CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
Be cautious of assuming native VLAN mismatches or protocol issues when the problem is specific to the allowed VLAN list.
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 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.
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 switch outputs show the native VLAN as VLAN 1, so there is no native VLAN mismatch demonstrated in the exhibit. While a native VLAN mismatch can cause unexpected behavior, it would not prevent VLAN 30 from crossing the trunk because VLAN 30 would be tagged with 802.1Q, not carried as untagged native traffic. Even if the native VLANs were mismatched, the error message or symptoms would usually implicate VLAN 1, not VLAN 30.
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 the native VLAN on an 802.1Q trunk is used for untagged traffic and does not need to be VLAN 30 for that VLAN to traverse the trunk. VLAN 30 can be carried normally as a tagged VLAN as long as it is included in the allowed list on both ends; moving it to the native VLAN would actually make it untagged, which is unnecessary and could create other issues. The correct fix is to add VLAN 30 to the allowed list on SwitchA's trunk, not to change the native VLAN.
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 functioning correctly for the other VLANs listed in the output, proving that the trunk encapsulation is not the cause of the problem. ISL is a Cisco-proprietary encapsulation that is largely obsolete and incompatible with 802.1Q; replacing 802.1Q with ISL would not help because both switches would need to support and agree on the same encapsulation, and the exhibit shows 802.1Q is already operating. The issue is specifically that VLAN 30 is not included in the allowed list, not the trunking protocol.
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 explicitly shows that VLAN 30 is absent from the allowed VLAN list on the trunk interface on SwitchA. For 802.1Q trunking, a VLAN must be permitted on both sides of the link; if it is missing from SwitchA's allowed list, that switch will not forward frames for VLAN 30 onto the trunk, breaking connectivity to the servers. The other VLANs are allowed and therefore work, isolating the issue to the missing VLAN 30 entry.
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 explicitly shows that VLAN 30 is absent from the allowed VLAN list on the trunk interface on SwitchA. For 802.1Q trunking, a VLAN must be permitted on both sides of the link; if it is missing from SwitchA's allowed list, that switch will not forward frames for VLAN 30 onto the trunk, breaking connectivity to the servers. The other VLANs are allowed and therefore work, isolating the issue to the missing VLAN 30 entry.
✗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?”
Visual reference
Go deeper
Related to this question
Learn chapter
Interpreting Packet Capture Output for Layer 2/3 Troubleshooting
Key term
Native VLAN
A native VLAN is the default VLAN assigned to a trunk port that carries untagged traffic for backwards compatibility with devices that do not understand VLAN tagging.
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
About these practice questions
Courseiva writes every 200-301 question from scratch — 1,389 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.