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).
Feature PVST PVST+
Trunking Support Cisco ISL only IEEE 802.1Q
Interoperability Cisco only Standards compatible
Default on Cisco No Yes
Per-VLAN STP Yes Yes
BPDU Dest MAC 0100.0CCC.CCCD 0100.0CCC.CCCD

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

Role Description
Root Port Best path to root bridge (per VLAN)
Designated Port Forwards traffic to segment (per VLAN)
Alternate/Backup Redundancy (blocks unless needed)
State Description
Blocking Prevents loops, does not forward frames
Listening Preparing to forward, not learning MACs
Learning Learning MAC addresses
Forwarding Active forwarding
Disabled Port not used

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

Advantages Disadvantages
  • Per-VLAN load balancing
  • Greater topology control
  • Enhanced redundancy
  • Consumes more resources (CPU/RAM)
  • More complex troubleshooting
  • Less scalable for many VLANs

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+

Aspect PVST+ Rapid PVST+
Protocol Base IEEE 802.1D (classic STP) IEEE 802.1w (RSTP)
Convergence ~30-50 seconds (slow) A few seconds (fast)
Per-VLAN operation Yes Yes

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+

Feature PVST PVST+
Trunking Cisco ISL only IEEE 802.1Q supported
Interoperability Cisco-only Standards compatible
Default on Cisco No (legacy) Yes
Per-VLAN STP Yes Yes
BPDU Dest MAC 0100.0CCC.CCCD 0100.0CCC.CCCD

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

1. What is the primary difference between traditional STP and PVST?

Correct answer is B. PVST runs one spanning tree instance per VLAN, unlike traditional STP which runs one for the entire network.

2. What advantage does PVST provide by running multiple STP instances?

Correct answer is D. Multiple STP instances allow different VLANs to have different root bridges, optimizing traffic flow.

3. What is PVST+ designed to support that PVST does not?

Correct answer is C. PVST+ adds support for industry-standard 802.1Q trunks and works with non-Cisco switches.

4. What destination MAC address do PVST+ BPDUs use?

Correct answer is A. PVST+ BPDUs use MAC 0100.0CCC.CCCD for VLAN-specific BPDU traffic.

5. How is the root bridge elected in PVST?

Correct answer is B. Root bridge election is determined by the lowest bridge ID, which includes priority and MAC address per VLAN.

6. What is the main disadvantage of using PVST/PVST+?

Correct answer is D. Running one STP instance per VLAN can cause resource and scalability challenges on switches.

7. What command sets a switch as the root bridge for VLAN 10?

Correct answer is A. The Cisco command to set root bridge priority for VLAN 10 is spanning-tree vlan 10 priority 4096.

8. What is BPDU Guard used for in PVST environments?

Correct answer is C. BPDU Guard disables ports that receive BPDUs where none are expected, protecting the network.

9. Which PVST variant offers faster convergence than PVST+?

Correct answer is B. Rapid PVST+ is based on IEEE 802.1w and provides faster convergence.

10. When should PVST/PVST+ generally be avoided?

Correct answer is D. PVST/PVST+ has scalability limits and slower convergence compared to MSTP or Rapid PVST+, so it's less suitable for large networks.

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