RSTP Backup vs Alternate Port Roles

H12-811 By islever Editorial Last updated

Spanning Tree Principles and Configuration

Short answer

B A Backup port replaces a faulty Designated port, not a Root port; the Alternate port handles Root port failures in RSTP.

Question

In RSTP, a backup port can replace a faulty root port.
  1. True
  2. False Correct Answer

Key Takeaways

The main trap is confusing RSTP's Alternate and Backup port roles; remember that Alternate backs up the Root Port, while Backup backs up the Designated Port.

This question tests knowledge of RSTP port roles. The correct answer is False because a Backup port replaces a Designated port, while an Alternate port replaces a Root port.

Common mistake: Choosing True because 'Backup' sounds like it should replace the primary upstream link (Root Port), but in RSTP, that specific function belongs to the Alternate Port.

Expert Analysis

The statement is False. In Rapid Spanning Tree Protocol (RSTP), the roles are strictly defined by their replacement targets. An Alternate Port serves as a backup for the Root Port, providing an alternative path to the root bridge if the primary root port fails. Conversely, a Backup Port serves as a backup for a Designated Port on the same segment, handling redundancy when two ports connect to the same collision domain or hub.

The most attractive wrong answer is True (A). This option appeals to the general understanding that RSTP provides faster convergence and redundancy compared to legacy STP. It satisfies the intuition that "backup" implies replacing any failed upstream link. However, it stops being correct because it conflates the specific RSTP role names with generic redundancy concepts. The single point of failure in this reasoning is ignoring the strict terminology: Backup replaces Designated, Alternate replaces Root. Comments in the source material correctly highlight this distinction, noting that while some may mistakenly believe Backup replaces Root, the protocol explicitly assigns Alternate to that duty.

Therefore, the claim that a Backup port replaces a faulty Root port is technically incorrect under IEEE 802.1w standards.

Exam Strategy

When answering RSTP role questions, map each role to its specific failure scenario rather than relying on semantic meaning. Memorize the pairings: Alternate ↔ Root Port and Backup ↔ Designated Port. Use this direct mapping to quickly eliminate options that swap these relationships.

Frequently Asked Questions

What is the difference between Alternate and Backup ports in RSTP?

An Alternate port provides a backup path to the Root Bridge and replaces a failed Root Port. A Backup port provides a backup path to a network segment and replaces a failed Designated Port.

Why does RSTP have both Alternate and Backup ports?

They handle different types of redundancy. Alternate ports deal with upstream failures toward the root, while Backup ports deal with downstream loop prevention on shared segments where two ports connect to the same LAN.

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Discussion 8 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 8

  1. IBB90704 👍 4
    La respuesta es la B An alternate port acts as a backup of the root port and provides an alternate path from the designated bridge to the root bridge. A backup port acts as a backup of the designated port and provides a backup path from the root bridge to the related network segment.
  2. CiscoCertified 👍 3
    Answer is B (False). - An alternate port acts as a backup of the root port (backup for the faulty root port). - A backup port acts as a backup of the designated port.
  3. Creyes1495 👍 2 Answered B
    B. False, Pag. 414
  4. hafizhou 👍 1
    A. True In Rapid Spanning Tree Protocol (RSTP), a backup port can indeed replace a faulty root port. RSTP introduces the concept of Alternate and Backup ports. An Alternate port is designated to replace a failed designated port, and a Backup port is designated to replace a failed root port. This feature helps in rapid convergence by quickly transitioning to an alternative path if a primary path fails.
  5. Fmoura17 👍 1 Answered B
    Answer: B. False A Backup Port in RSTP replaces a faulty Designated Port, not a Root Port. RSTP Port Roles: PortReplacesAlternate PortReplaces a faulty Root PortBackup PortReplaces a faulty Designated Port Key Point: It is the Alternate Port that takes over when a Root Port fails — not the Backup Port.
Show 3 more comments
  1. Creyes1495 👍 1
    Correct! Pag. 414
  2. TIConstecoin 👍 1 Answered B
    B is correct
  3. HCIA 👍 1 Answered A
    A. True

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