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 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.
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 servers in VLAN 30 on SwitchB. All other VLANs work across the trunk. 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
The native VLAN is mismatched between the switches.
Why wrong: This is wrong because both outputs show native VLAN 1. A native VLAN mismatch is not shown here.
B
VLAN 30 should be configured as the native VLAN on both ends.
Why wrong: This is wrong because VLAN 30 does not need to become the native VLAN in order to traverse the trunk.
C
SwitchB must use ISL instead of 802.1Q.
Why wrong: This is wrong because 802.1Q is already working for the other VLANs. The issue is not encapsulation type.
D
VLAN 30 is not allowed on the trunk from SwitchA.
This is correct because the exhibit clearly shows VLAN 30 missing from the allowed list on SwitchA. A VLAN must be permitted across the trunk on both sides for end-to-end communication.
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 and carrying multiple VLANs, but VLAN 30 is missing from the allowed list on one side. Since SwitchA and SwitchB both need to permit VLAN 30 on the trunk, if SwitchA's allowed VLAN list does not include VLAN 30, traffic for that VLAN cannot cross. Native VLAN mismatch or ISL vs 802.1Q issues would affect all VLANs, not just VLAN 30. Thus, the most likely cause is that VLAN 30 is not allowed on the trunk from SwitchA.
Key principle: A VLANtrunk carries multiple VLANs over a single physical link by tagging 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.
✗
The native VLAN is mismatched between the switches.
Why it's wrong here
This is wrong because both outputs show native VLAN 1. A native VLAN mismatch is not shown here.
When this WOULD be correct
In a different question where multiple VLANs are configured on a trunk, and the question specifies that the native VLAN is set differently on each switch, a native VLAN mismatch could cause communication failures across all VLANs, making this option correct.
✗
VLAN 30 should be configured as the native VLAN on both ends.
Why it's wrong here
This is wrong because VLAN 30 does not need to become the native VLAN in order to traverse the trunk.
When this WOULD be correct
In a different scenario where the question specifies that VLAN 30 is intended to be the native VLAN for both switches, and the native VLAN configuration is incorrect, this option would be correct. For example, if the question states that VLAN 30 is the native VLAN but it is not configured as such on one of the switches, it would lead to communication issues.
✗
SwitchB 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. The issue is not encapsulation type.
When this WOULD be correct
If the question specified that VLAN 30 was configured on both switches but that the trunk link was limited to only supporting ISL encapsulation, then stating that SwitchB must use ISL instead of 802.1Q would be correct. This would imply a compatibility issue between the two encapsulation methods.
✓
VLAN 30 is not allowed on the trunk from SwitchA.
Why this is correct
This is correct because the exhibit clearly shows VLAN 30 missing from the allowed list on SwitchA. A VLAN must be permitted across the trunk on both sides for end-to-end communication.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A VLANtrunk carries multiple VLANs over a single physical link by tagging frames with VLAN identifiers using 802.1Q encapsulation.
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 the exhibit clearly shows VLAN 30 missing from the allowed list on SwitchA. A VLAN must be permitted across the trunk on both sides for end-to-end communication.
✗The native VLAN is mismatched between the switches.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because a native VLAN mismatch would typically affect all VLANs across the trunk, not just VLAN 30. In this scenario, only VLAN 30 is experiencing connectivity issues, indicating a different problem.
★ When this WOULD be the correct answer
In a different question where multiple VLANs are configured on a trunk, and the question specifies that the native VLAN is set differently on each switch, a native VLAN mismatch could cause communication failures across all VLANs, making this option correct.
Why candidates choose this
Candidates might choose this option due to a common misconception that native VLAN mismatches are a frequent cause of VLAN connectivity issues, leading them to overlook the specific details of the question.
✗VLAN 30 should be configured as the native VLAN on both ends.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because configuring VLAN 30 as the native VLAN does not directly affect the ability of PCs in VLAN 30 on SwitchA to communicate with servers in VLAN 30 on SwitchB, especially if VLAN 30 is already properly configured on both switches.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that VLAN 30 is intended to be the native VLAN for both switches, and the native VLAN configuration is incorrect, this option would be correct. For example, if the question states that VLAN 30 is the native VLAN but it is not configured as such on one of the switches, it would lead to communication issues.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of native VLANs and their role in trunking, leading them to believe that any VLAN must be set as native to function correctly across trunks.
