PVST vs Rapid-PVST Comparison: Overview of the Two STPs

If you're working with Cisco networking equipment, you're likely familiar with Spanning Tree Protocol (STP).

STP is a protocol used to prevent loops in a network, ensuring that only one active path exists between any two network devices. However, STP can be slow to converge, leading to network downtime.

That's where PVST and Rapid-PVST come in.

Let’s compare PVST vs Rapid-PVST and explore their similarities and differences.

What is PVST?

PVST, or Per-VLAN Spanning Tree, is a Cisco proprietary protocol that operates on a per-VLAN basis. In PVST, a separate instance of STP runs for each VLAN, meaning that if there are 10 VLANs on a switch, there will be 10 instances of STP running simultaneously. PVST was introduced in Cisco IOS Release 11.1 and is widely used today.

What is Rapid-PVST?

Rapid-PVST, also known as RSTP, is an evolution of PVST that was introduced in Cisco IOS Release 12.2(14)SX. Rapid-PVST improves on PVST by reducing the time it takes for the network to reconverge after a topology change. Rapid-PVST operates on a per-port basis, meaning that instead of running a separate instance of STP for each VLAN, a single instance of STP is used for all VLANs on a given port.

PVST vs Rapid-PVST: A Comparison

Convergence Time

One of the biggest differences between PVST and Rapid-PVST is the time it takes for the network to reconverge after a topology change. PVST can take up to 50 seconds to reconverge, while Rapid-PVST can reconverge in as little as 3 seconds. This is because Rapid-PVST uses a different convergence algorithm that is more efficient than the one used by PVST.

CPU Utilization

Because PVST runs a separate instance of STP for each VLAN, it can be more CPU-intensive than Rapid-PVST. Rapid-PVST, on the other hand, uses a single instance of STP for all VLANs on a given port, reducing CPU utilization. This can be especially important in large networks with many VLANs.

Compatibility

PVST and Rapid-PVST are not directly compatible with each other. If you have a network that is running PVST, you cannot simply switch to Rapid-PVST without reconfiguring your network. However, Rapid-PVST does support PVST, meaning that you can have a network that is running Rapid-PVST with some switches running PVST.

Root Bridge Election

In both PVST and Rapid-PVST, a root bridge must be elected to ensure that the network is loop-free. However, the way in which the root bridge is elected is different between the two protocols. In PVST, the root bridge is elected based on the lowest bridge ID. In Rapid-PVST, the root bridge is elected based on the lowest path cost to the root bridge.

BPDU Format

PVST and Rapid-PVST use different BPDU (Bridge Protocol Data Unit) formats. PVST uses a 30-byte BPDU format, while Rapid-PVST uses a 32-byte BPDU format. This means that if you have a network that is running both PVST and Rapid-PVST, the switches will need to convert between the two BPDU formats, which can add some overhead to the network.

Conclusion

In conclusion, when it comes to PVST vs Rapid-PVST, the choice ultimately depends on the specific needs and requirements of your network. Both protocols have their advantages and disadvantages, and it's important to weigh them carefully before making a decision.

If you're looking to improve network convergence time and reduce CPU utilization, Rapid-PVST may be the better choice. However, if you have a network with many VLANs and require per-VLAN STP, PVST may be the way to go.

Regardless of which protocol you choose, having a solid understanding of STP is essential for any network engineer. That's why courses like Orhan Ergun's CCNP ENCOR 350-401 are so valuable.

With in-depth coverage of STP and other key networking topics, this course can help you become a more skilled and knowledgeable network engineer, better equipped to tackle the challenges of today's complex networks.