Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
Two switches are connected by a trunk. VLAN 50 exists on both switches, but traffic still fails across the link. The allowed VLAN list is correct. Which additional item should be checked next?
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
Check for a trunk mismatch such as native VLAN inconsistency or other trunk-parameter problems.
This is correct because VLAN permission alone does not guarantee the trunk is healthy end to end.
B
Distractor review
Reset OSPF process IDs on both switches.
This is wrong because OSPF process IDs are unrelated to a Layer 2 trunk issue.
C
Distractor review
Add ip helper-address under every access interface.
This is wrong because DHCP relay does not solve a trunk transport problem.
D
Distractor review
Disable the MAC address table.
This is wrong because MAC table operation is fundamental to switching and not the right next troubleshooting step.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is selecting options related to Layer 3 protocols like OSPF or DHCP relay when VLAN traffic fails on a trunk. Candidates might incorrectly think resetting OSPF or adding ip helper-address commands will fix the issue, but these do not influence Layer 2 trunk operation. Another trap is disabling the MAC address table, which is essential for frame forwarding and would cause more severe problems. The key mistake is overlooking trunk mismatches such as native VLAN inconsistencies, which are the actual root cause of VLAN traffic failure despite correct VLAN existence and allowed lists.
Technical deep dive
How to think about this question
A trunk link in Cisco networking is a Layer 2 connection between switches that carries traffic for multiple VLANs simultaneously. It uses tagging protocols like IEEE 802.1Q to insert VLAN identifiers into Ethernet frames, enabling switches to segregate and forward traffic correctly across VLAN boundaries. The native VLAN is a special VLAN whose frames are sent untagged on the trunk; both ends of the trunk must agree on the native VLAN to avoid misinterpretation of untagged frames. When troubleshooting VLAN traffic failures on a trunk, verifying that the VLAN exists and is allowed is necessary but not sufficient. The trunk must be operational with matching encapsulation types (e.g., 802.1Q) and consistent native VLAN settings on both switches. A mismatch in native VLAN or trunk parameters can cause frames to be dropped or misrouted, even if the VLAN is permitted. This layered verification approach ensures the trunk is healthy end to end before considering unrelated issues. A common exam trap is to jump to Layer 3 or service-related configurations such as OSPF process IDs or DHCP relay settings when VLAN traffic fails on a trunk. These do not affect Layer 2 trunk operation. Another mistake is disabling the MAC address table, which is fundamental to switching and would cause broader network disruption. The practical takeaway is to focus troubleshooting on trunk-specific parameters like native VLAN and encapsulation consistency before exploring other layers or features.
KKey Concepts to Remember
- A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers to maintain VLAN separation across switches.
- The native VLAN on a trunk must match on both ends to prevent untagged frames from being misclassified or dropped.
- Trunk encapsulation types such as IEEE 802.1Q must be consistent on both switches to ensure proper VLAN tagging and frame forwarding.
- Allowed VLAN lists on trunks control which VLANs can traverse the link but do not guarantee trunk operational status or consistency.
- A trunk mismatch, including native VLAN inconsistency or encapsulation differences, can cause VLAN traffic to fail despite correct VLAN configuration.
- Layer 2 protocols like VLAN tagging and trunk negotiation operate independently from Layer 3 routing protocols such as OSPF.
- MAC address tables are essential for switching decisions but disabling them disrupts normal frame forwarding and is not a troubleshooting step for trunk issues.
- DHCP relay configuration (ip helper-address) affects Layer 3 services and does not resolve Layer 2 trunk connectivity problems.
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 link carries traffic for multiple VLANs by tagging frames with VLAN identifiers to maintain VLAN separation across switches.
What is the correct answer to this question?
The correct answer is: Check for a trunk mismatch such as native VLAN inconsistency or other trunk-parameter problems. — After confirming that the VLAN exists on both switches and is allowed on the trunk, another important item to verify is whether the trunk itself is actually operational with the expected encapsulation and whether there is a native VLAN or other trunk inconsistency. In plain language, just because the VLAN is listed does not guarantee the trunk is healthy in every relevant way. Trunking problems can still occur because of broader configuration mismatches. This question is about disciplined troubleshooting. Once the obvious allowed-list issue is ruled out, the next step is to keep checking other trunk-related characteristics rather than jumping immediately to unrelated routing or service features. The correct answer is the one that stays grounded in trunk-specific verification.
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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