PVST+ Explained — Per-VLAN Root Bridge Election, Port Roles and Load Sharing

Imagine an enterprise network with two distribution switches and several access switches.

The network uses two VLANs:

• VLAN 10: Employees
• VLAN 20: Servers

Multiple physical connections exist between the switches to provide redundancy.

However, redundant Layer 2 links can create switching loops when they are not controlled correctly.

Spanning Tree Protocol (STP) solves this problem by creating a loop-free forwarding topology.

In a basic spanning-tree design, the root bridge provides the reference point for calculating forwarding paths.

But what if the network administrator wants:

• Distribution Switch SW1 to be the root bridge for VLAN 10
• Distribution Switch SW2 to be the root bridge for VLAN 20

This is where Per-VLAN Spanning Tree Plus (PVST+) becomes useful.

PVST+ allows compatible Cisco switches to maintain a separate spanning-tree instance for each VLAN.

As a result, different VLANs can elect different root bridges and potentially use different forwarding paths across the same physical topology.

This article focuses specifically on PVST+ root bridge election, Bridge ID calculation, VLAN-specific port roles and controlled Layer 2 load sharing.

1. What Is PVST+ and How Is It Different from STP?

PVST+ stands for Per-VLAN Spanning Tree Plus.

It is a Cisco implementation that maintains an independent spanning-tree topology for each participating VLAN.

Each VLAN has its own:

• Root bridge election
• Bridge ID comparison
• Root path cost
• Root-port selection
• Designated-port selection
• Alternate or blocked forwarding paths

Why Is This Useful?

Suppose a network uses one common spanning-tree forwarding topology.

The same redundant connection may remain unused for several VLANs.

With PVST+, an administrator can design different preferred paths for different VLANs.

This can provide better utilization of the physical topology when configured correctly.

PVST vs PVST+ vs Rapid PVST+

These terms are related, but they should not be treated as identical.

How Does PVST+ Operate?

Consider three VLANs:

Each instance can elect its own root bridge.

This does not mean each VLAN requires separate cables.

Multiple VLAN instances can operate over the same physical 802.1Q trunk connections.

Does PVST+ Automatically Load Balance?

No.

PVST+ provides the ability to create different forwarding topologies.

Actual load sharing depends on:

• Root bridge placement
• Bridge priorities
• Physical topology
• Link costs
• Port roles
• Traffic distribution

If the same switch becomes root for every VLAN, their forwarding paths may remain largely identical.

2. How PVST+ Root Bridge Election Works

Every PVST+ instance independently elects a root bridge.

The switch with the lowest Bridge ID (BID) becomes the root bridge for the VLAN.

What Is the Bridge ID?

In modern Cisco spanning-tree implementations using the extended system ID, the Bridge ID contains:

• Configurable bridge priority
• Extended system ID, normally the VLAN ID
• Switch MAC address used for STP

Conceptually:

The displayed priority component is calculated as:

[

4096 + 10 = 4106

]

This explains why a switch configured with priority 4096 may display priority 4106 in the VLAN 10 spanning-tree output.

Default Bridge Priority

On traditional Cisco Catalyst PVST+ platforms, the default configurable priority is:

32768

With the VLAN ID extension:

What Priority Values Can Be Configured?

With the extended system ID, configurable bridge priority values use increments of 4096.

Examples:

• 0
• 4096
• 8192
• 12288
• 16384
• 20480
• 24576
• 28672
• 32768

The supported range extends to 61440.

Root Bridge Election Order

For switches participating in the same VLAN instance:

First: Compare the complete Bridge IDs.

Second: The switch with the lower Bridge ID wins.

Third: When the priority components are equal, the switch with the lower STP MAC address wins.

The comparison is performed independently for each VLAN.

Example — Different Root Bridges

Consider two distribution switches:

For VLAN 10:

SW1 has the lower configured priority.

Therefore, SW1 becomes the root bridge.

For VLAN 20:

SW2 has the lower configured priority.

Therefore, SW2 becomes the root bridge.

Different roots can be elected even though the switches use the same physical connections.

Cisco Configuration

On SW1:

spanning-tree vlan 10 priority 4096
spanning-tree vlan 20 priority 8192

On SW2:

spanning-tree vlan 10 priority 8192
spanning-tree vlan 20 priority 4096

Alternative Root Configuration

Cisco also supports simplified root-selection commands:

spanning-tree vlan 10 root primary

And:

spanning-tree vlan 10 root secondary

On supported IOS platforms, these commands adjust bridge priorities to influence root selection.

root secondary does not guarantee that the switch will always become root after every possible topology or priority change.

