Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A switchport on one side of a link is configured as a trunk, but the peer side is configured as an access port. The physical link is up, but VLAN traffic behaves unexpectedly. 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
The two ends disagree on whether the link is a trunk or an access port.
This is correct because trunk/access mismatch causes VLAN handling problems.
B
Distractor review
The switches must both use the same hostname.
This is wrong because hostnames do not affect trunk versus access behavior.
C
Distractor review
The native VLAN must be set to 1 on both sides first.
This is wrong because the deeper issue is the trunk/access role mismatch itself.
D
Distractor review
The ports need OSPF enabled.
This is wrong because OSPF is not relevant to this Layer 2 switchport mismatch.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that because the physical link is up, the VLAN configuration must be correct. Candidates may mistakenly focus on native VLAN mismatches or routing protocols like OSPF, which are irrelevant at Layer 2 for switchport mode issues. The key mistake is overlooking that a trunk port expects tagged frames while an access port sends untagged frames, causing VLAN traffic to be mishandled even though the link is operational. This trap can lead to wasted time troubleshooting unrelated settings instead of verifying port mode consistency.
Technical deep dive
How to think about this question
A switchport configured as a trunk is designed to carry traffic for multiple VLANs by tagging frames with VLAN identifiers using the IEEE 802.1Q standard. This allows a single physical link to transport traffic from many VLANs simultaneously. Conversely, an access port is assigned to a single VLAN and does not tag frames, treating all traffic as belonging to that VLAN. When one side of a link is set as a trunk and the other as an access port, the two ends have incompatible expectations about frame tagging and VLAN handling. Cisco switches enforce strict VLAN tagging rules based on the port mode configuration. If one side expects tagged frames (trunk) but the other side sends untagged frames (access), VLAN traffic will not be properly forwarded or segregated. The physical link may still come up because Layer 1 and basic Layer 2 link integrity checks succeed, but the logical Layer 2 VLAN separation fails, causing unexpected traffic behavior and connectivity issues. This mismatch is a common exam trap because the link appears operational, misleading candidates to suspect other issues like native VLAN mismatches or routing protocols. However, the root cause is the fundamental disagreement on port mode. In practice, this misconfiguration can cause VLAN leaks, broadcast storms, or dropped traffic, making it critical to verify that both ends of a link agree on trunk or access mode to ensure proper VLAN traffic forwarding.
KKey Concepts to Remember
- A trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation.
- An access port belongs to a single VLAN and sends untagged frames without VLAN tags.
- Switchports must be configured consistently on both ends as either trunk or access to ensure proper VLAN traffic handling.
- A trunk/access mismatch causes VLAN traffic to be misinterpreted, leading to connectivity and forwarding problems despite the physical link being up.
- Cisco switches do not automatically resolve trunk/access mismatches, so manual configuration alignment is required.
- The native VLAN setting affects untagged frames on trunk ports but does not fix trunk/access mode mismatches.
- Layer 2 protocols like STP operate independently of trunk/access mismatches but rely on correct VLAN tagging for proper topology.
- Physical link status does not guarantee correct VLAN forwarding; logical configuration consistency is essential for network stability.
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 carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation.
What is the correct answer to this question?
The correct answer is: The two ends disagree on whether the link is a trunk or an access port. — The most likely cause is a switchport mode mismatch. In practical terms, one side expects the link to carry multiple VLANs with tagging behavior, while the other side treats it as a normal one-VLAN endpoint-style access connection. The physical interface can still come up, but the two ends do not agree on how the traffic should be handled. This is a classic Layer 2 troubleshooting pattern. The link may not be fully down, but the configuration disagreement causes logical forwarding problems.
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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