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: an 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.. 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.
Two switches are connected by an 802.1Q trunk. Hosts in VLAN 30 cannot communicate across the link, but VLAN 10 works. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 30 is missing from the allowed VLAN list on SW2
When one VLAN works across a trunk but another does not, the most likely cause is that the failing VLAN is missing from the allowed VLAN list on one or both switches. This is often confirmed by a 'show interfaces trunk' command. Option B is incorrect because removing VLAN 10 would break an already working VLAN and does not address VLAN 30. Option C is incorrect because both ISL and 802.1Q carry multiple VLANs; the issue is not the encapsulation protocol. Option D is incorrect because changing a trunk port to access mode would disable the trunk entirely, preventing all VLAN traffic.
Key principle: An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
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 missing from the allowed VLAN list on SW2
Why this is correct
SW2 is only allowing VLANs 10 and 20.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
✗
VLAN 10 must be removed for VLAN 30 to pass
Why it's wrong here
Multiple VLANs can be allowed on a trunk at the same time.
When this WOULD be correct
In a different scenario where the question states that only one VLAN can be active on the trunk due to a specific configuration limitation, removing VLAN 10 could be necessary to allow VLAN 30 to pass traffic. This could occur in a legacy system or a misconfigured switch environment.
✗
The trunk should use ISL instead of 802.1Q
Why it's wrong here
CCNA focuses on 802.1Q and this is not the issue shown.
When this WOULD be correct
In a scenario where a question specifies that a network is using only ISL for trunking and VLAN 30 is configured correctly on both switches, but communication still fails, the correct answer could be that ISL is required instead of 802.1Q due to compatibility issues with legacy equipment.
✗
The port on SW1 should be changed to access mode
Why it's wrong here
That would break trunking altogether.
When this WOULD be correct
In a different scenario where the question states that both VLANs are configured on the switches but VLAN 30 is not receiving any traffic, the correct answer could be that the port on SW1 needs to be set to access mode to allow VLAN 30 traffic to be processed correctly.
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 missing from the allowed VLAN list on SW2Correct answer▾
Why this is correct
SW2 is only allowing VLANs 10 and 20.
✗VLAN 10 must be removed for VLAN 30 to passWrong answer — click to see why▾
Why this is wrong here
Multiple VLANs can coexist on a trunk; removing VLAN 10 is not required for VLAN 30 to pass. The allowed VLAN list can include any combination of VLANs, and VLAN 10's presence does not block VLAN 30.
★ When this WOULD be the correct answer
In a different scenario where the question states that only one VLAN can be active on the trunk due to a specific configuration limitation, removing VLAN 10 could be necessary to allow VLAN 30 to pass traffic. This could occur in a legacy system or a misconfigured switch environment.
Why candidates choose this
A student might think that only one VLAN can be active at a time on a trunk, confusing the concept of native VLAN or misunderstanding that VLANs are independent.
✗The trunk should use ISL instead of 802.1QWrong answer — click to see why▾
Why this is wrong here
The problem is not related to the trunking protocol; 802.1Q is the industry standard and works correctly when configured properly. Changing to ISL would not resolve the issue if VLAN 30 is simply not allowed on the trunk.
★ When this WOULD be the correct answer
In a scenario where a question specifies that a network is using only ISL for trunking and VLAN 30 is configured correctly on both switches, but communication still fails, the correct answer could be that ISL is required instead of 802.1Q due to compatibility issues with legacy equipment.
Why candidates choose this
Some students may recall that ISL is an older Cisco proprietary trunking protocol and might think switching protocols could fix VLAN issues, but the root cause here is a missing VLAN in the allowed list.
✗The port on SW1 should be changed to access modeWrong answer — click to see why▾
Why this is wrong here
Changing the port to access mode would break the trunk entirely, preventing all VLAN traffic (including VLAN 10) from passing. The trunk is functioning for VLAN 10, so the issue is specific to VLAN 30, not the trunk mode.
