N10-009 Network Implementation Practice Question
A network administrator connects two switches with a trunk link that is configured to allow all VLANs. Workstations in VLAN 10 can communicate across the switches, but workstations in VLAN 20 cannot. Both VLANs are configured on the first switch. What is the most likely cause of the issue?
⚠ Common exam trap
The N10-009 exam often tests the misconception that a trunk configured to 'allow all VLANs' automatically makes every VLAN functional across the link, when in fact each switch must have the VLAN defined in its local database for traffic to be processed.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 20 is not created on the second switch.
VLAN 20 is configured on the first switch but not on the second switch. Even though the trunk link allows all VLANs, the second switch must have VLAN 20 created in its VLAN database for traffic to be forwarded. Without the VLAN existing on the second switch, frames tagged for VLAN 20 are dropped at the receiving switch because there is no corresponding VLAN interface or forwarding table entry.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The native VLAN is mismatched on the two switches.
Why it's wrong here
A native VLAN mismatch occurs when the untagged VLAN configured on one end of a trunk link differs from the other. This specific configuration error primarily impacts untagged frames, which are typically associated with the native VLAN, and can lead to security vulnerabilities or communication failures for devices in that particular VLAN. However, it would not selectively prevent tagged frames for a specific VLAN, such as VLAN 20, from traversing the trunk if VLAN 20 itself is properly tagged and allowed.
When this WOULD be correct
This would be correct if the question stated that all VLANs fail to communicate across the trunk, or if workstations in the native VLAN (e.g., VLAN 1) cannot communicate while other VLANs work. A native VLAN mismatch disrupts control traffic and untagged frames.
- ✓
VLAN 20 is not created on the second switch.
Why this is correct
When a switch receives a frame on a trunk link tagged with a specific VLAN ID, it expects that VLAN to be locally defined in its VLAN database. If VLAN 20 is not created on the second switch, the switch lacks the necessary context to process frames belonging to that VLAN, effectively discarding them. This prevents any devices connected to the second switch from participating in VLAN 20, even if the trunk link is otherwise operational.
- ✗
The trunk uses ISL instead of 802.1Q.
Why it's wrong here
Inter-Switch Link (ISL) and IEEE 802.1Q are two distinct trunking protocols designed to carry traffic for multiple VLANs over a single physical link. As long as both switches are configured to use the same trunking protocol (either ISL or 802.1Q) and that protocol is supported by their hardware, the choice between them does not inherently cause a specific VLAN, like VLAN 20, to fail while others function. A mismatch in trunking protocols would typically prevent the trunk from forming or passing any tagged traffic at all, not selectively affect one VLAN.
When this WOULD be correct
This would be correct if the question stated that the trunk is configured with ISL encapsulation and the switches do not support ISL, or if the question specified that ISL is incompatible with certain VLANs or switch models, causing connectivity issues for specific VLANs.
- ✗
The default gateway is missing for VLAN 20.
Why it's wrong here
A default gateway is an IP address configured on a Layer 3 device (like a router or a Layer 3 switch) that allows devices within a VLAN to communicate with devices in different VLANs or external networks. For devices within the same VLAN 20 to communicate across two switches connected by a trunk link, a default gateway is not required. Intra-VLAN communication relies solely on Layer 2 forwarding, where frames are switched based on MAC addresses within the broadcast domain, making the absence of a gateway irrelevant for this specific problem.
When this WOULD be correct
This option would be correct in a scenario where workstations in VLAN 20 cannot communicate with devices in other VLANs or the internet, but can communicate within VLAN 20 across switches. The question would specify that inter-VLAN routing is failing, and the default gateway is misconfigured or missing.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓VLAN 20 is not created on the second switch.Correct answer▾
Why this is correct
When a switch receives a frame on a trunk link tagged with a specific VLAN ID, it expects that VLAN to be locally defined in its VLAN database. If VLAN 20 is not created on the second switch, the switch lacks the necessary context to process frames belonging to that VLAN, effectively discarding them. This prevents any devices connected to the second switch from participating in VLAN 20, even if the trunk link is otherwise operational.
✗The native VLAN is mismatched on the two switches.Wrong answer — click to see why▾
Why this is wrong here
A native VLAN mismatch would cause issues for untagged traffic on the trunk, but VLAN 20 is a tagged VLAN. Since VLAN 10 works, the trunk is functioning; the problem is specific to VLAN 20, which is likely not present on the second switch.
★ When this WOULD be the correct answer
This would be correct if the question stated that all VLANs fail to communicate across the trunk, or if workstations in the native VLAN (e.g., VLAN 1) cannot communicate while other VLANs work. A native VLAN mismatch disrupts control traffic and untagged frames.
Why candidates choose this
Candidates often confuse native VLAN mismatch symptoms with VLAN absence, as both can cause connectivity issues. They may assume trunk misconfiguration is the default cause without verifying VLAN existence on both switches.
✗The trunk uses ISL instead of 802.1Q.Wrong answer — click to see why▾
Why this is wrong here
The trunk uses 802.1Q, not ISL, in modern networks. Even if ISL were used, it would affect all VLANs, not just VLAN 20, so it cannot explain why only VLAN 20 fails.
★ When this WOULD be the correct answer
This would be correct if the question stated that the trunk is configured with ISL encapsulation and the switches do not support ISL, or if the question specified that ISL is incompatible with certain VLANs or switch models, causing connectivity issues for specific VLANs.
Why candidates choose this
Candidates may confuse trunking protocol mismatches as a cause for selective VLAN failure, not realizing that a protocol mismatch would affect all VLANs equally, not just one.
✗The default gateway is missing for VLAN 20.Wrong answer — click to see why▾
Why this is wrong here
The issue is that VLAN 20 works on the first switch but not across the trunk, indicating the VLAN is missing on the second switch. A missing default gateway would prevent inter-VLAN routing but not affect Layer 2 communication within the same VLAN across switches.
★ When this WOULD be the correct answer
This option would be correct in a scenario where workstations in VLAN 20 cannot communicate with devices in other VLANs or the internet, but can communicate within VLAN 20 across switches. The question would specify that inter-VLAN routing is failing, and the default gateway is misconfigured or missing.
Why candidates choose this
Candidates often confuse Layer 2 VLAN issues with Layer 3 routing problems. They may think that because VLAN 20 cannot communicate across switches, it must be a routing issue, especially if they have seen scenarios where a missing default gateway causes connectivity failures.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.