- A
The native VLAN is mismatched on the two ends of the trunk.
Why wrong: Native VLAN mismatch primarily causes CDP and STP warnings and can impair traffic on the native VLAN, but tagged VLANs such as 20 use the 802.1Q tag and are forwarded normally across the trunk. So this would not prevent VLAN 20 communication.
- B
VLAN 20 has not been created in the VLAN database on SW2.
Why wrong: If VLAN 20 were missing from SW2's database, SW2 could not forward frames for that VLAN, and any access ports assigned to VLAN 20 would be down or non-functional. The stem explicitly states that hosts in VLAN 20 on SW2 can communicate with each other locally, confirming the VLAN exists.
- C
The trunk encapsulation is mismatched between SW1 and SW2.
Why wrong: An encapsulation mismatch (e.g., dot1q on one side, ISL on the other) prevents the trunk from coming up. The trunk in the scenario is up/up, so encapsulation is not mismatched.
- D
VLAN 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.
When a trunk port’s allowed VLAN list explicitly excludes a VLAN, the switch drops all frames tagged for that VLAN, even though the VLAN exists locally and the trunk is active. This results in the described symptom of local intra-VLAN communication working but no cross-switch communication for VLAN 20.
Quick Answer
The answer is that VLAN 20 is missing from the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2. This is the most likely cause because, even though the trunk link is up/up and VLAN 20 exists in both VLAN databases with spanning tree in a forwarding state, the switchport trunk allowed vlan command explicitly filters which VLANs can traverse the trunk. If VLAN 20 is omitted, frames from that VLAN are dropped at the trunk interface, preventing inter-switch communication while local connectivity within VLAN 20 on each switch remains intact. On the CCNA 200-301 v2 exam, this scenario tests your understanding of trunk configuration and the default behavior where all VLANs are allowed unless restricted; a common trap is assuming a working trunk means all VLANs pass traffic. A useful memory tip is: “Allowed list is a gate—if a VLAN isn’t on the list, it won’t make it through the gate.”
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. 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.
A technician is troubleshooting a network issue where hosts in VLAN 20 on SW1 cannot communicate with hosts in VLAN 20 on SW2. Both switches are connected by an Ethernet trunk link that is up/up and configured as a trunk. The VLAN databases on both switches include VLAN 20, and the spanning tree for VLAN 20 is in a forwarding state on all ports. Hosts within VLAN 20 on each switch can communicate with each other locally. 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 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
VLAN 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.
The most likely cause is that VLAN 20 is not included in the allowed VLAN list on the trunk port between SW1 and SW2. Even though the trunk is up/up and VLAN 20 exists in the VLAN database, the switchport trunk allowed vlan command restricts which VLANs can traverse the trunk. If VLAN 20 is omitted from this list, frames from VLAN 20 will be dropped at the trunk, preventing inter-switch communication for that VLAN.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 on the two ends of the trunk.
Why it's wrong here
Native VLAN mismatch primarily causes CDP and STP warnings and can impair traffic on the native VLAN, but tagged VLANs such as 20 use the 802.1Q tag and are forwarded normally across the trunk. So this would not prevent VLAN 20 communication.
- ✗
VLAN 20 has not been created in the VLAN database on SW2.
Why it's wrong here
If VLAN 20 were missing from SW2's database, SW2 could not forward frames for that VLAN, and any access ports assigned to VLAN 20 would be down or non-functional. The stem explicitly states that hosts in VLAN 20 on SW2 can communicate with each other locally, confirming the VLAN exists.
- ✗
The trunk encapsulation is mismatched between SW1 and SW2.
Why it's wrong here
An encapsulation mismatch (e.g., dot1q on one side, ISL on the other) prevents the trunk from coming up. The trunk in the scenario is up/up, so encapsulation is not mismatched.
- ✓
VLAN 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.
Why this is correct
When a trunk port’s allowed VLAN list explicitly excludes a VLAN, the switch drops all frames tagged for that VLAN, even though the VLAN exists locally and the trunk is active. This results in the described symptom of local intra-VLAN communication working but no cross-switch communication for VLAN 20.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
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 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.Correct answer▾
Why this is correct
When a trunk port’s allowed VLAN list explicitly excludes a VLAN, the switch drops all frames tagged for that VLAN, even though the VLAN exists locally and the trunk is active. This results in the described symptom of local intra-VLAN communication working but no cross-switch communication for VLAN 20.
