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Overview of Per VLAN Spanning Tree (PVST)

Networking is a critical component of modern business operations, and one protocol many companies rely on to ensure reliable communication between endpoints is PVST (Per-VLAN Spanning Tree). This powerful routing algorithm allows switches within a network to detect physical loops while providing multiple paths for data traffic. In this blog post, we'll go over the basics of PVST so you can understand how it works and why it's essential for your networks today.

What is PVST?

In a network, a spanning tree is a subset of the network's connections that connects all the nodes in the network without forming any loops. The idea is to create a tree-like structure that spans the entire network with the minimum number of connections possible while still ensuring that every node in the network is reachable.

PVST, or Per-VLAN Spanning Tree) is a network protocol that is used to prevent loops from forming in a network that is divided into multiple virtual local area networks (VLANs). In a typical network configuration, multiple devices are connected to a network through various switches and routers. Without a protocol like PVST, these devices can communicate with each other, but the network can become unstable if loops form, which can cause broadcast storms and other problems. 

How Does It Work?

PVST works by using the spanning tree algorithm to calculate the optimal network topology for each VLAN and then selectively blocking certain links in the network to prevent loops from forming. This allows each VLAN to have its unique network topology, ensuring stable and efficient communication between devices within the VLAN.

Advantages of PVST

One of the key advantages of PVST is that it allows multiple VLANs to coexist on a single physical network. This allows organizations to divide their network into separate VLANs for different departments or teams, which can improve security and network performance. For example, a company could create a VLAN for the sales department and another VLAN for the finance department, and each VLAN would have its own network topology. This allows for more efficient use of network resources and improved network performance.

Cisco-Proprietary Spanning Tree Protocols

Cisco-proprietary spanning tree protocols are network protocols developed and supported by Cisco Systems for use on Cisco switches and routers. Some examples of Cisco-proprietary spanning tree protocols include Per-VLAN Spanning Tree (PVST), Per-VLAN Spanning Tree Plus (PVST+), and Rapid PVST+ (RPVST+).

PVST+

Per-VLAN Spanning Tree Plus (PVST+) enhances the basic Per-VLAN Spanning Tree (PVST) protocol. It uses the same algorithm as the basic PVST protocol but with some enhancements to improve its performance and scalability in large networks.

So, what are some advantages of PVST+?

To begin with, PVST+ supports the IEEE 802.1w standard. This allows PVST+ to converge faster than PVST when changes are made to the network, such as when a link fails, or a new device is added. This can improve network reliability and reduce downtime.

Another difference between PVST and PVST+ is that PVST+ supports the Cisco Private VLAN (PVLAN) feature, which allows for more granular control over network traffic in a PVST+ environment. This can improve security and network performance by isolating devices within a PVLAN and limiting communication with other devices on the network.

RPVST+

Rapid Per-VLAN Spanning Tree Plus (RPVST+) is a network protocol similar to Per-VLAN Spanning Tree Plus (PVST+) but uses a different approach to calculate the spanning tree for each VLAN. RPVST+ is designed to improve the performance and scalability of PVST+ in large networks and supports additional features such as a multi-instance spanning tree.

One of the main differences between PVST+ and RPVST+ is how they calculate each VLAN's spanning tree. PVST+ uses the IEEE 802.1w standard, also known as Rapid Spanning Tree Protocol (RSTP), to converge faster when changes are made to the network. RPVST+ uses a different algorithm that allows it to converge even faster than PVST+, making it better suited for large networks.

Another difference between PVST+ and RPVST+ is that RPVST+ supports the multi-instance spanning tree feature, which allows multiple spanning tree instances to coexist on a single physical network. This can be useful in environments where different VLANs have different requirements for network connectivity or where multiple switches are connected in a hierarchy.

While RPVST+ is a Cisco-proprietary protocol, it is often compatible with PVST+ networks and can provide significant performance improvements in large, complex networks.

Final Words

In conclusion, PVST is a valuable tool for optimizing network performance and ensuring a stable, loop-free network topology. By operating on a per-VLAN basis, PVST allows each VLAN to have its own spanning tree, which can significantly improve the efficiency of network traffic and help prevent network congestion. Overall, understanding and implementing PVST can greatly benefit any network administrator looking to improve their network's performance and reliability.

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Stanley Avery

I am a certified network engineer with over 10 years of experience in the field. I have a deep understanding of networking and IT security, and I am always looking for new challenges.

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