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 trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.. 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 interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 10,20,40
SwitchB# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 10,20,30,40
PCs in VLAN 30 on SwitchA cannot reach PCs in VLAN 30 on SwitchB. VLAN 30 exists on both switches and all other VLANs work across the same link. Based on the exhibit, 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.
SwitchA# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 10,20,40
SwitchB# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 10,20,30,40
A
VLAN 30 is not allowed on the trunk from SwitchA.
This is correct because VLAN 30 is missing from SwitchA’s allowed list.
B
The native VLAN is mismatched.
Why wrong: This is wrong because both switches show native VLAN 1.
C
The trunk must use ISL instead of 802.1Q.
Why wrong: This is wrong because 802.1Q is already working for the other VLANs.
D
VLAN 30 must be configured as the native VLAN.
Why wrong: This is wrong because a VLAN does not have to be native to cross a trunk.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 30 is not allowed on the trunk from SwitchA.
The trunk is up, but VLAN 30 is missing from the allowed list on SwitchA. In plain language, the hallway between the switches is open, but one side is refusing to carry that specific VLAN through the hallway. Since the other VLANs are working, the failure is selective rather than total. That strongly points to an allowed-VLAN problem rather than a broader trunk outage.
This is a classic CCNA switching scenario because it tests whether you can separate trunk health from per-VLAN forwarding. A trunk can be operational and still block one VLAN if that VLAN is not permitted on one side. The native VLAN and encapsulation are not the issue shown here — the mismatch in the allowed list is.
Key principle: A trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
VLAN 30 is not allowed on the trunk from SwitchA.
Why this is correct
This is correct because VLAN 30 is missing from SwitchA’s allowed list.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.
✗
The native VLAN is mismatched.
Why it's wrong here
This is wrong because both switches show native VLAN 1.
When this WOULD be correct
In a scenario where a question specifies that PCs in VLAN 30 cannot communicate due to a native VLAN mismatch, and both switches are configured to use different native VLANs, this option would correctly identify the cause of the issue.
✗
The trunk must use ISL instead of 802.1Q.
Why it's wrong here
This is wrong because 802.1Q is already working for the other VLANs.
When this WOULD be correct
In a different scenario, if the question specified that the switches were configured to use ISL and that VLAN 30 was not configured to use ISL, then this option would be correct. For example, if the question stated that VLAN 30 was only configured for 802.1Q on both switches, this could lead to a failure in communication.
✗
VLAN 30 must be configured as the native VLAN.
Why it's wrong here
This is wrong because a VLAN does not have to be native to cross a trunk.
When this WOULD be correct
In a different scenario, if the question stated that PCs in VLAN 30 were unable to communicate due to untagged traffic being misrouted, then configuring VLAN 30 as the native VLAN could resolve the issue, making this option correct.
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.
✓VLAN 30 is not allowed on the trunk from SwitchA.Correct answer▾
Why this is correct
This is correct because VLAN 30 is missing from SwitchA’s allowed list.
✗The native VLAN is mismatched.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both switches have native VLAN 1 configured, so there is no mismatch. A native VLAN mismatch would cause all VLAN traffic to fail or be misdirected, not just a single VLAN.
★ When this WOULD be the correct answer
In a scenario where a question specifies that PCs in VLAN 30 cannot communicate due to a native VLAN mismatch, and both switches are configured to use different native VLANs, this option would correctly identify the cause of the issue.
Why candidates choose this
Students often confuse native VLAN issues with VLAN pruning or allowed lists, and may assume that a single VLAN problem is due to native VLAN mismatch, especially if they have seen such issues in labs.
✗The trunk must use ISL instead of 802.1Q.Wrong answer — click to see why▾
Why this is wrong here
Since other VLANs are working across the same trunk, the trunk encapsulation (802.1Q) is functioning correctly. Changing to ISL would not fix the issue and would break connectivity for all VLANs.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that the switches were configured to use ISL and that VLAN 30 was not configured to use ISL, then this option would be correct. For example, if the question stated that VLAN 30 was only configured for 802.1Q on both switches, this could lead to a failure in communication.
