VLAN Capacity Calculator
Find whether the VLAN table or the STP logical-interface limit caps how many VLANs a switch can really carry.
Inputs
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
Values used
Hardware VLAN table limit = 4,094 VLANs; VLANs configured = 320 VLANs; Spanning-tree mode = PVST+ / Rapid-PVST+ (one instance per VLAN); MST instances (MSTP only) = 8 instances; Trunk ports carrying the VLANs = 24 ports; Platform STP logical-interface limit = 16,000 instances
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.
PVST+ VLAN ceiling
max VLANs under PVST+ = logical-interface limit ÷ trunk ports.
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
How it works
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.
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.