For a deterministic lab, we will configure explicit priority values.

3. How PVST+ Selects Root Ports, Designated Ports and Blocked Paths

After the root bridge is selected, PVST+ calculates the forwarding topology for that VLAN.

Root Port

Each nonroot switch normally selects one root port for a given VLAN instance.

The root port provides the best path toward that VLAN’s root bridge.

The preferred path is primarily determined by the lowest cumulative path cost, followed by the standard STP tie-breakers.

Designated Port

For each Layer 2 segment, STP determines the designated bridge and designated port.

The designated port provides the best path from that segment toward the root bridge.

All eligible ports on the root bridge are designated ports for their respective segments.

Alternate or Blocking Port

Redundant paths that are not selected for forwarding remain in a non-forwarding state to prevent Layer 2 loops.

Classic PVST+ typically describes these as blocked ports.

Rapid PVST+ uses more explicit alternate-port roles and discarding states.

Example — Triangle Topology

Consider:

• SW1
• SW2
• SW3

All three switches connect to one another.

Every trunk carries VLAN 10 and VLAN 20.

If SW1 is root for VLAN 10:

• SW2 selects its best path to SW1.
• SW3 selects its best path to SW1.
• One redundant path is blocked for VLAN 10.

If SW2 is root for VLAN 20:

• SW1 selects its best path to SW2.
• SW3 selects its best path to SW2.
• A different redundant path may be blocked for VLAN 20.

Expected Forwarding Pattern

The exact non-forwarding ports depend on the topology, port costs and bridge identifiers.

This is the basis of per-VLAN path sharing.

Does PVST+ Make Both Links Forward for One VLAN?

Not necessarily.

STP still maintains a loop-free forwarding topology for each VLAN.

A redundant link may forward VLAN 10 while being blocked for VLAN 20.

PVST+ does not turn a Layer 2 loop into two unrestricted active paths for the same VLAN.

4. PVST+ Load Sharing, Limitations and MST Interoperability

How Per-VLAN Load Sharing Works

Imagine two redundant distribution switches.

The administrator configures:

• SW1 as the preferred VLAN 10 root
• SW2 as the preferred VLAN 20 root

Access switches then select VLAN-specific root paths.

This can distribute different VLANs across different links.

What Are the Benefits?

• VLAN-specific path control
• Redundant Layer 2 connectivity
• Better use of some alternate physical paths
• Flexibility in root bridge placement
• Clearer topology planning

What Are the Limitations?

1. Control-Plane Overhead

Every active VLAN can require its own spanning-tree instance.

A network with many VLANs may consume additional control-plane resources.

Some Catalyst platforms have explicit spanning-tree instance limits.

2. Classic STP Convergence

Traditional PVST+ can take noticeably longer to reconverge than Rapid PVST+ in suitable topologies.

3. Load Sharing Is Not Per-Packet Balancing

Different VLANs can prefer different paths.

That is not the same as distributing every packet or every flow evenly across all available links.

4. Root Placement Must Match the Network Design

Poor root bridge selection can create unnecessarily long forwarding paths.

PVST+ and MSTP Interoperability

PVST+ and MSTP can interoperate under supported conditions.

However, they use different spanning-tree models.

MSTP maps multiple VLANs into spanning-tree instances, while PVST+ maintains per-VLAN instances.

Cisco supports PVST simulation at appropriate MST/PVST+ boundaries.

This mechanism includes consistency checks and root-placement restrictions.

An inconsistent design can place boundary ports into a PVST simulation inconsistent state.

Therefore, PVST+ and MSTP interoperability should be explicitly designed and verified rather than assumed to be seamless.

Which Protocol Should Be Used?

5. Practical Cisco Lab — Elect Different Root Bridges for VLAN 10 and VLAN 20

This lab demonstrates the key advantage of PVST+.

You will configure a three-switch triangular topology and verify that each VLAN uses a different root bridge.

Lab Requirements

• Three Cisco Catalyst 2960 switches
• Four computers
• Cisco Packet Tracer
• Ethernet cables

Packet Tracer supports the basic PVST+ root bridge and port-state exercises on compatible switch models.

