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 trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.. 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
SW1# 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
SW2# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 10,20,30,40,50
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?
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.
SW1# 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
SW2# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/1 10,20,30,40,50
A
VLAN 50 is not allowed on the trunk from SW1.
This is correct because VLAN 50 is missing from the allowed list on SW1.
B
The native VLAN must be changed to 50 on both switches.
Why wrong: This is wrong because the problem is the allowed list, not native VLAN selection.
C
The trunk must be changed to an access port.
Why wrong: This is wrong because the link is intended to carry multiple VLANs.
D
The switches must run PPP on the uplink.
Why wrong: This is wrong because PPP is unrelated to Ethernet trunking.
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 SW1.
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.
Key principle: A trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
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 SW1.
Why this is correct
This is correct because VLAN 50 is missing from the allowed list on SW1.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
✗
The native VLAN must be changed to 50 on both switches.
Why it's wrong here
This is wrong because the problem is the allowed list, not native VLAN selection.
When this WOULD be correct
In a different scenario where the question specifies that both switches are configured with VLAN 50 as the native VLAN and that there is a requirement for untagged traffic to be sent on VLAN 50, this option would be correct. The question could ask about the implications of having mismatched native VLANs causing connectivity issues.
✗
The trunk must be changed to an access port.
Why it's wrong here
This is wrong because the link is intended to carry multiple VLANs.
When this WOULD be correct
In a different scenario where the question specifies that a switch is incorrectly configured to use an access port for a connection that should carry multiple VLANs, changing the trunk to an access port could be the correct answer if the goal is to isolate traffic to a single VLAN.
✗
The switches must run PPP on the uplink.
Why it's wrong here
This is wrong because PPP is unrelated to Ethernet trunking.
When this WOULD be correct
In a different scenario where the question specifies that the switches are connected via a serial link requiring PPP for communication, and the configuration is incorrectly set to Ethernet, this option would be correct. The question might ask about the necessary protocol for inter-switch communication in a non-Ethernet environment.
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 SW1.Correct answer▾
Why this is correct
This is correct because VLAN 50 is missing from the allowed list on SW1.
✗The native VLAN must be changed to 50 on both switches.Wrong answer — click to see why▾
Why this is wrong here
The native VLAN is used for untagged traffic on a trunk and does not affect the forwarding of tagged frames for other VLANs. Changing the native VLAN to 50 would not resolve the issue because VLAN 50 frames are still tagged and require inclusion in the allowed VLAN list.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that both switches are configured with VLAN 50 as the native VLAN and that there is a requirement for untagged traffic to be sent on VLAN 50, this option would be correct. The question could ask about the implications of having mismatched native VLANs causing connectivity issues.
Why candidates choose this
Students often confuse the native VLAN with the concept of a 'management VLAN' or think that setting the native VLAN to the desired VLAN will automatically allow that VLAN across the trunk, but the native VLAN only affects untagged frames.
✗The trunk must be changed to an access port.Wrong answer — click to see why▾
Why this is wrong here
An access port can only carry a single VLAN, so changing the trunk to an access port would prevent the link from carrying multiple VLANs, including VLAN 50. The link is intended to carry multiple VLANs, so a trunk is required.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that a switch is incorrectly configured to use an access port for a connection that should carry multiple VLANs, changing the trunk to an access port could be the correct answer if the goal is to isolate traffic to a single VLAN.
Why candidates choose this
Some test-takers might think that if a specific VLAN is not working, converting the link to an access port and assigning it to VLAN 50 could solve the problem, but this would break connectivity for other VLANs and is not a proper solution for inter-switch VLAN communication.
✗The switches must run PPP on the uplink.Wrong answer — click to see why▾
Why this is wrong here
PPP (Point-to-Point Protocol) is a WAN protocol used on serial links, not on Ethernet trunks. Ethernet trunks use 802.1Q or ISL encapsulation, and PPP has no relevance to VLAN trunking issues.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the switches are connected via a serial link requiring PPP for communication, and the configuration is incorrectly set to Ethernet, this option would be correct. The question might ask about the necessary protocol for inter-switch communication in a non-Ethernet environment.
Why candidates choose this
Students might recall that PPP can be used for authentication or encapsulation on point-to-point links and mistakenly think it could be applied to Ethernet trunks, but PPP is not used in Ethernet switching environments.
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 careful to distinguish between total trunk failures and selective VLAN issues. Check the allowed VLAN list on trunk links when only one VLAN is affected.
Detailed technical explanation
How to think about this question
VLAN trunking is a fundamental concept in Cisco switching that allows multiple VLANs to traverse a single physical link between switches. A trunk port carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q. This tagging enables switches to segregate traffic logically while sharing the same physical medium. Proper trunk configuration requires both ends to agree on allowed VLANs, native VLAN, and trunking mode to ensure seamless VLAN communication across switches.
The key rule in VLAN trunking is that the trunk port must explicitly allow the VLANs that need to pass through it. Cisco switches use the "switchport trunk allowed vlan" command to specify which VLANs are permitted on a trunk. If a VLAN is omitted from this allowed list on one side of the trunk, frames tagged with that VLAN are dropped and never forwarded across the trunk. This selective filtering causes connectivity issues only for the excluded VLAN, while other VLANs continue to function normally. This behavior is critical for troubleshooting VLAN reachability problems.
A common exam trap is to confuse native VLAN mismatches or trunk mode misconfigurations with VLAN filtering issues. While native VLAN mismatches can cause untagged frame drops or security risks, they do not selectively block a single VLAN’s traffic. Similarly, changing a trunk port to an access port disables VLAN tagging entirely, which is not the intended solution when multiple VLANs must traverse the link. Understanding that the allowed VLAN list controls which VLANs pass the trunk is essential to avoid these pitfalls and correctly diagnose VLAN communication failures.
KKey Concepts to Remember
A trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
The switchport trunk allowed vlan command controls which VLANs are permitted to traverse a trunk link.
If a VLAN is missing from the allowed VLAN list on a trunk, frames for that VLAN are dropped and cannot pass between switches.
Native VLAN mismatches cause untagged frame issues but do not selectively block individual VLAN traffic on a trunk.
Changing a trunk port to an access port disables VLAN tagging and prevents multiple VLANs from traversing the link.
Trunk ports must be consistently configured on both ends to ensure VLAN traffic flows correctly across switches.
Selective VLAN filtering on trunks causes connectivity loss only for the excluded VLAN, while other VLANs remain operational.
Troubleshooting VLAN reachability requires verifying allowed VLAN lists on trunk ports to identify omitted VLANs.
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 trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
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 trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches., 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 trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches..
What is the correct answer to this question?
The correct answer is: VLAN 50 is not allowed on the trunk from SW1. — 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.
What should I do if I get this 200-301 question wrong?
Review a trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches., 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 trunk port uses VLAN tagging to carry multiple VLANs over a single physical link between switches.
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