Cisco PVST+ Guide: Spanning Tree Made Simple

Introduction

In this lesson, I will explain how to implement and manage Per VLAN Spanning Tree (PVST+) on Cisco switches. PVST+ is Cisco’s enhanced version of Spanning Tree Protocol that creates separate spanning-tree instances for each VLAN, enabling better load balancing and more efficient network utilization.

Lesson Objectives

After completing this lesson, you will be able to:

  • Understand the fundamentals of PVST+ operation
  • Configure PVST+ on Cisco switches
  • Verify PVST+ operation and troubleshoot common issues
  • Implement PVST+ load balancing
  • Monitor and maintain PVST+ environments

Prerequisites

  • Basic understanding of switching concepts
  • Knowledge of basic Spanning Tree Protocol (STP)
  • Familiarity with VLANs and trunking
  • Access to Cisco switches (2960 or newer recommended)
  • Console cable and terminal emulator

Core Lesson Content

Part 1: PVST+ Fundamentals

Understanding PVST+

PVST+ creates a separate spanning tree instance for each VLAN in your network. This allows for:

  • Different root bridges per VLAN
  • Different path selections per VLAN
  • Better load balancing across the network
  • More granular control over spanning-tree behavior

Basic PVST+ Configuration

! Enable PVST+ (default on Cisco switches)
Switch(config)# spanning-tree mode pvst

! Configure root bridge priority for specific VLANs
Switch(config)# spanning-tree vlan 10 priority 24576
Switch(config)# spanning-tree vlan 20 priority 28672

! Configure secondary root bridge
Switch(config)# spanning-tree vlan 10 priority 28672
Understanding the Output

When you verify PVST+ configuration, check:

Switch# show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    24576
             Address     0023.ea9b.0b00
             This bridge is the root
  Bridge ID  Priority    24576
             Address     0023.ea9b.0b00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

What to Look For:

  • Root bridge priority and MAC address
  • Bridge role (root, secondary, normal)
  • Port states and roles
  • Timer values

Part 2: PVST+ Load Balancing

Configuration Example

! Configure different root bridges for different VLANs
Switch-1(config)# spanning-tree vlan 10,30,50 priority 24576
Switch-1(config)# spanning-tree vlan 20,40,60 priority 28672

Switch-2(config)# spanning-tree vlan 20,40,60 priority 24576
Switch-2(config)# spanning-tree vlan 10,30,50 priority 28672
Port Cost Manipulation
! Adjust port costs to influence path selection
Switch(config-if)# spanning-tree vlan 10 cost 100
Switch(config-if)# spanning-tree vlan 20 cost 50

Verification Steps:

Switch# show spanning-tree vlan 10 brief
Switch# show spanning-tree interface gi1/0/1 detail

Part 3: Advanced PVST+ Features

Path Cost Calculation

  • Short mode (default for older switches):
  • 10 Mbps: 100
  • 100 Mbps: 19
  • 1 Gbps: 4
  • 10 Gbps: 2
  • Long mode (recommended for modern networks):
Switch(config)# spanning-tree pathcost method long

Guard Features

! Enable BPDU Guard
Switch(config-if)# spanning-tree bpduguard enable

! Enable Root Guard
Switch(config-if)# spanning-tree guard root

! Enable Loop Guard
Switch(config)# spanning-tree loopguard default

Common Issues and Solutions

Problem 1: VLAN Not Load Balancing

Symptoms:

  • All VLANs taking same path
  • Underutilized links

Solution:

  1. Verify root bridge priorities
Switch# show spanning-tree summary
Switch# show spanning-tree root
  1. Check port costs
  2. Ensure VLANs exist and are active

Problem 2: Topology Changes

Symptoms:

  • Frequent TCNs
  • Intermittent connectivity

Solution:

  1. Enable PortFast on access ports
Switch(config)# spanning-tree portfast default
  1. Monitor topology changes
Switch# show spanning-tree detail | include topology

Practical Exercise

Lab: PVST+ Load Balancing Configuration

Objective: Configure PVST+ load balancing across two uplinks

Equipment:

  • 3 Cisco switches
  • Ethernet cables for interconnection

Tasks:

  1. Create VLANs 10, 20, 30
  2. Configure Switch-1 as root for VLANs 10, 30
  3. Configure Switch-2 as root for VLAN 20
  4. Verify traffic takes different paths
  5. Document port states for each VLAN

Verification Checklist:

  • [ ] Root bridge correct for each VLAN
  • [ ] Port states appropriate
  • [ ] Load balancing working as expected
  • [ ] No blocking ports on access interfaces

Key Takeaways

  1. PVST+ enables per-VLAN load balancing
  2. Root bridge placement critical for optimal traffic flow
  3. Port costs can fine-tune path selection
  4. Guard features protect topology stability
  5. Regular monitoring prevents issues

Next Steps

  1. Study Multiple Spanning Tree Protocol (MST)
  2. Learn about Rapid PVST+
  3. Practice more complex topologies
  4. Explore automation of PVST+ configuration

Quick Reference Commands

show spanning-tree summary
show spanning-tree root
show spanning-tree vlan X
show spanning-tree interface Y detail
show spanning-tree inconsistentports

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