example
With the default values below, VLAN Capacity Calculator returns VLANs Actually Supportable of 666. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Hardware VLAN table limitaccepts 1–4,094 VLANs
- 4,094 VLANs
- VLANs configuredaccepts 1–4,094 VLANs
- 320 VLANs
- Spanning-tree modeone of PVST+ / Rapid-PVST+ (one instance per VLAN), MSTP (VLANs mapped to instances), Single RSTP instance
- PVST+ / Rapid-PVST+ (one instance per VLAN)
- MST instances (MSTP only)accepts 1–64 instances
- 8 instances
- Trunk ports carrying the VLANsaccepts 1–512 ports
- 24 ports
- Platform STP logical-interface limitminimum 1 instances
- 16,000 instances
Results
- VLANs Actually Supportable
- 666
- STP Logical Interfaces Used
- 7,680
- Logical-Interface Limit Used
- 48.00%
- VLAN Table Used
- 7.82%
- Limiting Resource
- Spanning-tree logical interfaces
Step by step
- 1Values used
- 2VLAN Capacity
- 3PVST+ VLAN ceiling
- 4VLANs Actually Supportable= 666
- 5STP Logical Interfaces Used= 7,680
- 6Logical-Interface Limit Used= 48.00
- 7VLAN Table Used= 7.82
- 8Limiting Resource= Spanning-tree logical interfaces
Assumptions and limits
- The result is derived only from the 6 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 VLAN Capacity calculated?
STP logical interfaces = instances × trunk ports, where PVST+ runs one instance per VLAN and MSTP runs one per instance; supportable VLANs is the lower of the VLAN table and that limit. Switches publish a VLAN table size, but the number that usually bites first is logical interfaces: PVST+ builds a spanning tree for every VLAN on every trunk, so 320 VLANs on 24 trunks is 7680 instances. MSTP collapses that to instances × ports, which is why it scales.
Why does VLAN Capacity matter?
Exceeding the logical-interface limit does not fail cleanly — the switch stops running STP on new VLANs, and an unprotected VLAN is one cabling mistake away from a broadcast storm.
What values do I need to enter?
This calculator takes 6 inputs: Hardware VLAN table limit, VLANs configured, Spanning-tree mode, MST instances (MSTP only), Trunk ports carrying the VLANs, Platform STP logical-interface limit. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.