Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Switch SW1 sends traffic for VLAN 30 across a trunk to SW2, but hosts in VLAN 30 on SW2 cannot communicate with hosts in VLAN 30 on SW1. Other VLANs work across the trunk. Which trunk issue is most likely?
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 30 is pruned or missing from the allowed VLAN list
Native VLAN settings can matter, but they do not best explain why other VLANs still work while VLAN 30 alone fails.
B
Distractor review
The native VLAN is set to 1 on both switches
Correct. VLAN 30 likely is not being carried across the trunk.
C
Distractor review
The trunk uses 802.1Q encapsulation
Using 802.1Q is normal and not the cause here.
D
Distractor review
SW1 is the STP root bridge
Being the STP root bridge does not by itself block one VLAN on a trunk.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that native VLAN mismatches or STP root bridge roles cause a single VLAN to fail across a trunk. Candidates may incorrectly select native VLAN settings because they know it affects trunk behavior, but native VLAN mismatches typically impact untagged traffic or cause broader connectivity issues, not just one VLAN. Similarly, believing that the STP root bridge status blocks VLAN traffic is incorrect; STP manages loop prevention and port states but does not selectively block VLANs on trunks. The key trap is overlooking VLAN pruning or allowed VLAN list misconfigurations, which specifically cause one VLAN to be missing while others work fine.
Technical deep dive
How to think about this question
VLAN trunks carry multiple VLANs between switches using tagging protocols like IEEE 802.1Q. Each VLAN's traffic is tagged with a VLAN ID so that the receiving switch can segregate and forward frames to the correct VLAN. The trunk link must allow all VLANs that need to communicate across it; otherwise, traffic for missing VLANs will be dropped. Switches maintain a list of allowed VLANs on trunks, and pruning or misconfiguration can exclude specific VLANs from the trunk. When a VLAN is missing from the allowed VLAN list on a trunk, frames tagged with that VLAN ID are not forwarded across the link. This causes hosts in that VLAN on one switch to be unable to communicate with hosts in the same VLAN on the other switch, even though other VLANs function normally. The native VLAN setting affects untagged frames but does not selectively block a single VLAN's tagged traffic. Similarly, using 802.1Q encapsulation is standard and does not cause selective VLAN failures. A common exam trap is to confuse native VLAN mismatches or STP root bridge roles with VLAN pruning issues. Native VLAN mismatches typically cause untagged frame drops or VLAN hopping risks but do not isolate a single tagged VLAN while leaving others functional. STP root bridge status affects path selection and blocking ports but does not block VLANs on trunks. The practical impact of missing VLANs on trunks is a silent failure where traffic for that VLAN never reaches the other switch, causing communication failures isolated to that VLAN.
KKey Concepts to Remember
- A trunk port forwards traffic for multiple VLANs by tagging frames with VLAN IDs using protocols like IEEE 802.1Q.
- Switches maintain an allowed VLAN list on trunks that determines which VLANs are permitted to cross the trunk link.
- If a VLAN is pruned or missing from the allowed VLAN list, traffic for that VLAN is blocked from crossing the trunk.
- Native VLAN settings affect untagged frames but do not selectively block tagged VLAN traffic on trunks.
- Spanning Tree Protocol (STP) root bridge status influences port roles and states but does not block individual VLANs on trunks.
- Using 802.1Q encapsulation is standard for VLAN tagging and does not cause selective VLAN communication failures.
- A missing VLAN in the allowed VLAN list causes isolated VLAN communication failure despite other VLANs working normally.
- Troubleshooting VLAN issues on trunks requires verifying allowed VLAN lists and pruning configurations to ensure all needed VLANs are permitted.
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 port forwards traffic for multiple VLANs by tagging frames with VLAN IDs using protocols like IEEE 802.1Q.
What is the correct answer to this question?
The correct answer is: VLAN 30 is pruned or missing from the allowed VLAN list — If only one VLAN fails across an otherwise healthy trunk, a missing or filtered VLAN in the allowed list is a common cause. Native VLAN matching and encapsulation would affect broader trunk behavior, not usually just one VLAN in this way.
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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