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Question 794 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network administrator is troubleshooting…
This 200-301 practice question tests your understanding of 200-301 exam topics. 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.
Exhibit
SwitchA# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 32778
Address 0011.2233.4455
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0011.2233.4455
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1 Desg FWD 4 128.1 P2p
Gi0/2 Desg FWD 4 128.2 P2p
Gi0/3 Altn BLK 4 128.3 P2pA network administrator is troubleshooting connectivity issues in a switched network. Users on VLAN 10 report intermittent connectivity to the server farm. The network uses Rapid PVST+ as the spanning-tree protocol. The administrator examines the switch that is the root bridge for VLAN 10 and notices that one of the uplink interfaces to an access switch is in a blocking state. What is the most likely cause of this issue?
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
✓
Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.
The exhibit shows that SwitchA is the root bridge for VLAN 10, as indicated by 'This bridge is the root'. However, interface Gi0/3 is in an Alternate (Altn) blocking state. On a root bridge, all ports should be in the Designated forwarding state because the root bridge is the logical center of the spanning tree. An Alternate port on the root bridge indicates that another switch is sending superior BPDUs out of that interface, which is impossible unless there is a configuration issue such as a loop or a misconfigured priority. The most likely cause is that another switch in the network has a lower bridge priority for VLAN 10, causing it to be elected as the root bridge instead. This would result in some ports on the current root bridge transitioning to blocking states. The correct action is to check the spanning-tree configuration on all switches to ensure the desired root bridge has the lowest priority.
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.
- ✗
Change the port type of Gi0/3 to trunk to allow multiple VLANs.
Why it's wrong here
Changing the port type does not affect spanning-tree root bridge election or port roles.
- ✗
Configure spanning-tree portfast on Gi0/3 to speed up convergence.
Why it's wrong here
Portfast is used for access ports connected to end devices, not for uplinks. It would not resolve the blocking state.
- ✓
Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.
Why this is correct
The root bridge is elected based on the lowest bridge priority. If another switch has a lower priority, it becomes the root, causing ports on the current root to block. Verifying and adjusting priorities will ensure the correct root bridge election.
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.
- ✗
Enable BPDU guard on Gi0/3 to prevent unauthorized switches from affecting the network.
Why it's wrong here
BPDU guard is used on access ports to protect against rogue switches. It does not affect root bridge election or port roles on uplinks.
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.
✓Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10.Correct answer▾
Why this is correct
The root bridge is elected based on the lowest bridge priority. If another switch has a lower priority, it becomes the root, causing ports on the current root to block. Verifying and adjusting priorities will ensure the correct root bridge election.
✗Change the port type of Gi0/3 to trunk to allow multiple VLANs.Wrong answer — click to see why▾
Why this is wrong here
The issue is with spanning-tree root bridge election, not with VLAN trunking.
✗Configure spanning-tree portfast on Gi0/3 to speed up convergence.Wrong answer — click to see why▾
Why this is wrong here
Portfast does not change the port role or state determined by spanning-tree algorithm.
✗Enable BPDU guard on Gi0/3 to prevent unauthorized switches from affecting the network.Wrong answer — click to see why▾
Why this is wrong here
BPDU guard would not resolve the existing port role issue; it is a preventive measure for edge ports.
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?
Access ports place end devices into a single VLAN.
What is the correct answer to this question?
The correct answer is: Check the spanning-tree priority on other switches to ensure the intended root bridge has the lowest priority for VLAN 10. — The exhibit shows that SwitchA is the root bridge for VLAN 10, as indicated by 'This bridge is the root'. However, interface Gi0/3 is in an Alternate (Altn) blocking state. On a root bridge, all ports should be in the Designated forwarding state because the root bridge is the logical center of the spanning tree. An Alternate port on the root bridge indicates that another switch is sending superior BPDUs out of that interface, which is impossible unless there is a configuration issue such as a loop or a misconfigured priority. The most likely cause is that another switch in the network has a lower bridge priority for VLAN 10, causing it to be elected as the root bridge instead. This would result in some ports on the current root bridge transitioning to blocking states. The correct action is to check the spanning-tree configuration on all switches to ensure the desired root bridge has the lowest priority.
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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