One tree per VLAN
PVST+ is Cisco’s per-VLAN spanning tree mode based on classic STP. Rapid PVST+ applies RSTP behavior to each VLAN. Both allow different root switches and blocking decisions by VLAN. A trunk port can therefore forward for one VLAN while remaining blocked for another.
Per-VLAN operation and compatibility
Inspect the instance for the VLAN that is failing. A healthy forwarding state for another VLAN says little about the affected VLAN. A standard RSTP or MSTP neighbor may represent spanning tree differently. Check supported interoperability and the boundary design before relying on separate trees across vendors.
Try it in a lab
- On Cisco switches in a redundant triangle, enable a per-VLAN spanning tree mode and set different roots for VLANs 10 and 20.
- Compare the root ID and port roles for both VLANs. Find a redundant trunk that forwards in one VLAN but does not forward in the other. Then verify that hosts within each VLAN can still communicate.