Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Exhibit
SwitchA# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/2 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/2 10,20,30 SwitchB# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/2 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/2 10,20,30,50
Two switches are connected by a trunk. VLAN 50 exists on both switches, but hosts in VLAN 50 cannot communicate across the link. All other VLANs work. 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 50 is not allowed on the trunk from SwitchA.
This is correct because VLAN 50 is missing from SwitchA’s allowed list.
B
Distractor review
The native VLAN is mismatched.
This is wrong because both sides show native VLAN 1.
C
Distractor review
802.1Q cannot carry VLAN 50.
This is wrong because 802.1Q can carry normal VLAN tags, including VLAN 50.
D
Distractor review
VLAN 50 must be configured as the native VLAN first.
This is wrong because a VLAN does not need to be native to cross a trunk.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is to assume that if the trunk link is up and other VLANs are passing, then all VLANs must be allowed. Candidates often overlook the allowed VLAN list configuration, which can selectively block specific VLANs like VLAN 50. Another tempting mistake is blaming native VLAN mismatches or the 802.1Q protocol itself, but these do not cause selective VLAN blocking. The key is to focus on the allowed VLAN list on the trunk ports, as missing VLANs there cause partial connectivity failures rather than a complete trunk failure.
Technical deep dive
How to think about this question
A VLAN trunk is a point-to-point link between two switches that carries traffic for multiple VLANs simultaneously using tagging protocols such as IEEE 802.1Q. The trunk encapsulates frames with VLAN tags so that receiving switches can identify the VLAN membership of each frame and forward it appropriately. For hosts in the same VLAN to communicate across switches, the VLAN must be allowed and active on the trunk link. If a VLAN is not permitted on the trunk, frames tagged with that VLAN ID will be dropped or ignored, preventing inter-switch communication for that VLAN. Cisco switches use a default configuration that allows all VLANs on a trunk unless explicitly restricted by the "allowed VLAN" list. Network administrators can limit VLANs on trunks to reduce unnecessary traffic or for security reasons. When a VLAN is missing from the allowed VLAN list on one side of the trunk, frames tagged with that VLAN are filtered out, causing connectivity issues only for that VLAN. Troubleshooting such issues requires verifying the allowed VLAN lists on both trunk interfaces and ensuring VLAN consistency. The exam trap in this scenario is to confuse a trunk link being operational with all VLANs passing. The trunk can be up and carry traffic for many VLANs, but selective filtering of VLANs can cause partial failures. Candidates often mistake native VLAN mismatches or tagging protocol limitations as causes, but the key is the allowed VLAN list. Practically, this selective VLAN filtering is common in enterprise networks to control broadcast domains and enforce segmentation, making it a critical concept for CCNA candidates to master.
KKey Concepts to Remember
- A VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
- Cisco switches allow all VLANs on a trunk by default unless the allowed VLAN list is explicitly configured to restrict them.
- If a VLAN is missing from the allowed VLAN list on a trunk port, frames tagged with that VLAN are dropped and cannot cross the link.
- Hosts in the same VLAN on different switches require that VLAN to be allowed and active on the trunk connecting those switches.
- A trunk link can be operational and carry traffic for some VLANs while filtering out others due to allowed VLAN restrictions.
- Native VLAN mismatches do not prevent VLAN traffic from crossing a trunk unless the VLAN is explicitly filtered out.
- 802.1Q tagging supports all VLAN IDs from 1 to 4094, so it does not inherently block any VLAN traffic.
- Selective VLAN filtering on trunks is a common network design practice to control broadcast domains and improve security.
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 VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
What is the correct answer to this question?
The correct answer is: VLAN 50 is not allowed on the trunk from SwitchA. — The trunk itself is up, but VLAN 50 is missing from the allowed VLAN list on one side. In practical terms, this is a selective trunking failure rather than a complete one. Since other VLANs are already crossing successfully, the trunk is not generally down. The problem is that one particular VLAN is being filtered out. This is a common switching exam scenario because it checks whether you can separate link health from VLAN permission. The correct answer is the one focused on the missing VLAN in the allowed list.
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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