- A
The new switch was connected to a port configured as a trunk with a native VLAN mismatch.
Why wrong: A native VLAN mismatch can cause VLAN traffic to leak or STP inconsistency warnings, but it does not trigger BPDU Guard or change the root bridge. The root change and err-disable symptoms are specific to BPDU Guard and root priority events.
- B
The new switch’s bridge priority is lower than the existing root bridge, and it was connected to a port with BPDU Guard enabled.
A lower bridge priority causes the new switch to become the root for VLAN 10. Plugging it into a BPDU Guard-enabled port (which is normally an edge port with PortFast) results in the port receiving BPDUs and going err-disabled. This perfectly explains both symptoms.
- C
The new switch has PortFast enabled on its uplinks.
Why wrong: Enabling PortFast on uplinks can cause short-lived forwarding loops during convergence, but it does not trigger a root bridge change or BPDU Guard activation. It would also affect the whole switch, not just specific access ports.
- D
The BPDU Guard feature was globally enabled on all ports, including trunk ports.
Why wrong: Global BPDU Guard would disable all ports that receive BPDUs, including trunk links, leading to widespread network failure. The scenario specifically mentions only ‘several access ports’ going err-disabled, and the root bridge change would also not be explained by this alone.
Quick Answer
The answer is that the new switch’s lower bridge priority caused the root bridge change and triggered BPDU Guard err-disable on neighboring access ports. This happens because Rapid PVST+ elects the root bridge based on the lowest numerical bridge priority; when a new switch with a priority lower than the existing root (e.g., 0 or 4096) connects, it immediately becomes the new root for VLAN 10 and sends superior BPDUs out its uplinks. Those BPDUs are received by adjacent switches on ports configured with PortFast and BPDU Guard, which interprets any BPDU on a PortFast port as a potential loop and places the port into err-disable state, causing the reported intermittent connectivity loss. On the CCNA 200-301 v2 exam, this scenario tests your understanding of STP convergence, BPDU Guard behavior, and the impact of misconfigured bridge priority on a live network—a common trap is assuming a new switch will automatically behave as an access switch without verifying its default priority. Remember the mnemonic: “Lower priority wins the root, and BPDU Guard punishes the intruder.”
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 network engineer connects a new switch to an existing Rapid PVST+ campus network. The new switch is intended to serve as an additional access-layer switch, but after connecting its uplinks, the engineer discovers that the root bridge for VLAN 10 has changed to this new switch, and several access ports on other switches with PortFast and BPDU Guard enabled are now in err-disabled state. Some users report intermittent connectivity loss.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The new switch’s bridge priority is lower than the existing root bridge, and it was connected to a port with BPDU Guard enabled.
The new switch's bridge priority is lower (numerically smaller) than the existing root bridge, so it becomes the new root for VLAN 10. When it sends superior BPDUs out its uplinks, the neighboring switch's access ports with PortFast and BPDU Guard enabled receive these BPDUs, triggering err-disable state on those ports, causing connectivity loss.
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.
- ✗
The new switch was connected to a port configured as a trunk with a native VLAN mismatch.
Why it's wrong here
A native VLAN mismatch can cause VLAN traffic to leak or STP inconsistency warnings, but it does not trigger BPDU Guard or change the root bridge. The root change and err-disable symptoms are specific to BPDU Guard and root priority events.
- ✓
The new switch’s bridge priority is lower than the existing root bridge, and it was connected to a port with BPDU Guard enabled.
Why this is correct
A lower bridge priority causes the new switch to become the root for VLAN 10. Plugging it into a BPDU Guard-enabled port (which is normally an edge port with PortFast) results in the port receiving BPDUs and going err-disabled. This perfectly explains both symptoms.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The new switch has PortFast enabled on its uplinks.
Why it's wrong here
Enabling PortFast on uplinks can cause short-lived forwarding loops during convergence, but it does not trigger a root bridge change or BPDU Guard activation. It would also affect the whole switch, not just specific access ports.
- ✗
The BPDU Guard feature was globally enabled on all ports, including trunk ports.
Why it's wrong here
Global BPDU Guard would disable all ports that receive BPDUs, including trunk links, leading to widespread network failure. The scenario specifically mentions only ‘several access ports’ going err-disabled, and the root bridge change would also not be explained by this alone.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The new switch’s bridge priority is lower than the existing root bridge, and it was connected to a port with BPDU Guard enabled.Correct answer▾
Why this is correct
A lower bridge priority causes the new switch to become the root for VLAN 10. Plugging it into a BPDU Guard-enabled port (which is normally an edge port with PortFast) results in the port receiving BPDUs and going err-disabled. This perfectly explains both symptoms.
✗The new switch was connected to a port configured as a trunk with a native VLAN mismatch.Wrong answer — click to see why▾
Why this is wrong here
Attributing the issue to a native VLAN mismatch overlooks the root election change and the BPDU Guard-triggered err-disable state.
✗The new switch has PortFast enabled on its uplinks.Wrong answer — click to see why▾
Why this is wrong here
This answer ignores the root election shift and the BPDU Guard events; PortFast misconfiguration alone would not cause these symptoms.
✗The BPDU Guard feature was globally enabled on all ports, including trunk ports.Wrong answer — click to see why▾
Why this is wrong here
This fails to account for the selective err-disable of only access ports and the concurrent root bridge change, which points to a targeted misconfiguration rather than a blanket global setting.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that BPDU Guard only applies to access ports or that it prevents root bridge changes, when in fact it reacts to any BPDU received on a PortFast port, regardless of the BPDU's source or priority.
Trap categories for this question
Scenario analysis trap
Global BPDU Guard would disable all ports that receive BPDUs, including trunk links, leading to widespread network failure. The scenario specifically mentions only ‘several access ports’ going err-disabled, and the root bridge change would also not be explained by this alone.
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 determined by the lowest bridge priority (default 32768). A new switch with priority set to 0 or 4096 will immediately become root. BPDU Guard is designed to protect PortFast-enabled access ports from receiving BPDUs, which would indicate an unauthorized switch; upon receiving a BPDU, the port is placed in err-disable state. This is a common security mechanism to prevent rogue switches from becoming root.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The new switch’s bridge priority is lower than the existing root bridge, and it was connected to a port with BPDU Guard enabled. — The new switch's bridge priority is lower (numerically smaller) than the existing root bridge, so it becomes the new root for VLAN 10. When it sends superior BPDUs out its uplinks, the neighboring switch's access ports with PortFast and BPDU Guard enabled receive these BPDUs, triggering err-disable state on those ports, causing connectivity loss.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This 200-301 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 200-301 exam.
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