350-401 Question #828: Real Exam Question with Answer & Explanation

Question

Upon receipt of a configuration BPDU with the topology change flag set, how do the downstream switches react?

Options

• ABy moving all ports to a blocking state on all switches
• BBy flushing out all MAC addresses from the MAC address table
• CBy temporarily moving all non-root ports to a listening state
• DBy flushing out all old MAC addresses from the MAC address table
• EBy updating the Topology Change version flag on the local switch database

Explanation

Upon receiving a configuration BPDU with the topology change flag set, downstream switches react by flushing out their old MAC address table entries to quickly adapt to the new network topology.

Common mistakes.

• A. Moving all ports to a blocking state is an incorrect reaction to a TCN; it would cause network disruption.
• B. While flushing MAC addresses occurs, it's specifically 'old' or dynamically learned entries that are flushed to prevent blackholing, making 'all MAC addresses' less precise than 'all old MAC addresses'.
• C. Temporarily moving non-root ports to a listening state is part of the STP state transition process, not the direct and immediate action related to MAC address table management upon receiving a TCN flag.
• E. Updating a 'Topology Change version flag' is not the primary immediate action for downstream switches regarding their MAC address tables in response to a TCN.

Concept tested. STP Topology Change Notification (TCN) reaction

Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html

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