Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A trunk is up between two switches, but traffic for VLAN 40 fails while other VLANs work. Which output item should be checked first?
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
Whether VLAN 40 is included in the allowed VLAN list on the trunk
This is correct because a missing VLAN in the allowed list is a common cause of one-VLAN failure on an otherwise working trunk.
B
Distractor review
Whether the router ID matches on both switches
This is wrong because switches do not need matching router IDs for VLAN trunking.
C
Distractor review
Whether NetFlow is enabled on the VLAN
This is wrong because NetFlow is unrelated to a VLAN being permitted on a trunk.
D
Distractor review
Whether NTP is synchronized on the switches
This is wrong because time synchronization does not determine VLAN trunk forwarding.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that if a trunk is up, all VLANs automatically pass traffic. Candidates often overlook the allowed VLAN list on the trunk port and instead check unrelated parameters like router IDs or time synchronization. This mistake leads to confusion because the trunk link itself is operational, but selective VLAN filtering causes traffic failure for specific VLANs. The exam tests your ability to distinguish between a physical trunk link status and VLAN membership configuration, so focusing on allowed VLANs is critical.
Technical deep dive
How to think about this question
A VLAN trunk is a point-to-point link between switches that carries multiple VLANs simultaneously using tagging protocols like IEEE 802.1Q. The trunk encapsulates Ethernet frames with VLAN tags to distinguish traffic belonging to different VLANs across the same physical link. This allows switches to maintain VLAN separation while sharing a single physical connection, which is essential for scalable network design and segmentation. When troubleshooting a trunk where some VLANs pass traffic and others do not, the first step is to verify the allowed VLAN list configured on the trunk port. Cisco switches permit administrators to restrict which VLANs are allowed to traverse a trunk link using the "switchport trunk allowed vlan" command. If a VLAN is missing from this list, traffic for that VLAN will be dropped even though the trunk link itself is operational. This selective filtering explains why only VLAN 40 fails while others succeed. A common exam trap is to confuse trunk operational status with VLAN membership issues. Seeing the trunk as "up" might mislead candidates to check unrelated settings like router IDs, NetFlow, or NTP synchronization, which do not affect VLAN forwarding on trunks. In practice, the allowed VLAN list is a frequent cause of partial VLAN failures on trunks, and understanding this helps avoid wasting time on irrelevant configurations.
KKey Concepts to Remember
- A VLAN trunk carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
- Cisco switches allow administrators to restrict VLANs on a trunk using the allowed VLAN list, which controls which VLAN traffic is forwarded across the trunk.
- If a VLAN is missing from the trunk's allowed VLAN list, traffic for that VLAN will fail even though the trunk link remains operational for other VLANs.
- The operational status of a trunk interface does not guarantee all VLANs are permitted; selective VLAN filtering can cause partial traffic failures.
- Troubleshooting VLAN traffic issues on trunks requires checking VLAN membership and allowed VLAN lists before investigating unrelated parameters like router IDs or NTP.
- The "show interfaces trunk" command displays which VLANs are allowed and active on a trunk, helping identify VLAN filtering problems quickly.
- A missing VLAN in the allowed VLAN list is a common cause of selective VLAN failure on trunks and is a frequent exam scenario in CCNA VLAN troubleshooting.
- Understanding the difference between trunk link status and VLAN membership is essential to avoid misdiagnosing VLAN traffic failures in Cisco networks.
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 carries multiple VLANs over a single physical link by tagging Ethernet frames with VLAN identifiers using 802.1Q encapsulation.
What is the correct answer to this question?
The correct answer is: 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.
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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