Spanning TreeCCNA 200-301

Rapid PVST+ and PVST Compatibility Issues

Presenting Symptom

A switch in the network experiences frequent topology changes, causing MAC address table flapping and intermittent connectivity for end devices.

Network Context

A small enterprise campus network with three Cisco Catalyst 2960 switches (SW1, SW2, SW3) running IOS 15.0. SW1 is the root bridge for VLAN 10, SW2 and SW3 are non-root. SW1 runs Rapid PVST+ (spanning-tree mode rapid-pvst), while SW2 and SW3 run legacy PVST+ (spanning-tree mode pvst). The switches are interconnected via trunk links carrying VLAN 10.

Diagnostic Steps

Check spanning-tree mode on all switches

show spanning-tree summary

Switch is in rapid-pvst mode
Root bridge for: VLAN0010
Ethernet0/0 is root port
...

Look for the spanning-tree mode. If one switch shows 'rapid-pvst' and others show 'pvst', compatibility issues may cause BPDU mismatches.

Check for topology change notifications

show spanning-tree detail | include Topology

Number of topology changes: 25 last change occurred: 00:05:12 ago
Topology change flag: set
...

A high number of topology changes in a short time indicates instability. This is common when Rapid PVST+ and PVST+ interact, as they use different BPDU formats.

Check interface counters for BPDU errors

show interfaces trunk

Port        Mode         Encapsulation  Status        Native vlan
Et0/0       on           802.1q         trunking      1

Port        Vlans allowed on trunk
Et0/0       1-1005

Port        Vlans allowed and active in management domain
Et0/0       1,10

Port        Vlans in spanning tree forwarding state and not pruned
Et0/0       1,10
...

Look for 'BPDU' errors or 'inconsistent' states. If a port is in 'blocking' or 'listening' state unexpectedly, it may be due to BPDU version mismatch.

Check BPDU version on interfaces

show spanning-tree interface ethernet 0/0 detail

Port 1 (Ethernet0/0) of VLAN0010 is root port
  Port path cost 4, Port priority 128, Port Identifier 128.1.
  Designated root has priority 32768, address 0011.2233.4455
  Designated bridge has priority 32768, address 0011.2233.4455
  Designated port id is 128.1, designated path cost 0
  Timers: message age 2, forward delay 15, hold 1
  Number of transitions to forwarding state: 1
  BPDU: sent 3, received 5
  BPDU version: 2 (RSTP)
...

Check the BPDU version. If one side sends RSTP BPDUs (version 2) and the other sends STP BPDUs (version 0), the switch may revert to legacy STP on that port, causing suboptimal convergence.

Root Cause

The network has a mix of Rapid PVST+ and PVST+ switches. When a Rapid PVST+ switch receives a legacy PVST+ BPDU on a port, it reverts to legacy STP on that port, losing the fast convergence benefits. Additionally, the BPDU format mismatch can cause the legacy PVST+ switches to misinterpret the BPDUs, leading to frequent topology changes and MAC flapping.

Resolution

Configure all switches to use the same spanning-tree mode. To maintain Rapid PVST+ benefits, change the legacy PVST+ switches to Rapid PVST+: On SW2 and SW3: SW2(config)# spanning-tree mode rapid-pvst SW3(config)# spanning-tree mode rapid-pvst Alternatively, if Rapid PVST+ is not desired, change all to PVST+: SW1(config)# spanning-tree mode pvst After changing the mode, the switches will automatically renegotiate BPDU versions and converge.

Verification

Run 'show spanning-tree summary' on all switches to confirm the mode is consistent. Also check 'show spanning-tree detail' to see that topology change counts stabilize. Expected output: SW1# show spanning-tree summary Switch is in rapid-pvst mode Root bridge for: VLAN0010 ... SW2# show spanning-tree summary Switch is in rapid-pvst mode ... SW3# show spanning-tree summary Switch is in rapid-pvst mode ... Additionally, 'show spanning-tree detail | include Topology' should show no recent changes.

Prevention

1. Standardize the spanning-tree mode across all switches in the network to avoid compatibility issues. 2. Use Rapid PVST+ for all switches to benefit from faster convergence. 3. When adding new switches, ensure they are configured with the same spanning-tree mode as the existing network.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify the cause of frequent topology changes. The exam tests understanding of STP compatibility between Rapid PVST+ and PVST+. A key fact is that Rapid PVST+ and PVST+ are not compatible; a Rapid PVST+ switch will revert to legacy STP on ports receiving PVST+ BPDUs.

Exam Tips

Remember that Rapid PVST+ and PVST+ use different BPDU formats; mixing them causes the Rapid PVST+ switch to fall back to legacy STP on affected ports.

When troubleshooting frequent topology changes, always check the spanning-tree mode on all switches first.

The command 'show spanning-tree summary' quickly shows the mode and root bridge status.

Commands Used in This Scenario

show interfaces trunk

Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.

show spanning-tree detail

Displays detailed Spanning Tree Protocol (STP) information for all VLANs or a specific VLAN, including port roles, states, timers, and bridge IDs, used for troubleshooting STP convergence and topology changes.

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