CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
Cisco often tests the misconception that any trunk encapsulation mismatch will cause the interface to go down or show errors, but in reality the link can remain up while silently dropping all tagged traffic, leading to complete VLAN isolation.
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
✓
The trunk encapsulation is set to ISL on SW1, but the peer switch only supports 802.1Q.
The 'show interfaces trunk' output on SW1 would show the trunk as up but not passing traffic if the encapsulation is set to ISL while SW2 only supports 802.1Q. Cisco switches default to Dynamic Trunking Protocol (DTP) and may negotiate ISL on older hardware, but if the peer does not support ISL, the trunk fails to form correctly, isolating all VLANs. The mismatch in encapsulation prevents proper frame tagging, causing all VLAN traffic to be dropped.
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 shut down interface would display a status of "admin down" in the "show interfaces trunk" output, not "not-trunking". A shutdown port is in a lower layer state and cannot participate in trunk negotiation, so it would not appear as a candidate in the trunk table. The "not-trunking" status specifically indicates that the port is physically up and administratively enabled, but the trunk is not operational due to a protocol or encapsulation mismatch. Therefore, administrative shutdown is not the cause.
- ✓
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" while the Status is "not-trunking", which is the classic signature of an encapsulation mismatch. SW1 is sending ISL-framed traffic or negotiating with ISL, but the peer switch supports only 802.1Q, making trunk negotiation impossible. ISL and 802.1Q are fundamentally incompatible: ISL adds a 26-byte header and 4-byte trailer, whereas 802.1Q inserts a 4-byte tag into the Ethernet frame. For a trunk to form, both switches must agree on the exact same encapsulation, and since the peer cannot speak ISL, the link remains operationally "not-trunking".
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" while the Status is "not-trunking", which is the classic signature of an encapsulation mismatch. SW1 is sending ISL-framed traffic or negotiating with ISL, but the peer switch supports only 802.1Q, making trunk negotiation impossible. ISL and 802.1Q are fundamentally incompatible: ISL adds a 26-byte header and 4-byte trailer, whereas 802.1Q inserts a 4-byte tag into the Ethernet frame. For a trunk to form, both switches must agree on the exact same encapsulation, and since the peer cannot speak ISL, the link remains operationally "not-trunking".
✗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?”
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Related to this question
Learn chapter
Interpreting Packet Capture Output for Layer 2/3 Troubleshooting
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.