400-007 Question #41: Real Exam Question with Answer & Explanation

The correct answer is A: PVRSTP. Rapid PVST+ minimizes unicast flooding during topology changes by performing targeted, per-port MAC address flushing rather than clearing the entire table, and its faster convergence limits the flooding window in large multi-VLAN environments.

Question

While reviewing an existing network design, you are discussing the characteristics of different STP versions. Which protocol minimizes unicast flooding during a Topology Change Notification in a Layer 2 switched network with many VLANs?

Options

APVRSTP
BMST
CSTP
DPVSTP+

Explanation

Rapid PVST+ minimizes unicast flooding during topology changes by performing targeted, per-port MAC address flushing rather than clearing the entire table, and its faster convergence limits the flooding window in large multi-VLAN environments.

Common mistakes.

B. MST maps multiple VLANs to fewer spanning tree instances, so a topology change in one instance flushes MAC addresses for all VLANs sharing that instance simultaneously, causing broader unicast flooding across many VLANs than a per-VLAN rapid protocol.
C. Classic STP (IEEE 802.1D) flushes the entire MAC address table across all bridge ports upon receiving a Topology Change Notification, producing maximum unicast flooding until entries are relearned through normal traffic.
D. PVSTP+ is based on classic STP convergence behavior and flushes all MAC address entries for the affected VLAN when a TCN is processed, resulting in more extensive unicast flooding compared to the selective flushing used by Rapid PVST+.

Concept tested. Rapid PVST+ targeted MAC flushing during topology changes

Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html

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