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What is Per-VLAN Spanning Tree (PVST)?
Per-VLAN Spanning Tree (PVST) is an extension of the Spanning Tree Protocol (STP) designed for Ethernet networks. STP is used to prevent loop formation in Ethernet networks by blocking redundant links, ensuring a loop-free topology. However, STP operates at the VLAN-agnostic level, considering all VLANs as a single entity. This is where PVST comes in.
With PVST, a separate instance of STP is created for each VLAN in the network. This means that PVST provides independent loop prevention and topology control for each VLAN within an Ethernet network. By running a separate STP instance for each VLAN, PVST ensures that every active VLAN has a loop-free topology.
Basic Concepts of PVST
1. Instance: In PVST, each VLAN is associated with a separate STP instance. For example, if there are 10 VLANs, PVST creates 10 individual instances of STP. This allows for per-VLAN loop prevention and redundancy.
2. Root Bridge: In each STP instance, there is a designated Root Bridge. The Root Bridge is elected based on a Bridge ID, which consists of a Priority value and the MAC address of the switch.
3. Designated Port: Within each STP instance, one link per switch is designated as the Designated Port. This port is responsible for forwarding traffic towards the Root Bridge and is determined based on cost calculations.
4. Blocked Port: Any redundant link that could create loops is put into a Blocked state. These ports do not forward traffic and are only used as a backup in case of link failures.
Benefits of PVST
1. Enhanced Network Performance: By providing an independent STP instance for each VLAN, PVST optimizes network performance by allowing for faster convergence and better load balancing across VLANs.
2. Isolation and Redundancy: PVST ensures that a failure within one VLAN does not affect the connectivity and stability of other VLANs. Redundancy is provided by having blocked ports as backup links, which can become active when a primary link fails.
3. Flexibility: With PVST, network administrators have the flexibility to customize STP configurations on a per-VLAN basis, allowing for fine-tuning of network topologies and forwarding paths.
Overall, PVST enhances the stability, performance, and flexibility of Ethernet networks by extending the benefits of STP to individual VLANs. It is a crucial protocol for large-scale networks with multiple VLANs, providing efficient loop prevention and redundancy.
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