Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
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?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
Native VLAN mismatch between the two trunk ends
This is correct because a native VLAN mismatch is a common trunk-related issue worth checking after allowed VLANs have been verified.
B
Distractor review
Missing router ID in OSPF
This is wrong because OSPF router IDs are unrelated to an inter-switch VLAN trunk problem.
C
Distractor review
Incorrect NTP source interface
This is wrong because NTP source settings do not explain VLAN trunk transport issues.
D
Distractor review
Lack of PAT overload on the WAN router
This is wrong because WAN NAT design is unrelated to switch trunk VLAN carriage.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is to focus solely on verifying that VLAN 40 is allowed on both trunk ends and to overlook the native VLAN configuration. Candidates may incorrectly assume that allowed VLAN lists guarantee VLAN communication. However, if the native VLANs differ between the two switches, untagged frames can be misclassified, causing VLAN 40 traffic to fail despite the trunk being up. This subtle misconfiguration often leads to confusing symptoms and wasted troubleshooting time if native VLAN mismatch is not checked.
Technical deep dive
How to think about this question
A VLAN trunk is a point-to-point link between two switches that carries traffic for multiple VLANs simultaneously using tagging protocols like IEEE 802.1Q. The trunk encapsulates frames with VLAN tags to identify the VLAN membership of each frame as it traverses the link. The native VLAN is a special VLAN on a trunk port that carries untagged traffic, meaning frames sent on the native VLAN are transmitted without 802.1Q tags. This native VLAN must match on both ends of the trunk to ensure consistent handling of untagged frames. When troubleshooting VLAN communication issues over a trunk, verifying that the VLAN is allowed on both ends is necessary but not always sufficient. A native VLAN mismatch occurs when the two trunk ports are configured with different native VLAN IDs. This mismatch causes untagged frames sent by one switch to be interpreted as belonging to different VLANs by the other switch, leading to traffic loss or misdelivery. Cisco switches generate native VLAN mismatch warnings in logs, and this issue is a common cause of VLAN communication failures even when the VLAN is explicitly allowed. The exam trap here is to assume that allowed VLAN lists alone guarantee trunk functionality. Candidates often overlook the native VLAN configuration, which affects untagged traffic on the trunk. In practice, native VLAN mismatches can cause intermittent connectivity problems and unexpected traffic behavior, especially for management VLANs or protocols that send untagged frames. Understanding this subtlety helps avoid misdiagnosis and ensures reliable VLAN trunk operation in Cisco environments.
KKey Concepts to Remember
- A VLAN trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
- The native VLAN on a trunk port carries untagged traffic and must be identical on both trunk ends to prevent traffic misinterpretation.
- Allowed VLAN lists on trunk ports control which VLANs can traverse the link but do not affect native VLAN handling.
- A native VLAN mismatch causes untagged frames to be assigned to different VLANs on each switch, disrupting communication.
- Cisco switches generate native VLAN mismatch warnings in logs to help identify this common trunk configuration error.
- Troubleshooting VLAN connectivity issues requires checking both allowed VLANs and native VLAN consistency on trunk ports.
- Ignoring native VLAN mismatches can lead to intermittent VLAN communication failures despite trunks being operational.
- Proper native VLAN configuration ensures predictable forwarding of untagged traffic and stable VLAN trunk links.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
A VLAN trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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