200-101 Question #158: Real Exam Question with Answer & Explanation

The correct answer is D: in an improperly implemented redundant topology. Without Spanning Tree Protocol properly configured in a redundant switched network, switching loops form and cause the same unicast frames to be transmitted multiple times.

Question

In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?

Options

Aduring high traffic periods
Bafter broken links are re-established
Cwhen upper layer protocols require high reliability
Din an improperly implemented redundant topology
Ewhen a dual ring topology is in use

Explanation

Without Spanning Tree Protocol properly configured in a redundant switched network, switching loops form and cause the same unicast frames to be transmitted multiple times.

Common mistakes.

A. High traffic periods increase congestion and may cause drops, but do not inherently cause the same unicast frame to be duplicated and retransmitted.
B. Re-establishing a broken link may trigger a topology change and STP reconvergence, but does not by itself cause duplicate unicast frame transmission.
C. Upper-layer protocol reliability mechanisms (such as TCP retransmission) operate at Layer 4 and above and do not cause Layer 2 frame duplication in the switch fabric.
E. A dual ring topology is a valid, defined physical layout (as in FDDI or MAN designs) and does not inherently produce duplicate unicast frame transmission on its own.

Concept tested. Switching loops and redundant topology risks

Reference. https://www.cisco.com/c/en/us/tech/lan-switching/spanning-tree-protocol/index.html

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