CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: a VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.. 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 trunk is up between two switches, but traffic for VLAN 40 fails while other VLANs work. Which output item should be checked first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Whether VLAN 40 is included in the allowed VLAN list on the trunk
The first thing to check is whether VLAN 40 appears in the trunk’s allowed VLAN list. In practical terms, this is a selective failure, not a total trunk failure. Since other VLANs are crossing successfully, the link is operational. That strongly suggests one VLAN is being excluded rather than the trunk being generally broken.
This is one of the most common VLAN troubleshooting patterns in switching.
Key principle: A VLANtrunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Whether VLAN 40 is included in the allowed VLAN list on the trunk
Why this is correct
This is correct because a missing VLAN in the allowed list is a common cause of one-VLAN failure on an otherwise working trunk.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
A VLANtrunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
✗
Whether the router ID matches on both switches
Why it's wrong here
This is wrong because switches do not need matching router IDs for VLAN trunking.
When this WOULD be correct
In a question about OSPF or EIGRP configuration, where the focus is on ensuring proper routing between switches, checking the router ID would be crucial to confirm that both switches are configured to recognize each other in the routing domain.
✗
Whether NetFlow is enabled on the VLAN
Why it's wrong here
This is wrong because NetFlow is unrelated to a VLAN being permitted on a trunk.
When this WOULD be correct
In a question where the focus is on monitoring and analyzing traffic flows within a network, a scenario might ask about verifying configurations related to traffic analysis. If the question involved ensuring that traffic data is being collected for VLAN 40, then checking if NetFlow is enabled would be relevant.
✗
Whether NTP is synchronized on the switches
Why it's wrong here
This is wrong because time synchronization does not determine VLANtrunk forwarding.
When this WOULD be correct
In a different scenario where the question asks about the overall stability and performance of network services that rely on accurate timekeeping, such as logging or security protocols, verifying NTP synchronization could be crucial. For instance, if the question involved troubleshooting time-sensitive applications, this option could be relevant.
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.
✓Whether VLAN 40 is included in the allowed VLAN list on the trunkCorrect answer▾
Why this is correct
This is correct because a missing VLAN in the allowed list is a common cause of one-VLAN failure on an otherwise working trunk.
✗Whether the router ID matches on both switchesWrong answer — click to see why▾
Why this is wrong here
This option is wrong because the router ID is relevant for routing protocols, not for VLAN traffic issues on a trunk link. The problem specifically pertains to VLAN 40 traffic, which is not influenced by router IDs.
★ When this WOULD be the correct answer
In a question about OSPF or EIGRP configuration, where the focus is on ensuring proper routing between switches, checking the router ID would be crucial to confirm that both switches are configured to recognize each other in the routing domain.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of the relationship between VLANs and routing protocols, mistakenly believing that routing configuration issues could affect VLAN traffic.
✗Whether NetFlow is enabled on the VLANWrong answer — click to see why▾
Why this is wrong here
NetFlow is a network protocol used for collecting IP traffic information, but it does not directly affect VLAN traffic flow on a trunk link. Therefore, checking if NetFlow is enabled does not address the issue of VLAN 40 traffic failure.
★ When this WOULD be the correct answer
In a question where the focus is on monitoring and analyzing traffic flows within a network, a scenario might ask about verifying configurations related to traffic analysis. If the question involved ensuring that traffic data is being collected for VLAN 40, then checking if NetFlow is enabled would be relevant.
Why candidates choose this
Candidates may confuse traffic analysis tools like NetFlow with VLAN configurations, thinking that monitoring settings could impact VLAN traffic, leading them to incorrectly select this option.
✗Whether NTP is synchronized on the switchesWrong answer — click to see why▾
Why this is wrong here
NTP synchronization is not directly related to VLAN traffic issues on a trunk link; it primarily affects time-sensitive protocols. Therefore, checking NTP synchronization would not help diagnose why VLAN 40 traffic is failing.
★ When this WOULD be the correct answer
In a different scenario where the question asks about the overall stability and performance of network services that rely on accurate timekeeping, such as logging or security protocols, verifying NTP synchronization could be crucial. For instance, if the question involved troubleshooting time-sensitive applications, this option could be relevant.
Why candidates choose this
Candidates might choose this option due to a general understanding that synchronization issues can lead to various network problems, leading them to mistakenly believe it could impact VLAN traffic.
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
Avoid assuming a total trunk failure when only one VLAN is affected. Focus on VLAN-specific configurations.
Detailed technical explanation
How to think about this question
A VLAN trunk is a point-to-point link between switches that carries multiple VLANs simultaneously using tagging protocols like IEEE 802.1Q. The trunk encapsulates Ethernet frames with VLAN tags to distinguish traffic belonging to different VLANs across the same physical link. This allows switches to maintain VLAN separation while sharing a single physical connection, which is essential for scalable network design and segmentation.
When troubleshooting a trunk where some VLANs pass traffic and others do not, the first step is to verify the allowed VLAN list configured on the trunk port. Cisco switches permit administrators to restrict which VLANs are allowed to traverse a trunk link using the "switchport trunk allowed vlan" command. If a VLAN is missing from this list, traffic for that VLAN will be dropped even though the trunk link itself is operational. This selective filtering explains why only VLAN 40 fails while others succeed.
A common exam trap is to confuse trunk operational status with VLAN membership issues. Seeing the trunk as "up" might mislead candidates to check unrelated settings like router IDs, NetFlow, or NTP synchronization, which do not affect VLAN forwarding on trunks. In practice, the allowed VLAN list is a frequent cause of partial VLAN failures on trunks, and understanding this helps avoid wasting time on irrelevant configurations.
KKey Concepts to Remember
A VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
Cisco switches allow administrators to restrict VLANs on a trunk using the allowed VLAN list, which controls which VLAN traffic is forwarded across the trunk.
If a VLAN is missing from the trunk's allowed VLAN list, traffic for that VLAN will fail even though the trunk link remains operational for other VLANs.
The operational status of a trunk interface does not guarantee all VLANs are permitted; selective VLAN filtering can cause partial traffic failures.
Troubleshooting VLAN traffic issues on trunks requires checking VLAN membership and allowed VLAN lists before investigating unrelated parameters like router IDs or NTP.
The "show interfaces trunk" command displays which VLANs are allowed and active on a trunk, helping identify VLAN filtering problems quickly.
A missing VLAN in the allowed VLAN list is a common cause of selective VLAN failure on trunks and is a frequent exam scenario in CCNA VLAN troubleshooting.
Understanding the difference between trunk link status and VLAN membership is essential to avoid misdiagnosing VLAN traffic failures in Cisco networks.
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
A VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
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 200-301 question in full detail.
Review a VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — A VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation..
What is the correct answer to this question?
The correct answer is: Whether VLAN 40 is included in the allowed VLAN list on the trunk — The first thing to check is whether VLAN 40 appears in the trunk’s allowed VLAN list. In practical terms, this is a selective failure, not a total trunk failure. Since other VLANs are crossing successfully, the link is operational. That strongly suggests one VLAN is being excluded rather than the trunk being generally broken.
This is one of the most common VLAN troubleshooting patterns in switching.
What should I do if I get this 200-301 question wrong?
Review a VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation., then practise related 200-301 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
A VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
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