Why candidates choose this
Some test-takers might think that ISL is required for certain VLANs or that 802.1Q has limitations, but in modern networks 802.1Q is the standard and ISL is obsolete.
✗VLAN 30 must be configured as the native VLAN.Wrong answer — click to see why▾
Why this is wrong here
A VLAN does not need to be the native VLAN to traverse a trunk; native VLAN is only for untagged traffic. Making VLAN 30 the native VLAN would not solve the problem and could introduce other issues.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that PCs in VLAN 30 were unable to communicate due to untagged traffic being misrouted, then configuring VLAN 30 as the native VLAN could resolve the issue, making this option correct.
Why candidates choose this
Students may confuse the concept of native VLAN with the idea that a VLAN must be 'allowed' or 'special' to pass over a trunk, leading them to think that changing the native VLAN would help.
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
Be careful not to confuse general trunk issues with specific VLAN forwarding problems. Always check the allowed VLAN list when specific VLANs fail to pass.
Trap categories for this question
Command / output trap
This is wrong because both switches show native VLAN 1.
Detailed technical explanation
How to think about this question
VLAN trunks are essential in Cisco networks to carry multiple VLANs across a single physical link between switches. The trunk link uses tagging protocols like IEEE 802.1Q to identify VLAN frames so that traffic from different VLANs remains logically separated while traversing the same cable. Each switch must be configured to allow the VLANs that need to pass through the trunk. If a VLAN is not included in the allowed VLAN list on a trunk port, frames tagged with that VLAN ID will be dropped and not forwarded across the link.
In this scenario, the trunk between SwitchA and SwitchB is operational, and other VLANs are passing correctly, indicating that the physical and logical trunk configuration is mostly correct. However, VLAN 30 is missing from the allowed VLAN list on SwitchA’s trunk port, causing selective blocking of VLAN 30 traffic. Cisco switches enforce VLAN filtering on trunks based on the allowed VLAN list, so even if the VLAN exists on both switches, it must be explicitly permitted on the trunk to pass traffic.
A common exam trap is to confuse native VLAN mismatches or trunk encapsulation types as the cause of VLAN connectivity issues. While native VLAN mismatches can cause untagged traffic problems, they do not selectively block tagged VLANs. Similarly, 802.1Q is the standard trunking protocol and works fine if other VLANs pass. The practical takeaway is to always verify the allowed VLAN list on trunk ports when VLAN-specific connectivity issues arise, as this is a frequent cause of partial VLAN reachability failures in Cisco networks.
KKey Concepts to Remember
A trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.
If a VLAN is not included in the allowed VLAN list on a trunk port, traffic for that VLAN is blocked from crossing the trunk.
The native VLAN is used for untagged frames and does not affect the forwarding of tagged VLAN traffic on a trunk.
802.1Q is the standard trunk encapsulation protocol and supports multiple VLANs on a single link between Cisco switches.
VLANs must be configured and active on both ends of a trunk to enable communication between devices in the same VLAN.
A trunk link can be operational and carry some VLANs while selectively blocking others due to allowed VLAN filtering.
Exam scenarios often test the ability to distinguish between trunk operational status and per-VLAN forwarding issues.
Misconfiguring allowed VLAN lists on trunks is a common cause of VLAN connectivity problems in Cisco switching environments.
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 trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.
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 trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration., 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 trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration..
What is the correct answer to this question?
The correct answer is: VLAN 30 is not allowed on the trunk from SwitchA. — The trunk is up, but VLAN 30 is missing from the allowed list on SwitchA. In plain language, the hallway between the switches is open, but one side is refusing to carry that specific VLAN through the hallway. Since the other VLANs are working, the failure is selective rather than total. That strongly points to an allowed-VLAN problem rather than a broader trunk outage.
This is a classic CCNA switching scenario because it tests whether you can separate trunk health from per-VLAN forwarding. A trunk can be operational and still block one VLAN if that VLAN is not permitted on one side. The native VLAN and encapsulation are not the issue shown here — the mismatch in the allowed list is.
What should I do if I get this 200-301 question wrong?
Review a trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration., 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: "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?
A trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.
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