✗The native VLAN is mismatched on the two ends of the trunk.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that a native VLAN mismatch breaks all trunk functions.
✗VLAN 20 has not been created in the VLAN database on SW2.Wrong answer — click to see why▾
Why this is wrong here
Candidates may assume that a missing VLAN on one switch explains inter-switch failures, ignoring that local communication would also fail.
✗The trunk encapsulation is mismatched between SW1 and SW2.Wrong answer — click to see why▾
Why this is wrong here
Candidates might overlook that the trunk link is operational, which implies matching encapsulation.
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
Cisco often tests the distinction between VLAN existence in the database and VLAN permission on a trunk; candidates mistakenly think that if a VLAN is created and spanning tree is forwarding, it must work, but the trunk allowed list is an independent filter that can block traffic.
Trap categories for this question
Scenario analysis trap
An encapsulation mismatch (e.g., dot1q on one side, ISL on the other) prevents the trunk from coming up. The trunk in the scenario is up/up, so encapsulation is not mismatched.
Detailed technical explanation
How to think about this question
The switchport trunk allowed vlan command controls which VLANs are permitted to use the trunk link. By default, all VLANs (1-4094) are allowed, but an administrator can restrict this list. If VLAN 20 is not explicitly allowed, the switch will drop frames tagged with VLAN 20 at the egress port. This is a common misconfiguration when manually pruning VLANs from a trunk, and it can be verified with the 'show interfaces trunk' command, which displays the allowed VLAN list.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VLAN 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2. — The most likely cause is that VLAN 20 is not included in the allowed VLAN list on the trunk port between SW1 and SW2. Even though the trunk is up/up and VLAN 20 exists in the VLAN database, the switchport trunk allowed vlan command restricts which VLANs can traverse the trunk. If VLAN 20 is omitted from this list, frames from VLAN 20 will be dropped at the trunk, preventing inter-switch communication for that VLAN.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
3 more ways this is tested on 200-301
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Two switches, SW1 and SW2, are connected via a trunk link. Hosts in VLAN 50 on SW1 cannot communicate with hosts in VLAN 50 on SW2, while hosts in other VLANs communicate normally. What is the most likely cause?
hard- ✓ A.VLAN 50 is not allowed on the trunk from SW1.
- B.The native VLAN must be changed to 50 on both switches.
- C.The trunk must be changed to an access port.
- D.The switches must run PPP on the uplink.
Why A: The strongest explanation is that VLAN 50 is missing from the allowed VLAN list on one side of the trunk. In practical terms, the trunk is up and carrying other VLANs, so the problem is selective rather than total. When one VLAN is omitted from the allowed list, only that VLAN fails while others continue to work normally. This is a high-value switching troubleshooting pattern because it rewards careful reading of operational output rather than generic trunk theory.
Variation 2. Users in VLAN 60 on switch SW2 cannot reach the default gateway located on switch SW1. The trunk between SW1 and SW2 is operational and allows VLAN 60. What is the most likely reason for this issue?
hard- ✓ A.VLAN 60 does not exist locally on SW2.
- B.The native VLAN must be changed to 60 on both switches.
- C.VLAN 60 is not allowed on the trunk link.
- D.The default gateway must be configured as a loopback on SW2.
Why A: VLAN 60 has not been created locally on SW2, even though the trunk can carry its traffic. A switch never processes VLAN traffic for a VLAN it doesn't know about; it discards tagged frames from the trunk destined for that VLAN and prevents access ports from assigning frames to it. (A) is correct. (B) is incorrect because native VLAN configuration only affects untagged frames—changing it to 60 is unnecessary for tagged VLAN 60 traffic. (C) is incorrect because the trunk is already configured to allow VLAN 60, so trunk filtering isn't the problem. (D) is incorrect because a default gateway is simply an IP address on a router or Layer 3 switch interface (like SVIs) and does not require a loopback on SW2.
Variation 3. 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?
hard- ✓ A.VLAN 30 is not allowed on the trunk from SwitchA.
- B.The native VLAN is mismatched.
- C.The trunk must use ISL instead of 802.1Q.
- D.VLAN 30 must be configured as the native VLAN.
Why A: 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.
Last reviewed: Jun 25, 2026
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