Question 1hardmulti select
Full question →hardmulti selectObjective-mapped
Exhibit
Switch A trunk allowed VLANs: 10,20,30 Switch B trunk allowed VLANs: 10,20
A trunk link between two switches is up, but hosts in VLAN 30 on opposite switches cannot communicate. VLAN 10 works across the same trunk. Which two causes are the 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 not allowed on the trunk on one side
If the VLAN is missing from the allowed list on one side, traffic for that VLAN will not traverse the link.
B
Best answer
VLAN 30 may not exist in the VLAN database on the affected switch
The VLAN must exist locally to support access ports and forwarding.
C
Distractor review
The trunk native VLAN should always be 30
The native VLAN setting is unrelated to carrying tagged VLAN 30 traffic.
D
Distractor review
PortFast must be disabled on the access ports in VLAN 30
PortFast does not determine whether the trunk carries VLAN 30.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming the native VLAN must be VLAN 30 or that PortFast settings affect VLAN trunking. The native VLAN only applies to untagged frames and does not control whether VLAN 30 traffic is carried on the trunk. Similarly, PortFast affects STP port states on access ports but does not influence VLAN membership or trunking behavior. Candidates may incorrectly focus on these unrelated settings instead of verifying VLAN existence and allowed VLAN lists on trunks, leading to incorrect troubleshooting and exam answers.
Technical deep dive
How to think about this question
A VLAN (Virtual Local Area Network) segments a switched network logically, allowing devices in the same VLAN to communicate as if they were on the same physical network. When switches connect via a trunk link, multiple VLANs can traverse the same physical connection using tagging protocols like IEEE 802.1Q. For hosts in VLAN 30 on different switches to communicate, VLAN 30 must be configured and allowed on the trunk link, and the VLAN must exist in the VLAN database on both switches. Without these conditions, traffic for VLAN 30 will not be forwarded properly. The trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers. If VLAN 30 is not included in the allowed VLAN list on one side of the trunk, frames tagged for VLAN 30 will be dropped, preventing communication. Similarly, if VLAN 30 is not defined in the VLAN database on a switch, that switch cannot forward or recognize traffic for VLAN 30, causing connectivity issues. These two conditions are common causes when one VLAN fails to communicate across a trunk while others succeed. A common exam trap is confusing the native VLAN setting or PortFast configuration with VLAN trunking issues. The native VLAN only affects untagged frames and does not impact tagged VLAN 30 traffic. PortFast affects port state transitions and does not influence VLAN tagging or trunk membership. Misunderstanding these details can lead to incorrect troubleshooting steps. Practically, ensuring VLAN existence and trunk allowed VLAN lists are correct is the first step in resolving VLAN communication problems across trunks.
KKey Concepts to Remember
- A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
- A trunk link carries multiple VLANs by tagging frames, and only VLANs allowed on the trunk are forwarded across it.
- If a VLAN is not allowed on one side of a trunk, traffic for that VLAN will be blocked and not traverse the link.
- The native VLAN setting on a trunk affects untagged frames but does not impact tagged VLAN traffic like VLAN 30.
- PortFast enables immediate port forwarding on access ports but does not influence VLAN tagging or trunk VLAN membership.
- Switches drop frames for VLANs that are not configured locally, preventing communication for hosts in that VLAN.
- Troubleshooting VLAN communication issues across trunks requires verifying both VLAN existence and allowed VLAN lists on trunks.
- VLAN tagging protocols like IEEE 802.1Q enable multiple VLANs to share a single physical trunk link.
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
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FAQ
Questions learners often ask
What does this 200-301 question test?
A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
What is the correct answer to this question?
The correct answer is: VLAN 30 is not allowed on the trunk on one side — When one VLAN fails but others work across the same trunk, look first for VLAN-specific problems: whether the VLAN exists locally and whether it is allowed on the trunk.
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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