CCNA Switching and Network Access Practice Question
A trunk is up between two switches, but traffic for VLAN 40 fails while other VLANs work. Which output item should be checked first?
⚠ Common exam trap
Avoid assuming a total trunk failure when only one VLAN is affected. Focus on VLAN-specific configurations.
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
✓
Whether VLAN 40 is included in the allowed VLAN list on the trunk
The first thing to check is whether VLAN 40 appears in the trunk’s allowed VLAN list. In practical terms, this is a selective failure, not a total trunk failure. Since other VLANs are crossing successfully, the link is operational. That strongly suggests one VLAN is being excluded rather than the trunk being generally broken. This is one of the most common VLAN troubleshooting patterns in switching.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Whether VLAN 40 is included in the allowed VLAN list on the trunk
Why this is correct
A trunk being administratively up indicates the physical and data-link layers are functioning, but Layer 2 connectivity for a particular VLAN depends on the VLAN being present in the trunk's allowed list (via switchport trunk allowed vlan). If VLAN 40 is not included on one or both sides, its frames are silently discarded or not placed on the trunk, even though other VLANs pass normally. This configuration discrepancy is the classic cause of a single-VLAN outage on an otherwise healthy trunk.
- ✗
Whether the router ID matches on both switches
Why it's wrong here
Router IDs are used by control-plane routing protocols such as OSPF and BGP to identify a router, and they have no bearing on Layer 2 switching decisions. Two switches connected by a trunk do not need matching or even valid router IDs to forward VLAN traffic; in fact, a purely Layer 2 switch may have no router ID configured at all. Therefore this setting cannot be the root cause of VLAN 40 traffic failing over the trunk.
When this WOULD be correct
In a question about OSPF or EIGRP configuration, where the focus is on ensuring proper routing between switches, checking the router ID would be crucial to confirm that both switches are configured to recognize each other in the routing domain.
- ✗
Whether NetFlow is enabled on the VLAN
Why it's wrong here
NetFlow is a traffic-monitoring and accounting mechanism that samples and exports flow statistics to a collector; it does not filter, permit, or block traffic on any interface or VLAN. Even if VLAN 40 were not enabled for NetFlow, the switch would still forward that VLAN's frames across a trunk that allows it. Conversely, enabling NetFlow on a VLAN would not fix an exclusion from the trunk's allowed VLAN list, so this is irrelevant to the reported symptom.
When this WOULD be correct
In a question where the focus is on monitoring and analyzing traffic flows within a network, a scenario might ask about verifying configurations related to traffic analysis. If the question involved ensuring that traffic data is being collected for VLAN 40, then checking if NetFlow is enabled would be relevant.
- ✗
Whether NTP is synchronized on the switches
Why it's wrong here
NTP synchronization only aligns the switches' clocks for features like syslog timestamps, logging, and digital certificates; it is not a prerequisite for forwarding bridged frames. A switch with an unsynchronized or even completely wrong clock will still transport VLAN 40 traffic on a correctly configured trunk. Thus, while NTP misconfiguration can cause operational headaches, it has no causal relation to a specific VLAN failing to cross a trunk.
When this WOULD be correct
In a different scenario where the question asks about the overall stability and performance of network services that rely on accurate timekeeping, such as logging or security protocols, verifying NTP synchronization could be crucial. For instance, if the question involved troubleshooting time-sensitive applications, this option could be relevant.
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.
✓Whether VLAN 40 is included in the allowed VLAN list on the trunkCorrect answer▾
Why this is correct
A trunk being administratively up indicates the physical and data-link layers are functioning, but Layer 2 connectivity for a particular VLAN depends on the VLAN being present in the trunk's allowed list (via switchport trunk allowed vlan). If VLAN 40 is not included on one or both sides, its frames are silently discarded or not placed on the trunk, even though other VLANs pass normally. This configuration discrepancy is the classic cause of a single-VLAN outage on an otherwise healthy trunk.
✗Whether the router ID matches on both switchesWrong answer — click to see why▾
Why this is wrong here
This option is wrong because the router ID is relevant for routing protocols, not for VLAN traffic issues on a trunk link. The problem specifically pertains to VLAN 40 traffic, which is not influenced by router IDs.
★ When this WOULD be the correct answer
In a question about OSPF or EIGRP configuration, where the focus is on ensuring proper routing between switches, checking the router ID would be crucial to confirm that both switches are configured to recognize each other in the routing domain.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of the relationship between VLANs and routing protocols, mistakenly believing that routing configuration issues could affect VLAN traffic.
✗Whether NetFlow is enabled on the VLANWrong answer — click to see why▾
Why this is wrong here
NetFlow is a network protocol used for collecting IP traffic information, but it does not directly affect VLAN traffic flow on a trunk link. Therefore, checking if NetFlow is enabled does not address the issue of VLAN 40 traffic failure.
★ When this WOULD be the correct answer
In a question where the focus is on monitoring and analyzing traffic flows within a network, a scenario might ask about verifying configurations related to traffic analysis. If the question involved ensuring that traffic data is being collected for VLAN 40, then checking if NetFlow is enabled would be relevant.
Why candidates choose this
Candidates may confuse traffic analysis tools like NetFlow with VLAN configurations, thinking that monitoring settings could impact VLAN traffic, leading them to incorrectly select this option.
✗Whether NTP is synchronized on the switchesWrong answer — click to see why▾
Why this is wrong here
NTP synchronization is not directly related to VLAN traffic issues on a trunk link; it primarily affects time-sensitive protocols. Therefore, checking NTP synchronization would not help diagnose why VLAN 40 traffic is failing.
★ When this WOULD be the correct answer
In a different scenario where the question asks about the overall stability and performance of network services that rely on accurate timekeeping, such as logging or security protocols, verifying NTP synchronization could be crucial. For instance, if the question involved troubleshooting time-sensitive applications, this option could be relevant.
Why candidates choose this
Candidates might choose this option due to a general understanding that synchronization issues can lead to various network problems, leading them to mistakenly believe it could impact VLAN traffic.
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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Interpreting Packet Capture Output for Layer 2/3 Troubleshooting
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
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
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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.