✗SwitchB must use ISL instead of 802.1Q.Wrong answer — click to see why▾
Why this is wrong here
SwitchB using ISL instead of 802.1Q is not relevant to the connectivity issue between VLAN 30 on SwitchA and SwitchB. The problem is likely due to VLAN 30 not being allowed on the trunk, not the encapsulation method used.
★ When this WOULD be the correct answer
If the question specified that VLAN 30 was configured on both switches but that the trunk link was limited to only supporting ISL encapsulation, then stating that SwitchB must use ISL instead of 802.1Q would be correct. This would imply a compatibility issue between the two encapsulation methods.
Why candidates choose this
Candidates may choose this option due to a common misconception that encapsulation types directly affect VLAN communication, leading them to overlook the specific trunk configuration issue presented in the question.
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 cautious of assuming native VLAN mismatches or protocol issues when the problem is specific to the allowed VLAN list.
Trap categories for this question
Command / output trap
This is wrong because both outputs show native VLAN 1. A native VLAN mismatch is not shown here.
Detailed technical explanation
How to think about this question
VLAN trunking is a fundamental concept in Cisco networking that allows multiple VLANs to be carried over a single physical link between switches. The most common trunking protocol used in CCNA environments is IEEE 802.1Q, which tags Ethernet frames with VLAN identifiers to maintain VLAN separation across the trunk. For end-to-end VLAN communication, both switches must agree on which VLANs are allowed to traverse the trunk link. If a VLAN is not permitted on the trunk, frames tagged with that VLAN ID will be dropped, preventing communication between devices in that VLAN across switches.
When configuring trunks, Cisco switches use the "allowed VLAN" list to control which VLANs can pass through the trunk port. This list must be consistent on both ends of the trunk link to ensure VLAN traffic flows correctly. A mismatch where one switch allows a VLAN and the other does not will cause connectivity issues for that VLAN. The native VLAN setting, which is untagged on 802.1Q trunks, must also match but does not affect VLANs that are explicitly tagged. Therefore, missing VLAN 30 from the allowed VLAN list on SwitchA blocks VLAN 30 traffic, even though other VLANs work fine.
A common exam trap is to focus on native VLAN mismatches or encapsulation types like ISL versus 802.1Q, but these are not the cause when other VLANs traverse the trunk successfully. The practical impact is that VLAN 30 traffic is effectively filtered out by SwitchA’s trunk configuration, causing devices in VLAN 30 on SwitchA to be unable to reach VLAN 30 devices on SwitchB. Correcting the allowed VLAN list to include VLAN 30 on both switches restores proper VLAN communication across the trunk.
KKey Concepts to Remember
A VLAN trunk carries multiple VLANs over a single physical link by tagging frames with VLAN identifiers using 802.1Q encapsulation.
Both ends of a trunk link must have matching allowed VLAN lists to permit VLAN traffic to pass between switches.
If a VLAN is missing from the allowed VLAN list on one switch, traffic for that VLAN is blocked on the trunk, preventing inter-switch communication.
The native VLAN is the untagged VLAN on an 802.1Q trunk and must match on both ends, but it does not affect tagged VLAN traffic.
802.1Q is the standard trunking protocol in Cisco switches and does not require switching to ISL unless specifically needed.
A native VLAN mismatch typically causes broader connectivity issues and is unlikely if other VLANs are passing through the trunk successfully.
The show interfaces trunk command displays the allowed VLANs and native VLAN, aiding in troubleshooting VLAN trunk issues.
Consistent trunk configuration on both switches is essential to maintain VLAN segmentation and ensure proper inter-VLAN communication.
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 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 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 frames with VLAN identifiers using 802.1Q encapsulation..
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 and carrying multiple VLANs, but VLAN 30 is missing from the allowed list on one side. Since SwitchA and SwitchB both need to permit VLAN 30 on the trunk, if SwitchA's allowed VLAN list does not include VLAN 30, traffic for that VLAN cannot cross. Native VLAN mismatch or ISL vs 802.1Q issues would affect all VLANs, not just VLAN 30. Thus, the most likely cause is that VLAN 30 is not allowed on the trunk from SwitchA.
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 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: "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 VLAN trunk carries multiple VLANs over a single physical link by tagging frames with VLAN identifiers using 802.1Q encapsulation.
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