Multiple copies of the same unicast frame in a switched LAN: under which circumstance is this most likely?

Introduction / Context:
Switched Ethernet should deliver each unicast frame once. However, network design mistakes—especially loops without proper control—can cause duplicate unicast frames, confusing hosts and applications.

Given Data / Assumptions:

• Environment includes redundant links between switches.
• Spanning Tree Protocol may be misconfigured or disabled.
• MAC learning may flap due to looping traffic.

Concept / Approach:
In a faulty redundant topology, the same unicast frame can traverse multiple looped paths and be delivered more than once. STP, RSTP, or MSTP prevents this by blocking redundant links until needed.

Step-by-Step Solution:

Verification / Alternative check:
Packet captures show identical sequence numbers and data arriving repeatedly; switch logs may show STP disabled and MAC moves.

Why Other Options Are Wrong:
High traffic: Increases load but not duplication by itself.
After link restoration: STP convergence momentarily can cause brief anomalies, but persistent duplicates indicate a loop.
Upper-layer reliability: TCP retransmits missing segments, not duplicate Ethernet frames from the LAN.

Common Pitfalls:
Blaming servers or applications when the root cause is a Layer 2 loop. Always verify STP status and physical topology.

Final Answer:
In an improperly implemented redundant topology