STP Lab 7: Rapid-PVST+ Migration on Triangle Loop

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Migrate a three-switch triangle from legacy PVST to Rapid-PVST+ without changing the existing root or blocked port. SW1 remains the deterministic root for VLAN 70 (priority 4096). Verify protocol mode, link types (point-to-point vs edge), and end-to-end host reachability, then observe the faster reconvergence behavior of Rapid-PVST+.

Learning objectives

• Enable Rapid-PVST+ globally on all switches in a looped L2 topology.
• Maintain deterministic root control for VLAN 70 using bridge priority (SW1 at 4096).
• Verify protocol mode change with show spanning-tree summary.
• Confirm trunk ports are detected as point-to-point and access ports as edge with show spanning-tree vlan 70 detail.
• Validate connectivity between hosts across a spanning-tree blocked/alternate path scenario and observe faster convergence.

Troubleshooting focus

• One switch left in PVST while others run Rapid-PVST+ leads to mixed behavior and slower convergence.
• Trunk allow-list drift can silently prune VLAN 70 across part of the triangle.
• Access port missing PortFast/BPDU Guard risks long convergence or topology corruption if a host bridges BPDUs.
• Incorrect root priority on a non-root switch can steal root and change port roles unexpectedly.