The Lazy Networker

Cisco switches support Per-VLAN Spanning Tree (PVST) and Rapid Per-VLAN Spanning Tree (Rapid PVST) to prevent Layer 2 loops. Let’s compare them!

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1️⃣ What is PVST? (Per-VLAN Spanning Tree)

✅ Cisco proprietary version of STP.
✅ Runs one STP instance per VLAN.
✅ Based on IEEE 802.1D (classic STP) → Slow (50 sec convergence).
✅ Each VLAN can have a different Root Bridge, optimizing traffic flow.

How PVST Works:

• Each VLAN has its own STP topology.
• If there are 100 VLANs, the switch runs 100 STP instances.
• Can lead to high CPU usage on large networks.

Example Command to Enable PVST:

Switch(config)# spanning-tree mode pvst

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2️⃣ What is Rapid PVST? (Rapid Per-VLAN Spanning Tree)

✅ Cisco enhancement of Rapid Spanning Tree (RSTP – 802.1w).
✅ Runs one RSTP instance per VLAN.
✅ Faster convergence (<6 seconds) than PVST.
✅ Uses Port Roles & Link Types for quick transitions.

Key Features of Rapid PVST:

• Alternate & Backup Ports speed up recovery.
• Discards Listening State → Faster transition to Forwarding State.
• Works best with Point-to-Point links (Full-Duplex).

Example Command to Enable Rapid PVST:

Switch(config)# spanning-tree mode rapid-pvst

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3️⃣ PVST vs. Rapid PVST – Feature Comparison

Feature	PVST (802.1D)	Rapid PVST (802.1w)
Convergence Time	50 sec (Slow)	<6 sec (Fast)
Per-VLAN STP?	✅ Yes	✅ Yes
BPDU Exchange	Every 2 sec	Every 2 sec
Port Roles	Root, Designated, Blocking	Root, Designated, Alternate, Backup
Listening State?	✅ Yes	❌ No (Removed)
Loop Prevention	✅ Yes	✅ Yes (Faster Recovery)
CPU/Memory Usage	High (Multiple STP Instances)	High (Multiple RSTP Instances)

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4️⃣ When to Use PVST or Rapid PVST?

✅ Use PVST if:

• You have legacy Cisco switches that do not support RSTP.
• Your network is small, and convergence speed isn’t critical.

✅ Use Rapid PVST if:

• You need faster convergence to reduce downtime.
• You have a modern Cisco network with full RSTP support.
• Your network has high availability requirements.

🚀 Recommendation: Always use Rapid PVST for better performance and quick recovery.