Why Does Redundant Switch Connectivity Fail Without a Root Bridge?

N10-009 By islever Editorial Last updated Sep 1, 2026

Short answer

C A root bridge needs to be identified.

Question

A network engineer configures a new switch and connects it to an existing switch for expansion and redundancy. Users immediately lose connectivity to the network. The network engineer notes the following spanning tree information from both switches. Which of the following best describes the issue? - Exhibit: Why Does Redundant Switch Connectivity Fail Without a Root Bridge?

Exhibit: Why Does Redundant Switch Connectivity Fail Without a Root Bridge?

The port cost should not be equal.
The ports should use link aggregation.
A root bridge needs to be identified. Correct Answer
The switch should be configured for RSTP.

Key Takeaways

The question tests your understanding that STP uses a root bridge to decide which ports forward and which block; if no clear root is identified, loops form and connectivity collapses.

When a new switch is connected to an existing switch for redundancy, STP must elect a root bridge to block redundant paths. The CompTIA community identifies a missing/ambiguous root bridge as the primary cause of immediate connectivity loss.

Common mistake: Choosing link aggregation (B) confuses a potential solution with the root cause; while LAG can prevent STP from blocking redundant links, it does not fix the failure to have a defined root bridge.

Expert Analysis

Why the Answer Is Correct

In spanning tree protocol, every switch must agree on a single root bridge. The root bridge is the reference point used to determine port roles: root ports, designated ports, and blocked (alternate/backup) ports.

The scenario describes redundant links between two switches; without a clear root bridge, STP cannot block the redundant port, so a Layer 2 loop forms and broadcast frames circulate endlessly, causing users to lose connectivity.

The most-voted community answer supports C: "A root bridge is necessary to manage redundant paths." One user notes that when both switches show a cost of 2 on each port, they are "competing" for the root, creating instability. Thus identifying/configuring a root bridge is the correct fix.

Why the Other Options Are Wrong

A (port cost should not be equal): Equal port costs are normal and STP has deterministic tie-breakers (lowest bridge ID, then port priority), so equal costs alone do not cause failure.

B (ports should use link aggregation): Link aggregation is a useful redundancy feature, but it is a separate configuration technique; the immediate issue is STP's lack of a root bridge, not the absence of EtherChannel/LACP.

D (switch should be configured for RSTP): RSTP improves convergence speed but still relies on root bridge election. If no root bridge is identified, RSTP would also fail to break the loop.

Community Comment Notes

Comments with 7 and 3 likes both choose C, explaining that a root bridge is needed to manage redundant paths and prevent competing paths/instability.

A minority comment suggests B (link aggregation), and another suggests D (RSTP). While these are related to STP and redundancy, they do not describe the underlying cause captured in the spanning-tree output.

The official answer, as accepted by the largest vote group (92 votes for C), is consistent with standard STP troubleshooting.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

On exam day, when you see a redundancy question with two switches and STP output, immediately ask 'Is a root bridge clearly elected?' If not, the answer is almost always about root bridge identification. Remember that STP blocks redundant ports based on the root bridge, so a missing or contested root causes loops and loss of connectivity.

Community Votes

100%

100% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 5 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 5

st1a 👍 7 Answered C

a root bridge is necessary to manage redundant paths
a484b2b 👍 4

Answer C Since both switches have a cost of 2 on each port, they might be competing to determine which path is the preferred route to the root. This can create a loop or instability in the network if no clear root bridge is identified.
fd1eda1 👍 3

B Link aggregation is the best solution here to ensure both links are utilized effectively and STP does not block one of them, providing redundancy without causing connectivity issues.
callmetodd 👍 1 Answered D

The issue described in the scenario likely corresponds to the spanning tree protocol (STP) misconfiguration or inefficiency. Based on the options provided, the best answer is: D. The switch should be configured for RSTP. Explanation: RSTP (Rapid Spanning Tree Protocol): Unlike traditional STP, RSTP converges faster, reducing downtime when topology changes occur. Without RSTP, redundant links may cause a temporary loop until STP reconverges. The equal port cost isn't inherently an issue unless the root bridge is not correctly identified, but this doesn't seem the immediate issue here. Link aggregation (Option B) might help with bandwidth but is unrelated to the current STP problem. Identifying a root bridge (Option C) is essential, but modern switches should automatically determine this unless configured otherwise. Switches may be encountering loops due to delays in spanning tree reconvergence. Configuring RSTP would address this problem by allowing quicker recovery and preventing connectivity loss.
a484b2b 👍 4 Answered C

Answer C

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