200-150 Question #191: Real Exam Question with Answer & Explanation

The correct answer is B: Frames with an unknown destination address will be flooded.. When a switch MAC address table is full, it can no longer learn new addresses, so only frames with destination MACs not present in the table are flooded; known destinations are still forwarded normally.

Question

The MAC address table on a Cisco Nexus Server switch is full .What Layer 2 network behavior will occur while the network MAC address table remains full?

Options

AAll frames will be flooded.
BFrames with an unknown destination address will be flooded.
CThe switch reset the Mac address table. removin all MAC addresses.
DThe stitch will remove all dynamically MAC address from the table

Explanation

When a switch MAC address table is full, it can no longer learn new addresses, so only frames with destination MACs not present in the table are flooded; known destinations are still forwarded normally.

Common mistakes.

A. Not all frames are flooded - only frames with destination MACs missing from the table are flooded; frames matching existing table entries are still unicast forwarded to their learned ports.
C. Cisco switches do not automatically reset or clear the entire MAC address table when it reaches capacity; that would cause a severe disruption and is not standard behavior.
D. The switch does not proactively remove all dynamically learned addresses when the table is full; existing entries expire only when their aging timer (default 300 seconds) elapses.

Concept tested. MAC address table overflow and unknown unicast flooding

Reference. https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/23563-143.html

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