Question 1easymultiple choice
Full question →easymultiple choiceObjective-mapped
A network technician configures an 802.1Q trunk link between two switches. After the configuration, VLAN 20 traffic is not passing across the trunk, although VLAN 10 traffic works fine. The technician verifies that both switches have VLAN 20 created and that the trunk is up. 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
Distractor review
The native VLAN is mismatched between the switches.
A native VLAN mismatch causes issues for untagged frames, but it would not block a specific VLAN if the allowed list is properly configured.
B
Best answer
VLAN 20 is not included in the allowed VLAN list on the trunk.
By default, trunk ports allow all VLANs, but if an allowed VLAN list is configured, only listed VLANs are forwarded. VLAN 20 is likely missing.
C
Distractor review
The trunk port is set to dynamic desirable mode.
Dynamic Desirable is a DTP mode that negotiates trunking; it does not filter VLANs.
D
Distractor review
The encapsulation type is ISL instead of 802.1Q.
If both switches are configured for 802.1Q, the encapsulation matches, and this would not cause selective VLAN blocking.
Common exam trap
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Technical deep dive
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
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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 network engineer needs to connect two switches located 400 meters apart. The cable run includes high electromagnetic interference from nearby machinery. The engineer decides to use fiber optic cabling. Which transceiver type and fiber combination should be used to ensure the link reaches 400 meters while remaining cost-effective?
Question 2
A network engineer is designing a new switched network and needs to ensure that broadcast traffic from one department does not reach another department's workstations. The engineer plans to use VLANs. Which of the following must be configured on the switches to isolate broadcast domains as intended?
Question 3
A security engineer is configuring a site-to-site VPN between two branch offices. The requirement is to encrypt all traffic between the two networks using IPsec. Which IPsec mode should be used to encrypt the entire IP packet including the original header?
Question 4
A network administrator is connecting two switches to increase bandwidth and provide redundancy. Which technology should be used to combine multiple physical links into a single logical link?
Question 5
A network administrator is experiencing issues where unauthorized devices are offering IP addresses to clients, causing connectivity problems. Which security feature should be enabled on switches to prevent this?
Question 6
A network administrator is troubleshooting a connectivity issue and suspects the problem is related to the physical cabling. At which layer of the OSI model should the administrator begin their investigation?
FAQ
Questions learners often ask
What does this N10-009 question test?
Access ports place end devices into a single VLAN.
What is the correct answer to this question?
The correct answer is: VLAN 20 is not included in the allowed VLAN list on the trunk. — Trunk ports forward traffic for a set of allowed VLANs. If VLAN 20 is not explicitly included in the allowed VLAN list, frames from that VLAN are dropped. The native VLAN mismatch would affect untagged traffic, not specifically VLAN 20. DTP modes do not filter VLANs. ISL encapsulation would be a different issue if configured correctly.
What should I do if I get this N10-009 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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