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CCNP Practice Question: An engineer is troubleshooting a problem where a…
An engineer is troubleshooting a problem where a host in VLAN 20 cannot communicate with a host in VLAN 30, even though both are connected to the same access switch. The access switch is configured with VLANs 20 and 30, and the uplink to the distribution switch is a trunk that allows both VLANs. The distribution switch has SVIs for both VLANs and IP routing is enabled. The engineer verifies that the trunk is up and both VLANs are allowed. What is the most likely cause of the communication failure?
⚠ Common exam trap
Cisco often tests the misconception that a properly configured trunk and SVIs alone guarantee inter-VLAN communication, when in fact the hosts must have the correct default gateway configured to reach the SVI.
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 hosts are not configured with the correct default gateway pointing to the SVI on the distribution switch.
Hosts in different VLANs must communicate through a Layer 3 device. The correct default gateway for each host should be the IP address of the SVI on the distribution switch for its respective VLAN. If the hosts are configured with an incorrect or no default gateway, traffic cannot be routed between VLAN 20 and VLAN 30, even though the trunk and SVIs are properly configured.
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 hosts are not configured with the correct default gateway pointing to the SVI on the distribution switch.
Why this is correct
The hosts cannot reach the SVI because their default gateway is either absent or set to an incorrect IP address. For inter-VLAN communication, each host must send Layer 3 packets to its VLAN's SVI IP, which then routes them to other VLANs. If the gateway points to the wrong address (e.g., another host or a nonexistent IP), frames are sent to the wrong destination and never reach the distribution switch's routing engine.
- ✗
The trunk is not allowing VLAN 20 or VLAN 30.
Why it's wrong here
The engineer already verified that the trunk allows VLAN 20 and VLAN 30, so this cannot be the fault. If the trunk were pruning or blocking these VLANs, the access switch would not deliver frames from those VLANs to the distribution switch at all, causing a complete loss of inter-VLAN connectivity. Since the trunk is operationally up and both VLANs are in the allowed list, the failure is not at the Layer 2 trunking layer.
- ✗
Spanning Tree Protocol is blocking the SVI interfaces.
Why it's wrong here
Spanning Tree Protocol operates only on physical and logical Layer 2 switch ports, never on Layer 3 SVIs. An SVI is a routed interface that exists in software and is not subject to STP's blocking or forwarding states. Even if a physical access port were blocked by STP, the SVI itself remains up, so STP cannot be responsible for the hosts' inability to reach their default gateway.
- ✗
The native VLAN mismatch on the trunk is causing the issue.
Why it's wrong here
A native VLAN mismatch on a trunk does not affect tagged VLAN frames like those on VLAN 20 and 30, because native VLAN traffic is sent untagged. The trunk is already up and passing the required VLANs, meaning the native VLAN is configured consistently (otherwise CDP or STP would flag an inconsistency). Therefore, a native VLAN mismatch is not the root cause of the inter-VLAN routing failure.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.