CCNA Switching and Network Access Practice Question
A trunk between two switches is up, but users in VLAN 40 cannot communicate across it. The output shows both sides allow VLAN 40. What is another likely trunk-related cause to check next?
⚠ Common exam trap
Don't assume that allowed VLANs are the only trunk-related issue; native VLAN mismatches can also disrupt communication.
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
✓
Native VLAN mismatch between the two trunk ends
If VLAN 40 is allowed on both ends and the trunk is up, a native VLAN mismatch is still worth checking because trunk problems are not limited to the allowed VLAN list. In plain language, the link may be carrying traffic, but if the two switches disagree on how untagged traffic should be treated, behavior can still become unpredictable. Native VLAN mismatches are a well-known source of warnings and unexpected traffic handling on 802.1Q trunks. That does not mean every VLAN problem is caused by the native VLAN, but once the obvious allowed-list issue has been ruled out, it becomes a logical next trunk-specific item to verify.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Native VLAN mismatch between the two trunk ends
Why this is correct
A native VLAN mismatch on an 802.1Q trunk means each switch expects a different VLAN ID for untagged frames. If VLAN 40 is the native VLAN on one side but not the other, frames sent untagged from one switch are received and categorized into the wrong VLAN or dropped, breaking connectivity for that VLAN. This is a classic trunk misconfiguration to verify after confirming the allowed VLAN list includes VLAN 40 on both ends. Cisco best practice is to set the native VLAN to an unused VLAN or ensure it matches exactly on both trunk peers.
- ✗
Missing router ID in OSPF
Why it's wrong here
OSPF router IDs are used to identify routers in the OSPF process and to elect the designated router on multiaccess networks. A missing or misconfigured router ID could prevent OSPF adjacencies from forming, but that would affect Layer 3 routing between networks, not the Layer 2 transfer of VLAN 40 frames across a switch trunk. Since the problem is isolated to VLAN 40 communication over a trunk, OSPF router ID configuration is irrelevant.
When this WOULD be correct
In a different scenario, if the question were about OSPF routing issues where the router ID is not set, candidates might be asked to troubleshoot OSPF adjacency problems. In that case, a missing router ID would prevent OSPF neighbors from forming, making this option correct.
- ✗
Incorrect NTP source interface
Why it's wrong here
NTP source interface configuration determines which IP address is used as the source for NTP packets when a switch synchronizes its clock. An incorrect NTP source interface could cause time synchronization failures, but NTP is a management plane function and has no interaction with the data plane forwarding of VLAN-tagged frames. Users in VLAN 40 losing connectivity over a trunk cannot be caused by NTP settings because NTP does not affect the switch's MAC address table, STP state, or trunk encapsulation.
When this WOULD be correct
In a scenario where the question involves troubleshooting time-sensitive applications that rely on accurate timestamps for logging or coordination, a missing or incorrectly configured NTP source interface could lead to discrepancies in time, causing issues in those applications.
- ✗
Lack of PAT overload on the WAN router
Why it's wrong here
PAT overload on the WAN router is a NAT feature that allows many private addresses to share a single public IP by multiplexing on port numbers. This applies to traffic crossing the router's inside and outside interfaces, typically when clients access the internet. VLAN 40 users communicating across a switch trunk stay entirely within the Layer 2 domain, and no NAT is involved in that path, so lack of PAT overload cannot explain the trunk failure.
When this WOULD be correct
In a question focused on NAT configurations, if the scenario involves multiple internal networks needing to share a single public IP address, the lack of PAT overload on the WAN router would be a valid concern affecting outbound connectivity.
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.
✓Native VLAN mismatch between the two trunk endsCorrect answer▾
Why this is correct
A native VLAN mismatch on an 802.1Q trunk means each switch expects a different VLAN ID for untagged frames. If VLAN 40 is the native VLAN on one side but not the other, frames sent untagged from one switch are received and categorized into the wrong VLAN or dropped, breaking connectivity for that VLAN. This is a classic trunk misconfiguration to verify after confirming the allowed VLAN list includes VLAN 40 on both ends. Cisco best practice is to set the native VLAN to an unused VLAN or ensure it matches exactly on both trunk peers.
✗Missing router ID in OSPFWrong answer — click to see why▾
Why this is wrong here
This option is wrong because the question specifically addresses VLAN communication issues over a trunk link, which are unrelated to OSPF router ID configuration. OSPF router ID affects routing protocols, not VLAN traffic directly.
★ When this WOULD be the correct answer
In a different scenario, if the question were about OSPF routing issues where the router ID is not set, candidates might be asked to troubleshoot OSPF adjacency problems. In that case, a missing router ID would prevent OSPF neighbors from forming, making this option correct.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of how routing protocols interact with VLANs, mistakenly believing that OSPF configuration could impact VLAN communication across a trunk link.
✗Incorrect NTP source interfaceWrong answer — click to see why▾
Why this is wrong here
NTP source interface is unrelated to VLAN communication issues over a trunk link; it primarily affects time synchronization across devices. Since the question focuses on VLAN connectivity, this option does not address the core problem.
★ When this WOULD be the correct answer
In a scenario where the question involves troubleshooting time-sensitive applications that rely on accurate timestamps for logging or coordination, a missing or incorrectly configured NTP source interface could lead to discrepancies in time, causing issues in those applications.
Why candidates choose this
Candidates might confuse the importance of time synchronization in network operations with VLAN functionality, leading them to mistakenly believe that NTP configuration could impact VLAN communication.
✗Lack of PAT overload on the WAN routerWrong answer — click to see why▾
Why this is wrong here
Lack of PAT overload on the WAN router is unrelated to VLAN communication issues between switches. This option pertains to address translation for outbound traffic, not VLAN trunking problems.
★ When this WOULD be the correct answer
In a question focused on NAT configurations, if the scenario involves multiple internal networks needing to share a single public IP address, the lack of PAT overload on the WAN router would be a valid concern affecting outbound connectivity.
Why candidates choose this
Candidates may confuse general connectivity issues with NAT configurations, thinking that any routing or address translation problem could impact VLAN communication, leading them to select this option.
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
Interpreting Packet Capture Output for Layer 2/3 Troubleshooting
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
Native VLAN
A native VLAN is the default VLAN assigned to a trunk port that carries untagged traffic for backwards compatibility with devices that do not understand VLAN tagging.
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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.