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 algorithm allows switches within a network to detect physical loops while providing multiple paths for data traffic. In this 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, then dig into PVST+ and RPVST+, compare PVST with Rapid-PVST, and cover how to configure and troubleshoot it.
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 (Per-VLAN Spanning Tree) is a Cisco proprietary protocol 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 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. Underlying PVST is the Spanning Tree Protocol (STP), which prevents loops by ensuring only one active path exists between any two network devices.
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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 own unique network topology, ensuring stable and efficient communication between devices within the VLAN. In practice, each VLAN gets its own root bridge, designated ports, and backup ports, so it can operate independently without interfering with other VLANs on the network.
Advantages of PVST
One of the key advantages of PVST is that it allows multiple VLANs to coexist on a single physical network. This lets organizations divide their network into separate VLANs for different departments or teams. For example, a company could create a VLAN for the sales department and another for the finance department, and each VLAN would have its own network topology, allowing for more efficient use of network resources. The main benefits include:
- Improved network efficiency: a separate spanning tree instance per VLAN reduces the risk of data loss or network downtime.
- Enhanced network security: isolating traffic per VLAN helps contain issues and limit unauthorized access.
- Simplified network management: managing each VLAN separately reduces the risk of errors or misconfigurations.
How to Configure PVST
Configuring PVST on your network follows a straightforward sequence of steps:
- Enable STP on your switches.
- Configure the VLANs on your switches.
- Configure the root bridge for each VLAN.
- Configure the designated and backup ports for each VLAN.
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. PVST+ also adds support for IEEE 802.1Q VLAN tagging, which lets it work with both Cisco and non-Cisco switches, making it more versatile than the Cisco-only basic PVST.
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.
PVST vs Rapid-PVST: A Detailed Comparison
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.
Rapid-PVST (also referred to as Rapid-PVST+) is an evolution of PVST, introduced in Cisco IOS Release 12.2(14)SX, that builds on the Rapid Spanning Tree Protocol (RSTP) to reduce the time the network needs 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. Here is how the two compare across the dimensions that matter most.
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.
The choice ultimately depends on the specific needs of your network. If you want to improve 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.
Troubleshooting Common PVST Issues
While PVST is a robust technology, issues may arise that require troubleshooting. Below are the most common PVST problems, their typical causes, and how to resolve them. In most cases the root cause is either an incorrect STP or VLAN configuration or an underlying physical fault, such as a damaged cable, switch, or port.
Common Issues and Their Causes
- Blocked ports: A blocked port forwards no traffic but is not fully disabled either. Blocking is necessary for loop prevention, but it can cause problems if configured incorrectly. Check the STP configuration to confirm port priorities are set correctly, and inspect for physical issues such as a damaged cable or switch port.
- Root bridge issues: The root bridge is the central point of the STP topology and forwards traffic to all other switches in the network, so problems here can cause connectivity issues. Verify that the root bridge is selected correctly, and check for a damaged switch or cable.
- Inconsistent VLAN information / VLAN mismatch: When different switches carry different VLAN information, connectivity issues occur. Make sure the VLAN ranges are configured correctly and that all switches share consistent VLAN information.
- Inconsistent bridge priority: When switches have different bridge priorities, connectivity issues can occur. Check the STP configuration to confirm the bridge priorities are set correctly across all switches.
Useful Commands to Verify PVST Operation
To diagnose these issues, start by checking the PVST configuration and VLAN information on every switch, then confirm root bridge selection and identify any blocked or inconsistent ports. The following commands are useful for verifying PVST operation:
- show spanning-tree — displays the current STP configuration, including the root bridge, the bridge priority, and the VLAN ranges.
- show vlan — displays the current VLAN configuration, including the VLAN ranges and the ports assigned to each VLAN.
- show spanning-tree root — displays information about the root bridge, including the root bridge ID and the port used to connect to it.
- show spanning-tree blockedports — displays blocked ports, including the port ID and the reason the port is blocked.
- show spanning-tree inconsistentports — displays inconsistent ports, including the port ID and the reason for the inconsistency.
- show interfaces — displays interface status and VLAN information.
- show spanning-tree summary — displays a summary of the STP configuration, including the number of blocked ports and the root bridge ID.
- show spanning-tree detail — displays detailed STP information, including the bridge ID, the root bridge ID, and the port roles.
By following the best practices for PVST configuration and regularly checking the network with these commands, network administrators can identify problems quickly and prevent many issues from occurring in the first place.
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.
If you want to become an expert in PVST and other advanced networking technologies, consider taking Orhan Ergun's CCNP ENCOR 350-401 course, which covers advanced concepts including spanning tree, automation, security, and troubleshooting, along with hands-on labs and real-world scenarios.
