Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A switch trunk must carry VLANs 10, 20, and 30, but traffic for VLAN 20 is failing. The trunk allowed list on one side is `10,30`. What is the most likely cause?
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
VLAN 20 is missing from the allowed VLAN list on one side of the trunk.
This is correct because the trunk is explicitly not permitting VLAN 20 on that side.
B
Distractor review
The trunk must be converted to an access port for VLAN 20 to work.
This is wrong because a trunk is the correct inter-switch mechanism for carrying multiple VLANs.
C
Distractor review
VLAN 20 must always be the native VLAN.
This is wrong because a VLAN does not need to be native in order to traverse a trunk.
D
Distractor review
The switches must both use ISL instead of 802.1Q.
This is wrong because the key issue here is the allowed list, not a required change in encapsulation type.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is to assume that VLAN traffic failure on a trunk is caused by the native VLAN mismatch or the need to convert the trunk to an access port. Candidates may also mistakenly believe that encapsulation type must be changed to ISL for VLAN 20 to work. However, the real issue is the allowed VLAN list explicitly excluding VLAN 20 on one side. This selective filtering prevents VLAN 20 frames from passing, even though the trunk link itself is operational for other VLANs. Misidentifying the cause leads to incorrect configuration changes and wasted troubleshooting effort.
Technical deep dive
How to think about this question
A trunk link in Cisco networking is a point-to-point link between switches that carries traffic for multiple VLANs simultaneously. Trunks use tagging protocols like IEEE 802.1Q to identify VLAN membership of frames as they traverse the link. This allows switches to maintain VLAN separation across a shared physical connection, enabling devices in the same VLAN but on different switches to communicate. The trunk configuration includes specifying which VLANs are allowed to pass through the trunk, controlling VLAN traffic flow and security. When configuring trunks, the allowed VLAN list determines which VLANs can send traffic across the trunk link. If a VLAN is not included in the allowed list on either side of the trunk, traffic for that VLAN will be blocked and fail to traverse the link, even if the VLAN is active locally on both switches. This selective filtering is crucial for managing VLAN traffic and preventing unauthorized VLANs from crossing the trunk. The correct troubleshooting approach is to verify that the allowed VLAN lists match and include all required VLANs on both ends. A common exam trap is to confuse trunk operation with native VLAN or encapsulation type issues. While native VLAN mismatches or encapsulation differences can cause problems, the selective VLAN filtering caused by allowed VLAN lists is a frequent and specific cause of VLAN traffic failure. Practically, a trunk can appear operational for some VLANs but fail for others if the allowed list excludes certain VLANs. Understanding this selective filtering behavior helps avoid misdiagnosis and ensures VLAN traffic flows correctly across trunks.
KKey Concepts to Remember
- A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
- The allowed VLAN list on a trunk controls which VLANs can send traffic across the trunk link between switches.
- If a VLAN is missing from the allowed VLAN list on either side of the trunk, traffic for that VLAN will be blocked on the trunk.
- Trunks must have matching allowed VLAN lists on both ends to ensure VLAN traffic can traverse the link end to end.
- The native VLAN setting does not affect whether a VLAN is allowed on a trunk; it only determines untagged frame handling.
- Encapsulation type mismatches (e.g., ISL vs. 802.1Q) can cause trunk failures but are unrelated to VLAN allowed list filtering.
- A trunk can operate correctly for some VLANs while blocking traffic for others due to selective VLAN filtering in the allowed list.
- Troubleshooting VLAN traffic failures on trunks should start by verifying the allowed VLAN lists include all required VLANs.
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 trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
What is the correct answer to this question?
The correct answer is: VLAN 20 is missing from the allowed VLAN list on one side of the trunk. — The most likely cause is that VLAN 20 is not allowed across the trunk on that side. In plain language, the trunk is carrying some VLANs correctly, but one side has explicitly excluded VLAN 20 from the allowed list. That means VLAN 20 traffic cannot traverse the link end to end, even if the VLAN exists locally on both switches. This is one of the most common trunk troubleshooting patterns. A trunk can be operational overall while still failing for one specific VLAN because of the allowed list. The correct answer is the one that stays focused on that selective filtering behavior.
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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