Find whether the VLAN table or the STP logical-interface limit caps how many VLANs a switch can really carry.
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.
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.
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.
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.
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.