The answer is a trunk encapsulation mismatch, specifically that SW1 is configured for ISL while the peer switch only supports 802.1Q. This is the most likely cause because the output of 'show interfaces GigabitEthernet0/0 trunk' on SW1 shows encapsulation as 'isl' and status as 'not-trunking', meaning the port is not forming a trunk due to incompatible encapsulation protocols. On the CCNA 200-301 v2 exam, this scenario tests your understanding of trunking fundamentals and the critical difference between Cisco’s proprietary ISL and the industry-standard 802.1Q—a common trap is assuming that simply setting a port to trunk mode is enough, without verifying that both ends agree on the encapsulation type. Remember, ISL is legacy and rarely seen in modern networks, so if you see 'isl' in a troubleshooting context, suspect a mismatch with a peer that only supports 802.1Q. A quick memory tip: "ISL is old, 802.1Q is gold"—if the trunk won’t hold, check the encapsulation mold.
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.
Exhibit
SW1# show interfaces GigabitEthernet0/0 trunk
Port Mode Encapsulation Status Native vlan
Gi0/0 on isl not-trunking 1
Port Vlans allowed on trunk
Gi0/0 1-4094
Port Vlans allowed and active in management domain
Gi0/0 1,10,20,30,99
Port Vlans in spanning tree forwarding state and not pruned
Gi0/0 none
Refer to the exhibit. A network administrator is troubleshooting a trunk link between SW1 and SW2. The trunk on interface GigabitEthernet0/0 on SW1 is not passing traffic, and all VLANs are isolated. The administrator issues the command 'show interfaces GigabitEthernet0/0 trunk' on SW1. What is the most likely cause of the issue?
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 GigabitEthernet0/0 trunk
Port Mode Encapsulation Status Native vlan
Gi0/0 on isl not-trunking 1
Port Vlans allowed on trunk
Gi0/0 1-4094
Port Vlans allowed and active in management domain
Gi0/0 1,10,20,30,99
Port Vlans in spanning tree forwarding state and not pruned
Gi0/0 none
A
The native VLAN is mismatched between SW1 and SW2.
Why wrong: The exhibit shows Native vlan as 1, which is the default. There is no evidence of a native VLAN mismatch; the trunk would still attempt to form even with a mismatch, but the status would likely be 'trunking' with a warning.
B
The interface is configured as an access port instead of a trunk.
Why wrong: The 'Mode' field shows 'on', which means the port is statically set as a trunk. If it were an access port, the mode would display 'access' or the port would not appear in the trunk output.
C
The interface is administratively shut down.
Why wrong: An administratively down port would show 'admin down' in the Status column, not 'not-trunking'. The exhibit shows the port is not administratively disabled.
D
The trunk encapsulation is set to ISL on SW1, but the peer switch only supports 802.1Q.
The Encapsulation column shows 'isl', and the Status is 'not-trunking'. This confirms that SW1 is using ISL, which is incompatible with the peer's 802.1Q-only support, preventing trunk establishment.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The trunk encapsulation is set to ISL on SW1, but the peer switch only supports 802.1Q.
The 'show interfaces GigabitEthernet0/0 trunk' output explicitly displays encapsulation 'isl' and status 'not-trunking'. This indicates that the port is configured for ISL encapsulation, but the trunk is not operational. Since the peer switch only supports 802.1Q, the encapsulation mismatch prevents the trunk from forming, causing the traffic isolation.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both 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.
✗
The native VLAN is mismatched between SW1 and SW2.
Why it's wrong here
The exhibit shows Native vlan as 1, which is the default. There is no evidence of a native VLAN mismatch; the trunk would still attempt to form even with a mismatch, but the status would likely be 'trunking' with a warning.
✗
The interface is configured as an access port instead of a trunk.
Why it's wrong here
The 'Mode' field shows 'on', which means the port is statically set as a trunk. If it were an access port, the mode would display 'access' or the port would not appear in the trunk output.
✗
The interface is administratively shut down.
Why it's wrong here
An administratively down port would show 'admin down' in the Status column, not 'not-trunking'. The exhibit shows the port is not administratively disabled.
✓
The trunk encapsulation is set to ISL on SW1, but the peer switch only supports 802.1Q.
Why this is correct
The Encapsulation column shows 'isl', and the Status is 'not-trunking'. This confirms that SW1 is using ISL, which is incompatible with the peer's 802.1Q-only support, preventing trunk establishment.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
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.
✓The trunk encapsulation is set to ISL on SW1, but the peer switch only supports 802.1Q.Correct answer▾
Why this is correct
The Encapsulation column shows 'isl', and the Status is 'not-trunking'. This confirms that SW1 is using ISL, which is incompatible with the peer's 802.1Q-only support, preventing trunk establishment.
✗The native VLAN is mismatched between SW1 and SW2.Wrong answer — click to see why▾
Why this is wrong here
Native VLAN mismatches can cause traffic to leak between VLANs, but they do not prevent a trunk from becoming operational. The output clearly shows the encapsulation type as ISL, not a native VLAN problem.
✗The interface is configured as an access port instead of a trunk.Wrong answer — click to see why▾
Why this is wrong here
Candidates might assume a misconfigured mode causes the issue, but the exhibit confirms the port is in trunk mode ('on' mode).
✗The interface is administratively shut down.Wrong answer — click to see why▾
Why this is wrong here
New learners might equate 'not-trunking' with a shutdown state, but 'admin down' is a distinct status. The port is operationally unable to trunk, not manually disabled.
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
The exhibit shows Native vlan as 1, which is the default. There is no evidence of a native VLAN mismatch; the trunk would still attempt to form even with a mismatch, but the status would likely be 'trunking' with a warning.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both 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 VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
Switching and Network Access — This question tests Switching and Network Access — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The trunk encapsulation is set to ISL on SW1, but the peer switch only supports 802.1Q. — The 'show interfaces GigabitEthernet0/0 trunk' output explicitly displays encapsulation 'isl' and status 'not-trunking'. This indicates that the port is configured for ISL encapsulation, but the trunk is not operational. Since the peer switch only supports 802.1Q, the encapsulation mismatch prevents the trunk from forming, causing the traffic isolation.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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