PVST and PVST+ – In-Depth Guide

What is PVST (Per VLAN Spanning Tree)?

• Overview and Purpose: PVST (Per VLAN Spanning Tree) is a Cisco proprietary enhancement of IEEE 802.1D STP. PVST runs a separate instance of STP for each VLAN, enabling granular optimization of traffic flow and redundancy.
• Purpose:
  • Prevent Layer 2 loops in switched networks
  • Optimize root bridge selection and paths per VLAN
  • Improve load balancing and redundancy

How It Differs from Traditional STP

• Traditional STP (IEEE 802.1D): One instance for the entire network, all VLANs share a single spanning tree.
• PVST: Each VLAN runs its own instance, root, and topology. You can optimize paths differently for each VLAN.

Example: VLAN 10, 20, and 30 each have their own root bridge and STP topology. VLAN 10 may have Switch A as root; VLAN 20 may use Switch B as root.

What is PVST+?

• PVST+ is an enhancement to PVST for compatibility with IEEE 802.1Q trunks.
• Allows separate STP per VLAN over 802.1Q trunks and interoperability with standards-based switches using CST (Common Spanning Tree).

Operation of PVST and PVST+

• Each VLAN runs its own STP instance and elects its own root bridge.
• Example: Switch A is root for VLAN 10; Switch B is root for VLAN 20. You can balance traffic by manual root selection.
• Benefit: Load balancing – different VLANs can use different network paths, optimizing bandwidth and redundancy.

Bridge Protocol Data Units (BPDUs)

• PVST and PVST+ use BPDUs with VLAN-specific info, sent to MAC 0100.0CCC.CCCD.
• For VLAN 1, PVST+ uses standard IEEE BPDUs, enabling interoperability.

Interaction with Legacy STP and MSTP

• PVST+ can coexist with CST and MSTP. It maps all VLANs into the native VLAN spanning tree when interoperating with standards-based (non-Cisco) devices.

Root Bridge Election in PVST

• Each VLAN elects its own root bridge, based on lowest bridge ID (priority + MAC).
• Example: VLAN 10 – Switch A set to priority 4096; VLAN 20 – Switch B set to 4096.

Impact on Network Topology

• Different VLANs may have different root bridges and network paths. This reduces bottlenecks and increases resilience.

Port Roles and States in PVST

Interoperability and Compatibility

• PVST+ tags BPDUs for each VLAN (except native VLAN).
• Works over 802.1Q trunks and can interoperate with CST/MSTP via native VLAN.
• For rapid convergence, consider Rapid PVST+ (IEEE 802.1w).

Advantages and Disadvantages

Configuration Basics

• PVST+ is default on Cisco switches.
• To view STP status per VLAN:

  Switch# show spanning-tree

• Set root bridge for a VLAN:

  Switch(config)# spanning-tree vlan 10 priority 4096

Troubleshooting PVST/PVST+

• Loops: May occur if BPDUs are filtered/blocked
• Blocked Ports: Could result from suboptimal root bridge election
• Useful commands:

  Switch# show spanning-tree vlan 10
  Switch# show spanning-tree summary
  Switch# show spanning-tree detail
      

PVST+ vs. Rapid PVST+

Security Considerations

• BPDU Guard: Shuts down port if BPDU seen on access port (spanning-tree bpduguard enable).
• Root Guard: Prevents unauthorized root bridge (spanning-tree guard root).

When and Where to Use PVST/PVST+

• Best in Cisco-only networks needing granular, per-VLAN load balancing and redundancy.
• Avoid in large/multi-vendor networks—use MSTP or Rapid PVST+ for better scalability/convergence.

Key Points & Exam Tips

• PVST = Separate STP instance per VLAN; Classic STP = one instance only.
• PVST+ = Supports 802.1Q trunks, interoperability.
• Root bridge is elected per VLAN; optimize per-VLAN traffic flow.
• PVST+ is default on Cisco switches.
• BPDU guard and root guard recommended for security.
• Use show spanning-tree vlan [id] for troubleshooting.
• Scalability overhead grows with number of VLANs.

Sample Scenario (with Example)

• You have three switches: S1, S2, S3. Two VLANs: 10 and 20.
• Set S1 as root bridge for VLAN 10, S2 for VLAN 20 (lower the priority).
• Connect all switches together: VLAN 10’s traffic prefers S1, VLAN 20’s prefers S2—bandwidth is optimized per VLAN.

S1(config)# spanning-tree vlan 10 priority 4096
S2(config)# spanning-tree vlan 20 priority 4096

Summary Table: PVST vs. PVST+

Conclusion

• Use PVST+ in Cisco-centric, VLAN-rich networks for optimal, per-VLAN spanning tree control and load balancing.
• For faster convergence, use Rapid PVST+ (based on RSTP).
• Secure with BPDU guard/root guard.
• For large/multi-vendor environments, prefer MSTP for scalability.

PVST and PVST+ Quiz

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