Real convergence timing and hardware behavior should be validated using appropriate physical equipment.

Topology

The three inter-switch links form a triangle.

All three links are 802.1Q trunks carrying VLAN 10 and VLAN 20.

VLAN and IP Plan

No gateway is required for the same-VLAN tests.

PVST+ Root Plan

Step 1 — Create VLANs and Enable PVST+ on SW1

enable
configure terminal

hostname SW1
spanning-tree mode pvst

vlan 10
 name EMPLOYEES
exit

vlan 20
 name SERVERS
exit

end

Step 2 — Configure SW1 Trunks and Access Port

configure terminal

interface range GigabitEthernet0/1 - 2
 switchport mode trunk
 switchport trunk allowed vlan 10,20
 no shutdown
exit

interface FastEthernet0/1
 description PC-A-VLAN10
 switchport mode access
 switchport access vlan 10
 spanning-tree portfast
 no shutdown
exit

end

Step 3 — Configure SW1 Bridge Priorities

configure terminal

spanning-tree vlan 10 priority 4096
spanning-tree vlan 20 priority 8192

end

SW1 is designed to become the root for VLAN 10.

Step 4 — Configure SW2

enable
configure terminal

hostname SW2
spanning-tree mode pvst

vlan 10
 name EMPLOYEES
exit

vlan 20
 name SERVERS
exit

interface range GigabitEthernet0/1 - 2
 switchport mode trunk
 switchport trunk allowed vlan 10,20
 no shutdown
exit

interface FastEthernet0/1
 description PC-C-VLAN20
 switchport mode access
 switchport access vlan 20
 spanning-tree portfast
 no shutdown
exit

spanning-tree vlan 10 priority 8192
spanning-tree vlan 20 priority 4096

end

SW2 is designed to become the root for VLAN 20.

Step 5 — Configure SW3

enable
configure terminal

hostname SW3
spanning-tree mode pvst

vlan 10
 name EMPLOYEES
exit

vlan 20
 name SERVERS
exit

interface range GigabitEthernet0/1 - 2
 switchport mode trunk
 switchport trunk allowed vlan 10,20
 no shutdown
exit

interface FastEthernet0/1
 description PC-B-VLAN10
 switchport mode access
 switchport access vlan 10
 spanning-tree portfast
 no shutdown
exit

interface FastEthernet0/2
 description PC-D-VLAN20
 switchport mode access
 switchport access vlan 20
 spanning-tree portfast
 no shutdown
exit

end

SW3 retains the default bridge priority for both VLANs.

Step 6 — Configure the Computers

In Packet Tracer:

PC → Desktop → IP Configuration

Assign the IP addresses from the lab table.

Use subnet mask:

255.255.255.0

Leave the gateway field empty.

Step 7 — Verify the Spanning-Tree Mode

On each switch:

show spanning-tree summary

Confirm that the switch is operating in PVST+ mode.

On applicable Catalyst IOS releases, the output may identify the active mode as pvst.

Step 8 — Verify the VLAN 10 Root Bridge

On SW1:

show spanning-tree vlan 10

Expected relevant information:

VLAN0010
Spanning tree enabled protocol ieee

Root ID    Priority 4106
           This bridge is the root

The displayed priority is:

4096 + VLAN ID 10 = 4106

Step 9 — Verify the VLAN 20 Root Bridge

On SW2:

show spanning-tree vlan 20

Expected relevant information:

VLAN0020
Spanning tree enabled protocol ieee

Root ID    Priority 4116
           This bridge is the root

The displayed priority is:

4096 + VLAN ID 20 = 4116

The exact output includes additional Bridge ID and port information.

Step 10 — Check SW3 Root Ports

On SW3:

show spanning-tree vlan 10

Then:

show spanning-tree vlan 20

With the intended topology, equal-speed Gigabit trunks and configured priorities:

The direct links provide the lowest-cost paths toward their respective roots.

Step 11 — Compare Blocked Ports

On SW3, examine the spanning-tree port information for both VLANs.

In this specific triangular topology, the expected stable forwarding behavior is:

For classic PVST+, the CLI typically displays non-forwarding ports with status BLK and an applicable port-role indication.

This demonstrates why different VLANs can use different active paths.

Step 12 — Test VLAN 10 Connectivity

From PC-A:

ping 192.168.10.20

Expected:

Successful communication with PC-B.

