STP - PVST+ and Rapid PVST+ interaction

Per-VLAN Spanning Tree Plus (PVST+) and Rapid Per-VLAN Spanning Tree Plus (Rapid PVST+) are both Cisco-proprietary enhancements of the Spanning Tree Protocol (STP), each designed to provide specific improvements in network environments. Understanding their interoperability requires a look at how they function and their key differences:

  1. PVST+:

    • Based on the original STP, PVST+ allows a separate spanning tree for each VLAN in the network. This means each VLAN can have a different root bridge and optimized paths, reducing the likelihood of network congestion.
    • PVST+ is backward compatible with standard STP.
  2. Rapid PVST+

    • An enhancement of PVST+, Rapid PVST+ is based on the Rapid Spanning Tree Protocol (RSTP), which is a faster version of STP.
    • It provides faster convergence than PVST+ by quickly reconfiguring the spanning tree in case of topology changes.
    • Rapid PVST+ maintains a separate spanning tree for each VLAN, just like PVST+.

Interoperation between PVST+ and Rapid PVST+:

  • Backward Compatibility: Both protocols are designed to be backward compatible. This means that a network that utilizes both PVST+ and Rapid PVST+ can operate effectively, with each protocol functioning correctly on their respective VLANs.
  • Convergence and Performance: In a mixed environment, Rapid PVST+ VLANs benefit from faster convergence times compared to PVST+ VLANs. However, the overall network performance may be impacted by the slower convergence of PVST+ segments.
  • Network Design Considerations: When designing a network that incorporates both protocols, careful planning is necessary to ensure optimal performance. It's important to understand the topology and traffic patterns to decide where to deploy each protocol.
  • Transition Strategy: In environments transitioning from PVST+ to Rapid PVST+, interoperability allows for a gradual migration. Networks can run both protocols simultaneously during the transition period.

In summary, while PVST+ and Rapid PVST+ can interoperate in the same network environment, the mixed deployment should be carefully managed to optimize network performance and take advantage of the faster convergence times offered by Rapid PVST+.