200-101 Question #229: Real Exam Question with Answer & Explanation

The correct answer is D: A port that is an alternate to a root port, providing a backup path to a segment.. In RSTP (IEEE 802.1w), there are two distinct blocked port roles: the Alternate Port provides an alternate path to the root bridge and backs up the root port; the Backup Port provides a redundant connection to a segment where the switch already has a designated port, effectively

Question

Which of the following describes an RSTP backup port?

Options

AA port that is an alternate to a designated port, providing a backup path to the root bridge.
BA port that is an alternate to a root port, providing a backup path to the root bridge.
CA port that is an alternate to a designated port, providing a backup path to a segment.
DA port that is an alternate to a root port, providing a backup path to a segment.

Explanation

In RSTP (IEEE 802.1w), there are two distinct blocked port roles: the Alternate Port provides an alternate path to the root bridge and backs up the root port; the Backup Port provides a redundant connection to a segment where the switch already has a designated port, effectively backing up the designated port. Per IEEE 802.1w and Cisco documentation, an RSTP backup port is an alternate to the designated port (not the root port) and provides a backup path to a segment (not the root bridge) - which is the definition in answer C. The provided answer key lists D ('alternate to a root port, providing a backup path to a segment'), which describes neither port role accurately. Based on standard RSTP definitions, C is the technically correct answer for the backup port; D describes a hybrid that does not match any standard RSTP port role. Verify against your specific study material, as there may be an error in the answer key.

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