CCNA Switching and Network Access Practice Question
A technician is troubleshooting a network issue where hosts in VLAN 20 on SW1 cannot communicate with hosts in VLAN 20 on SW2. Both switches are connected by an Ethernet trunk link that is up/up and configured as a trunk. The VLAN databases on both switches include VLAN 20, and the spanning tree for VLAN 20 is in a forwarding state on all ports. Hosts within VLAN 20 on each switch can communicate with each other locally. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between VLAN existence in the database and VLAN permission on a trunk; candidates mistakenly think that if a VLAN is created and spanning tree is forwarding, it must work, but the trunk allowed list is an independent filter that can block traffic.
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 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.
The most likely cause is that VLAN 20 is not included in the allowed VLAN list on the trunk port between SW1 and SW2. Even though the trunk is up/up and VLAN 20 exists in the VLAN database, the switchport trunk allowed vlan command restricts which VLANs can traverse the trunk. If VLAN 20 is omitted from this list, frames from VLAN 20 will be dropped at the trunk, preventing inter-switch communication for that VLAN.
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 on the two ends of the trunk.
Why it's wrong here
Native VLAN mismatch primarily causes CDP and STP warnings and can impair traffic on the native VLAN, but tagged VLANs such as 20 use the 802.1Q tag and are forwarded normally across the trunk. So this would not prevent VLAN 20 communication.
- ✗
VLAN 20 has not been created in the VLAN database on SW2.
Why it's wrong here
If VLAN 20 were missing from SW2's database, SW2 could not forward frames for that VLAN, and any access ports assigned to VLAN 20 would be down or non-functional. The stem explicitly states that hosts in VLAN 20 on SW2 can communicate with each other locally, confirming the VLAN exists.
- ✗
The trunk encapsulation is mismatched between SW1 and SW2.
Why it's wrong here
A trunk encapsulation mismatch occurs when one switch is configured for IEEE 802.1Q and the other for Cisco ISL, or when a trunk is set to 'negotiate' but only one side supports a given protocol. Because each switch inserts and expects a different frame tag format, the trunk cannot form successfully—the port typically remains in a down/down state, becomes err-disabled, or fails to reach the operational 'trunking' mode. The scenario explicitly states that the trunk is up/up, which proves both ends are using a compatible encapsulation (normally 802.1Q). Therefore, this option cannot explain the loss of VLAN 20 traffic across the trunk.
- ✓
VLAN 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.
Why this is correct
When a trunk port’s allowed VLAN list explicitly excludes a VLAN, the switch drops all frames tagged for that VLAN, even though the VLAN exists locally and the trunk is active. This results in the described symptom of local intra-VLAN communication working but no cross-switch communication for VLAN 20.
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 20 is not in the switchport trunk allowed VLAN list on the trunk port between SW1 and SW2.Correct answer▾
Why this is correct
When a trunk port’s allowed VLAN list explicitly excludes a VLAN, the switch drops all frames tagged for that VLAN, even though the VLAN exists locally and the trunk is active. This results in the described symptom of local intra-VLAN communication working but no cross-switch communication for VLAN 20.
✗The native VLAN is mismatched on the two ends of the trunk.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that a native VLAN mismatch breaks all trunk functions.
✗VLAN 20 has not been created in the VLAN database on SW2.Wrong answer — click to see why▾
Why this is wrong here
Candidates may assume that a missing VLAN on one switch explains inter-switch failures, ignoring that local communication would also fail.
✗The trunk encapsulation is mismatched between SW1 and SW2.Wrong answer — click to see why▾
Why this is wrong here
Candidates might overlook that the trunk link is operational, which implies matching encapsulation.
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
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Forwarding state
A Spanning Tree Protocol port state where the port is fully operational, forwarding both data frames and BPDUs, and is part of the active loop-free topology.
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
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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.