★ When this WOULD be the correct answer
In a different scenario where the question states that both VLANs are configured on the switches but VLAN 30 is not receiving any traffic, the correct answer could be that the port on SW1 needs to be set to access mode to allow VLAN 30 traffic to be processed correctly.
Why candidates choose this
A student might think that if a VLAN is not working, the port should be in access mode for that VLAN, but that would isolate the port to a single VLAN and not solve the inter-switch communication problem.
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
Beware of confusing native VLAN issues with allowed VLAN list configurations. Native VLAN problems affect untagged traffic, not specific VLANs.
Trap categories for this question
Command / output trap
CCNA focuses on 802.1Q and this is not the issue shown.
Detailed technical explanation
How to think about this question
802.1Q trunking is a fundamental VLAN technology that allows multiple VLANs to share a single physical link between switches by tagging Ethernet frames with VLAN IDs. This tagging enables switches to segregate and forward traffic based on VLAN membership, maintaining logical separation of broadcast domains across the network. Each trunk port has a configuration that includes which VLANs are allowed to pass through it, ensuring only authorized VLAN traffic traverses the link.
The allowed VLAN list on a trunk port acts as a filter. Even if a VLAN exists on both switches, if it is not included in the allowed VLAN list on one side, frames tagged for that VLAN will be dropped and not forwarded across the trunk. This is why VLAN 30 traffic fails to communicate across the trunk while VLAN 10 works, indicating VLAN 30 is likely missing from the allowed VLAN list on SW2. The native VLAN is typically untagged and does not affect tagged VLAN traffic, so it is rarely the cause when some VLANs pass and others do not.
A common exam trap is to confuse trunking protocols or port modes. For example, suggesting ISL instead of 802.1Q is incorrect because CCNA focuses on 802.1Q as the standard. Similarly, changing a trunk port to access mode disables trunking and prevents multiple VLANs from passing, which is not the solution here. The practical behavior in real networks is to verify allowed VLAN lists on both ends of the trunk to ensure all required VLANs are permitted, which resolves communication issues for specific VLANs like VLAN 30 in this scenario.
KKey Concepts to Remember
An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
Switches use an allowed VLAN list on trunk ports to control which VLANs can traverse the trunk; VLANs not in this list are blocked from passing.
If a VLAN is missing from the allowed VLAN list on one switch, hosts in that VLAN cannot communicate across the trunk despite the physical link being up.
The native VLAN on an 802.1Q trunk is untagged and usually does not affect VLAN traffic that is tagged and allowed on the trunk.
Multiple VLANs can coexist on a single 802.1Q trunk link, so removing one VLAN does not enable another VLAN to pass traffic.
ISL is a Cisco proprietary trunking protocol and is not required or recommended in modern CCNA environments where 802.1Q is standard.
Access mode ports carry traffic for a single VLAN and do not tag frames; changing a trunk port to access mode disables trunking and blocks multiple VLANs.
Troubleshooting VLAN communication issues over trunks often involves verifying VLAN membership, trunk configuration, and allowed VLAN lists on both ends.
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
An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
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 an 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link., 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 — An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link..
What is the correct answer to this question?
The correct answer is: VLAN 30 is missing from the allowed VLAN list on SW2 — When one VLAN works across a trunk but another does not, the most likely cause is that the failing VLAN is missing from the allowed VLAN list on one or both switches. This is often confirmed by a 'show interfaces trunk' command. Option B is incorrect because removing VLAN 10 would break an already working VLAN and does not address VLAN 30. Option C is incorrect because both ISL and 802.1Q carry multiple VLANs; the issue is not the encapsulation protocol. Option D is incorrect because changing a trunk port to access mode would disable the trunk entirely, preventing all VLAN traffic.
What should I do if I get this 200-301 question wrong?
Review an 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link., 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?
An 802.1Q trunk link carries multiple VLANs by tagging Ethernet frames with VLAN identifiers to segregate traffic across the same physical link.
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