Cisco PVST Guide: Spanning Tree for Every VLAN

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

2. Check port costs
3. 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

2. 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