What is Per-VLAN Spanning Tree (PVST), and why is it useful?

PVST (Per-VLAN Spanning Tree) runs a separate STP (Spanning Tree Protocol) instance for each VLAN (Virtual Local Area Network), allowing different root bridges and topologies per VLAN.

In a multi-VLAN environment, a single spanning tree might block links that could be useful for some VLANs. PVST solves this by running independent spanning tree instances.

How it works:

• Each VLAN gets its own root bridge election
• Each VLAN can have different ports in blocking/forwarding state
• This enables load balancing — traffic for VLAN 10 might use one path while VLAN 20 uses another

Example:

• If all ports are in VLAN 1, there's only one spanning tree instance
• With VLANs 10, 20, and 30, there are three separate spanning tree instances running simultaneously

Tip: This is why the extended system ID (VLAN number) is part of the BID (Bridge ID) — it ensures each VLAN's STP election is independent.

Go deeper:

• Introduction to Rapid PVST+ (NetworkAcademy.IO) — explains the per-VLAN instance model and why Cisco runs a separate tree per VLAN.