- A
The new switch is running PVST+ while the existing switches use Rapid PVST+
Why wrong: PVST+ and Rapid PVST+ interoperate using 802.1D BPDUs on native VLANs and per VLAN. Root election is still based on bridge priority and MAC address. A protocol mismatch does not cause a different switch with a higher MAC to become the root.
- B
Root Guard is enabled on the new switch’s uplink ports facing the existing root
Why wrong: Root Guard on a port causes the port to become root-inconsistent if superior BPDUs are received. If the new switch has a lower MAC and default priority, the existing root’s BPDUs would be inferior, not superior. Thus Root Guard would not be triggered, and even if it were, it would block the port on the new switch, not make another switch become the root.
- C
The existing root bridge has a bridge priority lower than the default value of 32768
If the existing root bridge’s priority is less than 32768 (e.g., 4096 or 0), its Bridge ID is lower than the new switch’s default 32768 + lower MAC. STP always elects the switch with the lowest Bridge ID as the root bridge. Thus, despite the new switch’s lower MAC, the manually lowered priority keeps the existing switch as root.
- D
The new switch was added with a bridge priority of 4096
Why wrong: A bridge priority of 4096 is much lower than the default 32768. If the new switch had a priority of 4096, it would become the root bridge, not a different switch. This contradicts the observed symptom where a different switch becomes root.
Quick Answer
The answer is that the existing root bridge has a bridge priority lower than the default value of 32768. In STP root election, bridge priority is the primary parameter, evaluated before MAC address; the switch with the lowest bridge priority wins, and only if priorities are equal does the lowest MAC address break the tie. This means a new switch with a default priority of 32768 and a lower MAC cannot unseat an existing root that has a priority of, say, 4096, because the existing switch’s Bridge ID is numerically smaller. On the CCNA 200-301 v2 exam, this concept tests your understanding that STP root election is not simply about the lowest MAC—a common trap where students assume a lower MAC always wins. Remember the mnemonic: “Priority first, MAC last—if the root’s priority is lower, the new switch won’t be faster.”
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 notices that after adding a new switch to the network, a different switch unexpectedly becomes the STP root bridge, disrupting all VLANs. The new switch has the default priority (32768) but has a lower MAC address than all existing switches. What is the most likely cause?
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 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 existing root bridge has a bridge priority lower than the default value of 32768
In STP, bridge priority is the primary parameter for root election. If the existing root bridge has a bridge priority lower than the default 32768, it will have a lower Bridge ID regardless of its MAC address, so it remains the root. The new switch’s lower MAC would only win if all bridge priorities are equal (default). This explains why a different switch becomes root even though the new one has a lower MAC.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 is running PVST+ while the existing switches use Rapid PVST+
Why it's wrong here
PVST+ and Rapid PVST+ interoperate using 802.1D BPDUs on native VLANs and per VLAN. Root election is still based on bridge priority and MAC address. A protocol mismatch does not cause a different switch with a higher MAC to become the root.
- ✗
Root Guard is enabled on the new switch’s uplink ports facing the existing root
Why it's wrong here
Root Guard on a port causes the port to become root-inconsistent if superior BPDUs are received. If the new switch has a lower MAC and default priority, the existing root’s BPDUs would be inferior, not superior. Thus Root Guard would not be triggered, and even if it were, it would block the port on the new switch, not make another switch become the root.
- ✓
The existing root bridge has a bridge priority lower than the default value of 32768
Why this is correct
If the existing root bridge’s priority is less than 32768 (e.g., 4096 or 0), its Bridge ID is lower than the new switch’s default 32768 + lower MAC. STP always elects the switch with the lowest Bridge ID as the root bridge. Thus, despite the new switch’s lower MAC, the manually lowered priority keeps the existing switch as root.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The new switch was added with a bridge priority of 4096
Why it's wrong here
A bridge priority of 4096 is much lower than the default 32768. If the new switch had a priority of 4096, it would become the root bridge, not a different switch. This contradicts the observed symptom where a different switch becomes root.
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 existing root bridge has a bridge priority lower than the default value of 32768Correct answer▾
Why this is correct
If the existing root bridge’s priority is less than 32768 (e.g., 4096 or 0), its Bridge ID is lower than the new switch’s default 32768 + lower MAC. STP always elects the switch with the lowest Bridge ID as the root bridge. Thus, despite the new switch’s lower MAC, the manually lowered priority keeps the existing switch as root.
✗The new switch is running PVST+ while the existing switches use Rapid PVST+Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that STP version incompatibility disrupts root election, but both versions use the same BPDU format and root election rules.
✗Root Guard is enabled on the new switch’s uplink ports facing the existing rootWrong answer — click to see why▾
Why this is wrong here
Candidates often associate Root Guard with preventing a switch from becoming the root. However, it does not cause another switch to become root; it just protects the network from unexpected superior BPDUs.
✗The new switch was added with a bridge priority of 4096Wrong answer — click to see why▾
Why this is wrong here
Candidates may confuse the symptom and think that a low priority on the new switch causes the problem, but this would make the new switch the root, not another switch.
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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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 — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The existing root bridge has a bridge priority lower than the default value of 32768 — In STP, bridge priority is the primary parameter for root election. If the existing root bridge has a bridge priority lower than the default 32768, it will have a lower Bridge ID regardless of its MAC address, so it remains the root. The new switch’s lower MAC would only win if all bridge priorities are equal (default). This explains why a different switch becomes root even though the new one has a lower MAC.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 14, 2026
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