CCNA Switching and Network Access Practice Question
Exhibit
Switch A trunk allowed VLANs: 10,20,30 Switch B trunk allowed VLANs: 10,20
A trunk link between two switches is up, but hosts in VLAN 30 on opposite switches cannot communicate. VLAN 10 works across the same trunk. Which two causes are the most likely?
⚠ Common exam trap
Don't assume trunk issues affect all VLANs equally; check for VLAN-specific settings.
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 30 is not allowed on the trunk on one side
When one VLAN fails but others work across the same trunk, the problem is likely VLAN-specific. VLAN 30 may not exist on one switch or may not be allowed on the trunk. Option C is incorrect because the native VLAN does not need to be 30; a native VLAN mismatch would typically cause connectivity issues on all VLANs, not just VLAN 30. Option D is incorrect because PortFast only affects the speed at which an access port enters the forwarding state and does not impact communication across an already-up trunk.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
VLAN 30 is not allowed on the trunk on one side
Why this is correct
A trunk port has an allowed VLAN list, and both ends must include VLAN 30 in that list for its traffic to cross the link. If one switch's trunk configuration omits VLAN 30 (via the allowed vlan command), the trunk remains up and carries other VLANs, but frames tagged with VLAN 30 are discarded at that port. This mismatch is a frequent cause of a single VLAN failing while the trunk itself appears operational.
- ✓
VLAN 30 may not exist in the VLAN database on the affected switch
Why this is correct
A switch only forwards frames for a VLAN that exists in its local VLAN database. If VLAN 30 has not been created or has been shut down on the switch hosting the access ports, those ports become inactive and cannot pass traffic, even though the trunk link is administratively up. The VLAN must be defined and active on every switch that needs to forward that VLAN's frames.
- ✗
The trunk native VLAN should always be 30
Why it's wrong here
The native VLAN only governs untagged frames on a trunk; it is not the mechanism that permits tagged VLAN 30 traffic. Setting the native VLAN to 30 would change which VLAN is sent untagged, but VLAN 30 traffic in a normal trunk is tagged, so it would not help and could create a native VLAN mismatch with the neighbor. Correctly carrying VLAN 30 depends on it being allowed on the trunk, not on the native VLAN number.
When this WOULD be correct
This option would be correct if the question stated that hosts in VLAN 30 cannot communicate because the native VLAN mismatch on the trunk is causing the trunk to be down or not forwarding traffic for any VLAN.
- ✗
PortFast must be disabled on the access ports in VLAN 30
Why it's wrong here
PortFast is a Spanning Tree Protocol feature for access ports that immediately transitions them to forwarding, skipping the listening and learning states. It does not filter or enable VLANs, and it applies only to the access port itself, not to the trunk link. Disabling or enabling PortFast on access ports in VLAN 30 cannot affect whether the trunk carries that VLAN, so this is not a valid cause of the problem.
When this WOULD be correct
In a scenario where a host in VLAN 30 cannot get an IP address via DHCP and the issue is that PortFast is not enabled on the access port, causing spanning-tree delays that prevent DHCP from working. The question would specify that other VLANs work and the trunk is fine.
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 30 is not allowed on the trunk on one sideCorrect answer▾
Why this is correct
A trunk port has an allowed VLAN list, and both ends must include VLAN 30 in that list for its traffic to cross the link. If one switch's trunk configuration omits VLAN 30 (via the allowed vlan command), the trunk remains up and carries other VLANs, but frames tagged with VLAN 30 are discarded at that port. This mismatch is a frequent cause of a single VLAN failing while the trunk itself appears operational.
✗The trunk native VLAN should always be 30Wrong answer — click to see why▾
Why this is wrong here
The native VLAN ID is not required to match the VLAN that is having connectivity issues; native VLAN is used for untagged traffic on the trunk and does not affect communication for specific VLANs like VLAN 30.
★ When this WOULD be the correct answer
This option would be correct if the question stated that hosts in VLAN 30 cannot communicate because the native VLAN mismatch on the trunk is causing the trunk to be down or not forwarding traffic for any VLAN.
Why candidates choose this
Candidates may confuse the native VLAN with the VLAN that is having problems, or think that setting the native VLAN to the problematic VLAN ID is a troubleshooting step.
✗PortFast must be disabled on the access ports in VLAN 30Wrong answer — click to see why▾
Why this is wrong here
PortFast is used on access ports to speed up the transition to forwarding state and is unrelated to inter-VLAN communication across a trunk. Disabling PortFast would not resolve connectivity issues for VLAN 30 hosts on different switches.
★ When this WOULD be the correct answer
In a scenario where a host in VLAN 30 cannot get an IP address via DHCP and the issue is that PortFast is not enabled on the access port, causing spanning-tree delays that prevent DHCP from working. The question would specify that other VLANs work and the trunk is fine.
Why candidates choose this
Candidates may confuse PortFast with features that affect VLAN connectivity, or think that disabling PortFast could fix issues related to VLAN propagation or trunking, due to a misunderstanding of spanning-tree operations.
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
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Access port
An access port is a switch port that connects to a single end device, like a computer or printer, and carries traffic for only one VLAN.
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.
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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.