Rapid PVST+

Per-VLAN Spanning Tree Plus (PVST+) with Rapid Spanning Tree Protocol (RSTP) convergence - Cisco proprietary enhancement that runs separate spanning tree instances for each VLAN while using RSTP fast convergence mechanisms
Combines the VLAN flexibility of PVST+ with the rapid convergence of IEEE 802.1w RSTP (typically 1-3 seconds vs. 30-50 seconds for legacy STP)
Backward compatible with legacy PVST+ and can interoperate in mixed environments (will fall back to slower convergence when needed)

Port States and Roles

Uses three port states instead of legacy STP’s five states:
- Discarding - equivalent to blocking/listening/disabled (no frame forwarding or MAC learning)
- Learning - builds MAC address table but doesn’t forward frames
- Forwarding - normal operation (forwards frames and learns MACs)
Enhanced port roles for faster convergence:
- Root Port - best path to root bridge (same as legacy STP)
- Designated Port - forwarding port on each segment (same as legacy STP)
- Alternate Port - backup path to root bridge (immediate failover capability)
- Backup Port - backup designated port on same segment (rare in switched networks)

Fast Convergence Mechanisms

Mechanism	Purpose	Convergence Time
Edge Ports	End-device connections skip listening/learning	Immediate to forwarding
Point-to-Point Links	Full-duplex links between switches	Sub-second failover
Proposal/Agreement	Rapid convergence negotiation	1-3 seconds
Sync Process	Coordinated state changes	Prevents temporary loops

Edge ports (PortFast equivalent) - ports connected to end devices transition immediately to forwarding state
Proposal/Agreement mechanism - when topology changes, switches propose new topology and wait for agreement before forwarding
Link-type detection automatically identifies point-to-point links (full-duplex) for rapid convergence

Configuration and Verification

Enable globally: spanning-tree mode rapid-pvst
Configure edge ports: spanning-tree portfast (interface level)
Manual link-type: spanning-tree link-type point-to-point or shared
Per-VLAN root bridge: spanning-tree vlan <vlan-id> root primary|secondary

Key verification commands:

show spanning-tree - displays all VLAN instances
show spanning-tree vlan <vlan-id> - specific VLAN details
show spanning-tree interface <interface> - port-specific information
show spanning-tree summary - protocol mode and statistics

Vocabulary

BPDU (Bridge Protocol Data Unit) - control frames exchanged between switches to build spanning tree topology; Rapid PVST+ uses enhanced BPDUs with additional flags

Proposal/Agreement - rapid convergence mechanism where switches negotiate topology changes instead of waiting through timer-based states

Edge Port - port connected to end device that can immediately transition to forwarding (equivalent to PortFast)

Alternate Port - pre-calculated backup path that can immediately become root port if primary path fails

Sync Process - mechanism ensuring all non-edge ports go to discarding state during topology changes to prevent loops

Notes

Each VLAN maintains separate spanning tree instance - allows per-VLAN load balancing but increases CPU overhead with many VLANs (consider MST for 50+ VLANs)
Edge port configuration is critical for fast convergence - misconfigured edge ports connected to switches can cause temporary loops
Rapid PVST+ automatically falls back to legacy timers when interoperating with older STP implementations (losing rapid convergence benefits)
Point-to-point link detection usually works automatically but may need manual configuration in unusual scenarios (half-duplex links, certain media converters)
Still uses same STP priorities and path cost calculations as legacy STP - only convergence mechanism is enhanced
Maximum 4096 VLAN instances supported - practical limit much lower due to memory and CPU constraints
Consider Multiple Spanning Tree (MST) for networks with many VLANs to reduce overhead while maintaining rapid convergence