VLAN 10 forwarding reaches SW3 through its preferred path toward SW1.

Step 13 — Test VLAN 20 Connectivity

From PC-C:

ping 192.168.20.20

Expected:

Successful communication with PC-D.

VLAN 20 forwarding reaches SW3 through its preferred path toward SW2.

Step 14 — Simulate a VLAN 10 Preferred Link Failure

On SW3, shut down the connection toward SW1.

configure terminal

interface GigabitEthernet0/1
 shutdown

end

The direct SW3-to-SW1 link is now unavailable.

Step 15 — Observe the VLAN 10 Topology Change

On SW3:

show spanning-tree vlan 10

After convergence, the surviving link toward SW2 should become SW3’s forwarding root path for VLAN 10.

SW3 can reach the VLAN 10 root through the remaining triangle path:

SW3 → SW2 → SW1

SW1 remains the VLAN 10 root bridge.

The root bridge does not change merely because SW3 loses one direct link.

Step 16 — Verify Connectivity After Convergence

From PC-A:

ping 192.168.10.20

Expected:

Connectivity returns after STP reconvergence.

Traditional PVST+ may require several seconds or tens of seconds, depending on the failure detection and topology.

Do not expect the same recovery time as a carefully designed Rapid PVST+ network.

Step 17 — Restore the Link

On SW3:

configure terminal

interface GigabitEthernet0/1
 no shutdown

end

Wait for spanning tree to stabilize.

Verify:

show spanning-tree vlan 10
show spanning-tree vlan 20

The intended VLAN-specific root-port selections should return.

Step 18 — Save Configurations

On all three switches:

copy running-config startup-config

Expected Lab Results

6. PVST+ Troubleshooting and Best Practices

Important Verification Commands

Check STP mode:

show spanning-tree summary

Verify the VLAN 10 root:

show spanning-tree vlan 10

Verify the VLAN 20 root:

show spanning-tree vlan 20

Check STP root summary:

show spanning-tree root

Check an interface’s STP information:

show spanning-tree interface GigabitEthernet0/1 detail

Verify trunks:

show interfaces trunk

Common PVST+ Problems

Recommended Practices

• Define the intended root bridge for important VLANs.
• Configure secondary root choices deliberately.
• Keep root bridges on suitable aggregation or distribution infrastructure.
• Verify the full trunk topology before changing priorities.
• Avoid unnecessary Layer 2 loops.
• Use BPDU Guard on appropriate edge ports.
• Consider Root Guard and Loop Guard where the design requires them.
• Prefer Rapid PVST+ or another appropriate modern design when recovery requirements justify it.
• Consider MSTP when many VLANs make per-VLAN instance overhead undesirable.
• Verify the actual forwarding topology after every root bridge change.

7. Key Takeaways — PVST+ Root Bridge Election

Core Concepts

• PVST+ maintains separate spanning-tree instances for participating VLANs.
• Each VLAN independently selects a root bridge.
• The switch with the lowest complete Bridge ID wins the election.
• Modern Bridge ID calculations include a VLAN-based extended system ID.

Root Selection and Forwarding

• Lower configured bridge priorities have higher root preference.
• Priority values use increments of 4096.
• Root ports are selected independently for each VLAN.
• Different VLANs can use different forwarding paths over the same physical topology.

Operations and Design

• PVST+ load sharing requires intentional root and path design.
• STP still prevents loops independently for each VLAN.
• Classic PVST+ and Rapid PVST+ differ in convergence behavior.
• MSTP interoperability requires supported boundary behavior and careful root placement.

Conclusion — Why PVST+ Matters in Enterprise Switching

PVST+ provides VLAN-specific control over Layer 2 spanning-tree topology.

Rather than forcing every VLAN to use the same logical forwarding tree, compatible Cisco switches can maintain independent root bridge elections and port roles.

This allows administrators to select different preferred root bridges for different VLANs.

For example, VLAN 10 can prefer one distribution switch while VLAN 20 prefers another.

Such a design can improve the use of redundant links while preserving the loop-free forwarding behavior required by Ethernet switching.

However, PVST+ does not automatically distribute traffic evenly.

Effective deployment requires correct bridge priorities, path costs, trunk configuration and careful topology verification.

Understanding per-VLAN root election is therefore an important step toward mastering enterprise Layer 2 redundancy.

Useful Links and References

SanchitGurukul Static Resources

Official External References