Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
SW1 show interfaces trunk Port Vlans allowed on trunk: 10,30 SW2 show interfaces trunk Port Vlans allowed on trunk: 10
Exhibit: Two switches are connected by an 802.1Q trunk. Hosts in VLAN 30 on opposite sides cannot communicate, but VLAN 10 works. 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 30 is pruned or not allowed on one side of the trunk
That directly explains why VLAN 10 works and VLAN 30 does not.
B
Distractor review
The native VLAN must be set to 30
User VLAN traffic does not need to be the native VLAN.
C
Distractor review
STP blocks all VLANs on the trunk when one VLAN fails
STP is per-VLAN or per-instance and would not automatically block all VLANs because one VLAN has an allow-list issue.
D
Distractor review
The trunk must be changed to access mode for VLAN 30
That would break the multi-VLAN link entirely.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that the native VLAN must be set to the VLAN experiencing connectivity issues, such as VLAN 30 in this case. The native VLAN is simply the untagged VLAN on a trunk and does not need to match the user VLAN for communication to work. Another trap is thinking that Spanning Tree Protocol (STP) blocks all VLANs on a trunk if one VLAN fails; STP runs per VLAN or per instance and would not block unrelated VLANs. Lastly, switching the trunk to access mode for a single VLAN breaks multi-VLAN trunking, which is another common misconception.
Technical deep dive
How to think about this question
802.1Q trunking is a Cisco and IEEE standard method that allows multiple VLANs to traverse a single physical link between switches by tagging Ethernet frames with VLAN identifiers. This tagging enables switches to segregate traffic logically, ensuring that frames belonging to different VLANs remain isolated even when sharing the same trunk link. The native VLAN is untagged by default, but all other VLANs must be explicitly allowed on the trunk to pass traffic. When configuring trunks, each side must agree on which VLANs are allowed to traverse the link. Cisco switches use VLAN pruning or allowed VLAN lists to restrict which VLANs can send traffic over the trunk. If a VLAN is not included in the allowed VLAN list on one side, frames tagged with that VLAN ID are dropped, preventing communication for hosts in that VLAN. This explains why VLAN 10 communication works while VLAN 30 fails if VLAN 30 is pruned or disallowed on one side. A common exam trap is confusing native VLAN settings or Spanning Tree Protocol (STP) behavior with VLAN pruning issues. Native VLAN mismatches do not block VLAN traffic but can cause security or tagging problems. STP operates per VLAN and does not block all VLANs due to one VLAN's failure. Understanding the allowed VLAN list on trunks and verifying VLAN membership is critical for troubleshooting VLAN connectivity issues in Cisco networks.
KKey Concepts to Remember
- 802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
- Each side of a trunk link must allow the same VLANs for traffic to pass between switches for those VLANs.
- If a VLAN is pruned or not allowed on one side of the trunk, traffic for that VLAN is dropped, causing communication failure.
- The native VLAN is untagged on a trunk and does not need to match the VLAN experiencing connectivity issues.
- Spanning Tree Protocol operates per VLAN and does not block all VLANs on a trunk due to one VLAN's failure.
- Changing a trunk port to access mode disables multi-VLAN trunking and breaks connectivity for all VLANs except the access VLAN.
- Cisco switches use VLAN pruning and allowed VLAN lists to control which VLANs can traverse a trunk link.
- Verifying allowed VLANs on trunks is essential for troubleshooting VLAN connectivity issues 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?
802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
What is the correct answer to this question?
The correct answer is: VLAN 30 is pruned or not allowed on one side of the trunk — If a VLAN is not allowed on one side of the trunk, traffic for that VLAN is dropped even though other VLANs continue to work. The output shows VLAN 30 missing from the allowed list on one side.
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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