CCNA Switching and Network Access Practice Question
An administrator notices that hosts in VLAN 30 on SW1 cannot communicate with hosts in VLAN 30 on SW2, even though both switches are connected via an 802.1Q trunk. Traffic for VLANs 10 and 20 passes without issues across the same trunk. The trunk is configured to allow all VLANs, and the allowed VLAN list explicitly includes VLAN 30. What is the most likely cause of the problem?
⚠ Common exam trap
Cisco often tests the native VLAN mismatch scenario by describing a trunk that works for most VLANs but fails for one specific VLAN, leading candidates to incorrectly suspect STP blocking or SVI issues instead of recognizing the native VLAN mismatch.
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 native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side.
The most likely cause is a native VLAN mismatch. When the native VLAN is mismatched on an 802.1Q trunk, traffic for the native VLAN is not tagged, so frames from VLAN 30 on one switch are received as untagged frames on the other switch and placed into the switch's configured native VLAN. If the native VLAN on one side is VLAN 30 and on the other side is a different VLAN (e.g., VLAN 1), the hosts in VLAN 30 cannot communicate because the frames are interpreted as belonging to different VLANs. Traffic for VLANs 10 and 20 passes because they are not the native VLAN and are properly tagged.
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, and VLAN 30 is the native VLAN on one side.
Why this is correct
A native VLAN mismatch causes one switch to send untagged frames for VLAN 30 while the other expects tagged frames, so the receiving switch cannot associate the untagged traffic with VLAN 30, leading to a communication failure only for that VLAN.
- ✗
The trunk encapsulation is set to ISL on one switch and 802.1Q on the other.
Why it's wrong here
A trunk encapsulation mismatch between ISL and 802.1Q would prevent the trunk link from coming up entirely, because the two switches would be unable to agree on a frame tagging format for any VLAN. Since the problem states that other VLANs are communicating successfully over the same link, an encapsulation mismatch cannot be the cause. Moreover, modern Cisco switches default to 802.1Q and auto-negotiate encapsulation via DTP, making a unilateral ISL configuration extremely unlikely in a standard CCNA scenario. The selective failure of only VLAN 30 instead indicates a native VLAN tagging mismatch, not a global trunking protocol incompatibility.
- ✗
Spanning Tree Protocol has placed VLAN 30 in a blocking state on the trunk link.
Why it's wrong here
STP operates per VLAN, but there is no inherent reason for STP to block only one VLAN while others forward, especially without a topology change. The scenario gives no indication of a loop or STP change.
- ✗
The switched virtual interface (SVI) for VLAN 30 on SW1 is administratively down.
Why it's wrong here
An SVI is a Layer 3 interface used for routing between VLANs, not for Layer 2 frame forwarding across a trunk. Disabling the SVI for VLAN 30 on SW1 would only remove the switch's ability to route traffic to or from that VLAN locally; it does not affect the bridging or trunking of VLAN 30 frames between SW1 and SW2. Even with the SVI administratively down, access ports and trunk ports carrying VLAN 30 remain active at Layer 2. Therefore, this misconfiguration would impact inter-VLAN routing, not the intra-VLAN connectivity of hosts in VLAN 30 across the trunk, so it cannot explain a failure that is specific to one VLAN on a trunk.
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 native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side.Correct answer▾
Why this is correct
A native VLAN mismatch causes one switch to send untagged frames for VLAN 30 while the other expects tagged frames, so the receiving switch cannot associate the untagged traffic with VLAN 30, leading to a communication failure only for that VLAN.
✗The trunk encapsulation is set to ISL on one switch and 802.1Q on the other.Wrong answer — click to see why▾
Why this is wrong here
This would cause a complete trunk failure, not a failure limited to a single VLAN.
✗Spanning Tree Protocol has placed VLAN 30 in a blocking state on the trunk link.Wrong answer — click to see why▾
Why this is wrong here
STP would not randomly block one VLAN on a point-to-point trunk while the rest are forwarding; this is not a typical behavior.
✗The switched virtual interface (SVI) for VLAN 30 on SW1 is administratively down.Wrong answer — click to see why▾
Why this is wrong here
Layer 2 switching within the same VLAN does not require an SVI; an SVI is only needed for routing between VLANs or management.
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
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
About these practice questions
This 200-301 question is part of Courseiva's 1,389-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam 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.