Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
SwitchA# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,40 SwitchB# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30,40
PCs in VLAN 30 on SwitchA cannot reach PCs in VLAN 30 on SwitchB. VLAN 30 exists on both switches and all other VLANs work across the same link. Based on the exhibit, 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 not allowed on the trunk from SwitchA.
This is correct because VLAN 30 is missing from SwitchA’s allowed list.
B
Distractor review
The native VLAN is mismatched.
This is wrong because both switches show native VLAN 1.
C
Distractor review
The trunk must use ISL instead of 802.1Q.
This is wrong because 802.1Q is already working for the other VLANs.
D
Distractor review
VLAN 30 must be configured as the native VLAN.
This is wrong because a VLAN does not have to be native to cross a trunk.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that a trunk link failure or native VLAN mismatch is the cause of VLAN connectivity issues when only one VLAN is affected. Candidates may mistakenly think that the trunk encapsulation protocol must be changed or that the native VLAN must be set to the problematic VLAN. However, if other VLANs pass successfully, the trunk is operational, and the native VLAN mismatch is irrelevant. The real issue is often that the VLAN is not included in the trunk’s allowed VLAN list, causing selective blocking. Overlooking this selective filtering leads to incorrect troubleshooting and answer selection.
Technical deep dive
How to think about this question
VLAN trunks are essential in Cisco networks to carry multiple VLANs across a single physical link between switches. The trunk link uses tagging protocols like IEEE 802.1Q to identify VLAN frames so that traffic from different VLANs remains logically separated while traversing the same cable. Each switch must be configured to allow the VLANs that need to pass through the trunk. If a VLAN is not included in the allowed VLAN list on a trunk port, frames tagged with that VLAN ID will be dropped and not forwarded across the link. In this scenario, the trunk between SwitchA and SwitchB is operational, and other VLANs are passing correctly, indicating that the physical and logical trunk configuration is mostly correct. However, VLAN 30 is missing from the allowed VLAN list on SwitchA’s trunk port, causing selective blocking of VLAN 30 traffic. Cisco switches enforce VLAN filtering on trunks based on the allowed VLAN list, so even if the VLAN exists on both switches, it must be explicitly permitted on the trunk to pass traffic. A common exam trap is to confuse native VLAN mismatches or trunk encapsulation types as the cause of VLAN connectivity issues. While native VLAN mismatches can cause untagged traffic problems, they do not selectively block tagged VLANs. Similarly, 802.1Q is the standard trunking protocol and works fine if other VLANs pass. The practical takeaway is to always verify the allowed VLAN list on trunk ports when VLAN-specific connectivity issues arise, as this is a frequent cause of partial VLAN reachability failures in Cisco networks.
KKey Concepts to Remember
- A trunk port forwards traffic for VLANs that are explicitly allowed in its allowed VLAN list configuration.
- If a VLAN is not included in the allowed VLAN list on a trunk port, traffic for that VLAN is blocked from crossing the trunk.
- The native VLAN is used for untagged frames and does not affect the forwarding of tagged VLAN traffic on a trunk.
- 802.1Q is the standard trunk encapsulation protocol and supports multiple VLANs on a single link between Cisco switches.
- VLANs must be configured and active on both ends of a trunk to enable communication between devices in the same VLAN.
- A trunk link can be operational and carry some VLANs while selectively blocking others due to allowed VLAN filtering.
- Exam scenarios often test the ability to distinguish between trunk operational status and per-VLAN forwarding issues.
- Misconfiguring allowed VLAN lists on trunks is a common cause of VLAN connectivity problems in Cisco switching environments.
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 VLANs that are explicitly allowed in its allowed VLAN list configuration.
What is the correct answer to this question?
The correct answer is: VLAN 30 is not allowed on the trunk from SwitchA. — The trunk is up, but VLAN 30 is missing from the allowed list on SwitchA. In plain language, the hallway between the switches is open, but one side is refusing to carry that specific VLAN through the hallway. Since the other VLANs are working, the failure is selective rather than total. That strongly points to an allowed-VLAN problem rather than a broader trunk outage. This is a classic CCNA switching scenario because it tests whether you can separate trunk health from per-VLAN forwarding. A trunk can be operational and still block one VLAN if that VLAN is not permitted on one side. The native VLAN and encapsulation are not the issue shown here — the mismatch in the allowed list is.
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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