CCNA Switching and Network Access Practice Question
Exhibit
SwitchA# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/2 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/2 10,20,30 SwitchB# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/2 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/2 10,20,30,50
Two switches are connected by a trunk. VLAN 50 exists on both switches, but hosts in VLAN 50 cannot communicate across the link. All other VLANs work. Based on the exhibit, what is the most likely cause?
⚠ Common exam trap
Ensure you differentiate between general trunk issues and VLAN-specific configurations. Don't confuse access port settings with trunk link issues.
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 50 is not allowed on the trunk from SwitchA.
VLAN 50 is missing from the allowed VLAN list on the trunk, which blocks only that VLAN while allowing others. Option B is incorrect: a native VLAN mismatch would affect the native VLAN (usually VLAN 1) or cause STP issues, not specifically VLAN 50. Option C is incorrect: 802.1Q encapsulation supports all VLANs from 1 to 4094, including VLAN 50. Option D is incorrect: VLANs do not need to be the native VLAN to be transported over a trunk; any VLAN can traverse a trunk if it is allowed.
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 50 is not allowed on the trunk from SwitchA.
Why this is correct
On a Cisco trunk, the 'switchport trunk allowed vlan' list on SwitchA determines which VLANs may traverse the link. Even though VLAN 50 is defined on both switches, if SwitchA's trunk interface does not explicitly include VLAN 50 in its allowed list, the switch will not forward VLAN 50 frames across the trunk. You must add VLAN 50 to the allowed list with 'switchport trunk allowed vlan add 50' on SwitchA's trunk port to restore connectivity.
- ✗
The native VLAN is mismatched.
Why it's wrong here
The two switches both have native VLAN 1 on their trunk interfaces, so the native VLAN is consistent and not the cause of the problem. A native VLAN mismatch would generate CDP messages and cause spanning-tree errors, but here both sides agree. Since VLAN 50 is not the native VLAN and both sides are set to VLAN 1, a mismatch cannot be the reason VLAN 50 traffic fails.
When this WOULD be correct
In a scenario where two switches are configured with different native VLANs and a question specifies that only one VLAN is affected, a mismatch in the native VLAN would cause communication issues for that specific VLAN while allowing others to function normally.
- ✗
802.1Q cannot carry VLAN 50.
Why it's wrong here
802.1Q is the IEEE trunking standard that inserts a 4-byte tag with a 12-bit VLAN ID, which supports any normal VLAN from 1 to 4094. VLAN 50 falls well within that range and is perfectly encapsulated and carried by 802.1Q. Therefore, the failure cannot be due to a protocol limitation of 802.1Q; the problem is specific to the allowed VLAN configuration on the trunk.
When this WOULD be correct
In a different scenario, if the question stated that the switch was using a proprietary trunking protocol that does not support VLAN 50 or has a limitation on the VLAN range, then option C would be correct, indicating a protocol limitation.
- ✗
VLAN 50 must be configured as the native VLAN first.
Why it's wrong here
The native VLAN is simply the VLAN that carries untagged frames on a trunk; every other VLAN, including VLAN 50, must be tagged when traversing an 802.1Q trunk. There is no requirement for a VLAN to be native in order to be carried across a trunk link. Forcing VLAN 50 to be native would change the tagging behavior but would not fix the root cause, which is its absence from SwitchA's allowed list.
When this WOULD be correct
In a different scenario where the question specifies that VLAN 50 must be the native VLAN for proper communication, this option would be correct. For example, if the question stated that all VLANs except the native VLAN are unable to communicate, then configuring VLAN 50 as the native VLAN would resolve the issue.
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 50 is not allowed on the trunk from SwitchA.Correct answer▾
Why this is correct
On a Cisco trunk, the 'switchport trunk allowed vlan' list on SwitchA determines which VLANs may traverse the link. Even though VLAN 50 is defined on both switches, if SwitchA's trunk interface does not explicitly include VLAN 50 in its allowed list, the switch will not forward VLAN 50 frames across the trunk. You must add VLAN 50 to the allowed list with 'switchport trunk allowed vlan add 50' on SwitchA's trunk port to restore connectivity.
✗The native VLAN is mismatched.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, not just VLAN 50. Since other VLANs are functioning correctly, it indicates that the native VLAN configuration is not the issue.
★ When this WOULD be the correct answer
In a scenario where two switches are configured with different native VLANs and a question specifies that only one VLAN is affected, a mismatch in the native VLAN would cause communication issues for that specific VLAN while allowing others to function normally.
Why candidates choose this
Candidates may choose this option due to a common misconception that native VLAN mismatches universally disrupt all VLAN traffic, leading them to overlook the specific context of the question.
✗802.1Q cannot carry VLAN 50.Wrong answer — click to see why▾
Why this is wrong here
802.1Q is a standard that can carry VLANs up to 4096, including VLAN 50. Therefore, the issue of VLAN 50 not communicating is not due to the protocol's inability to carry it.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that the switch was using a proprietary trunking protocol that does not support VLAN 50 or has a limitation on the VLAN range, then option C would be correct, indicating a protocol limitation.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of VLAN tagging protocols and their capabilities, leading them to incorrectly assume that certain VLANs cannot be carried over 802.1Q.
✗VLAN 50 must be configured as the native VLAN first.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because VLAN 50 can exist on a trunk without being the native VLAN; it does not need to be configured as such to communicate across the trunk link.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that VLAN 50 must be the native VLAN for proper communication, this option would be correct. For example, if the question stated that all VLANs except the native VLAN are unable to communicate, then configuring VLAN 50 as the native VLAN would resolve the issue.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of VLAN configurations and the role of native VLANs, thinking that all VLANs must be treated the same without recognizing the specific requirements for trunk links.
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
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
Encapsulation
Encapsulation is the process of wrapping data with protocol headers and trailers before sending it across a network.
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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.