Configure STP mode PVST on a Cisco Switch

Spanning Tree Protocol (STP) prevents network loops in a Layer 2 switching environment by selectively blocking redundant paths. In this article, we configure the Per-VLAN Spanning Tree (PVST) mode on Cisco switches using a three-switch topology. PVST allows a separate STP instance for each VLAN, enabling more granular control of traffic flow in VLAN-based networks.

STP

Topology

The topology consists of three Cisco switches, S1, S2, and S3, interconnected as shown below:

S1 is connected to S2 via GigabitEthernet 0/1.
S1 is connected to S3 via GigabitEthernet 0/2.
S2 is connected to S3 via GigabitEthernet 0/1.

In this configuration:

STP will block one redundant port to prevent network loops.
The topology ensures high availability by automatically reactivating the blocked port if an active link fails.

Configuration Steps

Set the Hostnames
Assign descriptive hostnames to each switch to make network management easier.
```
Switch>enable
Switch#configure terminal
Switch(config)#hostname S1
```
Repeat the same steps for the other two switches, replacing S1 with S2 and S3.
Enable PVST Mode
Configure each switch to operate in PVST mode. PVST provides one STP instance per VLAN, optimizing the path selection process.
For S1:
```
S1(config)#spanning-tree mode pvst
```
For S2:
```
S2(config)#spanning-tree mode pvst
```
For S3:
```
S3(config)#spanning-tree mode pvst
```
Verify and Monitor STP
Use the following commands to verify the STP configuration and monitor the status of the ports:
```
Switch#show spanning-tree
```
This command displays information about the root bridge, blocked ports, and active ports. Confirm that one of the redundant links is in the blocking state, ensuring no loops occur in the network.

Example Output

After configuration, you should see:

One port in the blocking state to prevent loops (e.g., Gig0/1 on S3).
The remaining ports in the forwarding state.

Conclusion

Configuring PVST mode on Cisco switches ensures loop-free and efficient VLAN-based network traffic. By following the steps outlined above, you can set up a reliable and robust spanning-tree topology.

Next Steps

Explore advanced STP configurations like Rapid PVST+ or MSTP for more complex network environments.