The answer is to configure the new switch with a higher bridge priority, such as 28672, to prevent it from becoming the root bridge. This is correct because the new access switch, likely using its default bridge priority of 32768, had a lower numerical priority than the existing root bridge for VLAN 100, causing it to be elected as the new root upon connection. Root Guard on the Layer 3 switch port then detected the superior BPDU from this unintended root and blocked the port, triggering the SPANTREE-2-ROOTGUARD_BLOCK messages and intermittent connectivity. On the ENCOR 350-401 exam, this scenario tests your understanding of STP root guard behavior and bridge priority manipulation; a common trap is assuming the new switch’s default priority is safe, but any switch with a priority lower than the current root can trigger a root guard violation. Remember the memory tip: “Lower priority wins the root election, so raise the priority on access switches to keep them humble.”
350-401 Network Assurance Practice Question
This 350-401 practice question tests your understanding of network assurance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company has a network with multiple VLANs connected via a Layer 3 switch acting as the gateway for all VLANs. The network uses Rapid PVST+ for spanning tree. Recently, the network team added a new access switch to VLAN 100. After the switch was connected, users in VLAN 100 experienced intermittent connectivity, and the Layer 3 switch logs show 'SPANTREE-2-ROOTGUARD_BLOCK' messages for the port connected to the new switch. The new switch is intended to provide additional access ports for VLAN 100. The network team ensured that the new switch's configuration is correct for VLAN 100 access. What is the most likely cause of the issue, and what action should be taken to resolve it?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the new switch with a higher bridge priority (e.g., 28672) to prevent it from becoming the root bridge.
The issue is that the new switch, intended as an access switch, has a lower bridge priority (or default priority of 32768) than the existing root bridge for VLAN 100. When connected, it becomes the new root bridge, causing topology changes and intermittent connectivity. Root Guard on the Layer 3 switch port detects this superior BPDU and blocks the port to protect the root bridge position. Configuring the new switch with a higher bridge priority (e.g., 28672) ensures it cannot become the root bridge, resolving the Root Guard blocks.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Change the port configuration on the new switch to access mode for VLAN 100.
Why it's wrong here
This does not address the BPDU attack or the root guard blocking.
✗
Disable Root Guard on the Layer 3 switch port connected to the new switch.
Why it's wrong here
Disabling Root Guard would allow the new switch to become root, potentially causing suboptimal paths.
✓
Configure the new switch with a higher bridge priority (e.g., 28672) to prevent it from becoming the root bridge.
Why this is correct
Setting a higher bridge priority ensures the new switch does not attempt to become root, resolving the root guard blocking.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Remove the new switch from the network because it is causing a BPDU attack.
Why it's wrong here
The new switch is intended to be part of the network; it is not a rogue switch.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that Root Guard is the problem and should be disabled, when in fact the root cause is the new switch's bridge priority being too low, and the correct fix is to adjust the priority on the new switch.
Detailed technical explanation
How to think about this question
In Rapid PVST+, each VLAN runs its own spanning-tree instance, and the root bridge is elected based on the lowest bridge priority (default 32768). Root Guard, when enabled on a port, places the port into a root-inconsistent state if it receives a superior BPDU that would cause the switch to become a non-root. This prevents unauthorized switches from becoming root. The new switch's default priority (32768) may be lower than the current root's priority (e.g., 4096 or 8192), triggering the block. Setting the new switch's priority to 28672 (or higher than the current root) ensures it will never win the election.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure the new switch with a higher bridge priority (e.g., 28672) to prevent it from becoming the root bridge. — The issue is that the new switch, intended as an access switch, has a lower bridge priority (or default priority of 32768) than the existing root bridge for VLAN 100. When connected, it becomes the new root bridge, causing topology changes and intermittent connectivity. Root Guard on the Layer 3 switch port detects this superior BPDU and blocks the port to protect the root bridge position. Configuring the new switch with a higher bridge priority (e.g., 28672) ensures it cannot become the root bridge, resolving the Root Guard blocks.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.