CCNA Switching and Network Access Practice Question
Exhibit
SW1# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi1/0/24 on 802.1q trunking 1 Port Vlans allowed on trunk Gi1/0/24 10,20,40 Port Vlans in spanning tree forwarding state and not pruned Gi1/0/24 10,20,40
After a switch replacement, users in VLAN 30 cannot reach devices in other VLANs. The replacement switch has a trunk link to the distribution switch that shows as up/up. What is the most likely cause?
⚠ Common exam trap
Ensure you distinguish between management settings and VLAN configuration. Focus on trunk settings when inter-VLAN issues arise.
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 permitted on the trunk link
The trunk is up, but VLAN 30 is not allowed on it. Traffic from that VLAN never crosses the uplink, so inter-VLAN reachability fails for users in VLAN 30 even though the local access ports may still look fine.
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 must be changed to 30
Why it's wrong here
A native VLAN mismatch causes symptoms like misplaced traffic on the native VLAN and possible STP/BPDU issues, but it does not explain why VLAN 30 specifically is unreachable if the allowed list is correct. The native VLAN is used for untagged frames on the trunk; changing it to 30 would simply alter which VLAN is untagged, not permit VLAN 30 to cross the link. VLAN 30 failure here is a filtering problem, not a tagging or native-VLAN problem, so this option is incorrect.
When this WOULD be correct
In a different scenario where a network administrator is troubleshooting a switch that has been configured with a native VLAN of 30, and the requirement is to ensure that untagged traffic is handled correctly, changing the native VLAN to 30 would be appropriate if the design specifies that all untagged traffic should belong to VLAN 30.
- ✓
VLAN 30 is not permitted on the trunk link
Why this is correct
The trunk allowed list controls which VLANs traverse the link, and without an explicit 'allowed vlan add 30' statement, frames belonging to VLAN 30 are discarded at the trunk. Because the access switch and distribution switch only carry permitted VLANs, VLAN 30 has no path to the rest of the network. This exactly matches the symptom that only VLAN 30 suffers while other VLANs work normally on the same uplink.
- ✗
The uplink should be configured as an access port
Why it's wrong here
Configuring the uplink as an access port would restrict it to a single VLAN, so it cannot carry the multiple VLANs that are presumably already operating across this trunk. An access port places the link in the designated access VLAN and strips tags, which would isolate VLAN 30 even further rather than connecting it to other devices. Since the issue is that VLAN 30 is blocked on a trunk, converting the uplink into an access port would not fix the allowed-list omission and would break other VLANs.
When this WOULD be correct
In a different question context where the prompt indicates that a switch port must connect to a single VLAN and no inter-VLAN routing is required, stating that the uplink should be configured as an access port would be correct. For example, if the question specifies that all devices on the same VLAN need to communicate without routing, this option would be valid.
- ✗
STP must be disabled on VLAN 30
Why it's wrong here
STP is designed to prevent Layer 2 loops by blocking redundant paths, and it does not filter traffic based on VLAN membership on a trunk. Disabling STP would only risk creating broadcast storms and bridging loops, and it would not add VLAN 30 to the trunk's allowed list. The connectivity failure described is caused by the trunk pruning VLAN 30, not by any spanning-tree state, so this option does not address the root cause.
When this WOULD be correct
In a scenario where a question states that VLAN 30 is experiencing broadcast storms or loops, and the exam asks for a method to stabilize the network, disabling STP on VLAN 30 could be considered correct to allow for immediate traffic flow, assuming there are no other VLANs affected.
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 permitted on the trunk linkCorrect answer▾
Why this is correct
The trunk allowed list controls which VLANs traverse the link, and without an explicit 'allowed vlan add 30' statement, frames belonging to VLAN 30 are discarded at the trunk. Because the access switch and distribution switch only carry permitted VLANs, VLAN 30 has no path to the rest of the network. This exactly matches the symptom that only VLAN 30 suffers while other VLANs work normally on the same uplink.
✗The native VLAN must be changed to 30Wrong answer — click to see why▾
Why this is wrong here
Changing the native VLAN to 30 would not resolve the issue because the native VLAN is used for untagged traffic on a trunk, and the problem is that VLAN 30 traffic is not being allowed at all. Additionally, native VLAN mismatch can cause connectivity issues, but it does not specifically prevent only VLAN 30 from reaching other VLANs.
★ When this WOULD be the correct answer
In a different scenario where a network administrator is troubleshooting a switch that has been configured with a native VLAN of 30, and the requirement is to ensure that untagged traffic is handled correctly, changing the native VLAN to 30 would be appropriate if the design specifies that all untagged traffic should belong to VLAN 30.
Why candidates choose this
Students might confuse native VLAN with the VLAN that is allowed on the trunk, or think that setting the native VLAN to 30 would automatically permit VLAN 30 traffic.
✗The uplink should be configured as an access portWrong answer — click to see why▾
Why this is wrong here
Configuring the uplink as an access port would place it in a single VLAN, preventing traffic from multiple VLANs (including VLAN 30) from traversing the link. Since the switch needs to carry traffic for multiple VLANs, the uplink must remain a trunk port.
★ When this WOULD be the correct answer
In a different question context where the prompt indicates that a switch port must connect to a single VLAN and no inter-VLAN routing is required, stating that the uplink should be configured as an access port would be correct. For example, if the question specifies that all devices on the same VLAN need to communicate without routing, this option would be valid.
Why candidates choose this
Students might think that using an access port simplifies configuration, but they overlook the need to carry multiple VLANs between switches.
✗STP must be disabled on VLAN 30Wrong answer — click to see why▾
Why this is wrong here
Disabling STP on VLAN 30 would be dangerous as it could cause bridging loops and network instability. Moreover, STP does not control whether a VLAN is allowed on a trunk; it only prevents loops. The issue is a missing VLAN in the trunk allowed list, not a spanning-tree problem.
★ When this WOULD be the correct answer
In a scenario where a question states that VLAN 30 is experiencing broadcast storms or loops, and the exam asks for a method to stabilize the network, disabling STP on VLAN 30 could be considered correct to allow for immediate traffic flow, assuming there are no other VLANs affected.
Why candidates choose this
Students might associate STP with VLAN issues because STP operates per VLAN, but they incorrectly assume that disabling STP can fix connectivity problems caused by trunk misconfiguration.
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
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
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.