N10-009 Network Troubleshooting Practice Question
After replacing a faulty switch, several users in the same VLAN report that they cannot communicate with the server that is on a different subnet. The switch is connected to the router via a trunk port. Which command should the administrator run on the router to verify that the VLAN is allowed on the trunk?
⚠ Common exam trap
Test-takers frequently confuse `show vlan` (which shows local switch VLANs) with `show interfaces trunk` (which shows VLAN filtering on the trunk), leading them to pick A when the real issue is trunk permission, not VLAN existence.
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
✓
show interfaces trunk
The `show interfaces trunk` command displays which VLANs are allowed on each trunk port. Since the switch is connected to the router via a trunk, this command verifies whether the VLAN of the affected users is permitted on that trunk. If the VLAN is missing from the allowed list, traffic to the server on a different subnet will be dropped at the router interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
show vlan
Why it's wrong here
The `show vlan` command displays the local VLAN database on the switch, outlining which access ports are assigned to specific VLANs and the overall status of each VLAN. While it confirms the existence of a VLAN and its local port memberships, it does not provide any information about the configuration of trunk links between switches, nor does it indicate which VLANs are permitted to traverse those trunks. Therefore, it cannot identify if a VLAN is blocked from passing between switches.
When this WOULD be correct
If the question asked 'Which command on a switch displays the VLANs configured and their port assignments?' then 'show vlan' would be correct, as it lists VLANs and associated ports on the switch.
- ✓
show interfaces trunk
Why this is correct
The `show interfaces trunk` command is indispensable for diagnosing inter-switch connectivity issues affecting specific VLANs. It provides a comprehensive overview of all trunking interfaces on the switch, detailing their operational status, encapsulation type (e.g., IEEE 802.1Q), and, critically, the list of VLANs explicitly allowed to traverse each trunk link. If the VLAN where users are experiencing problems is not included in the allowed list on the trunk connecting the new switch to the network, traffic for that VLAN will be dropped, preventing communication.
- ✗
show mac address-table
Why it's wrong here
The `show mac address-table` command reveals the MAC addresses learned by the switch, indicating which MACs are reachable via specific ports and VLANs. While a lack of learned MAC addresses for affected users might suggest a connectivity problem, this command primarily reflects the *result* of traffic flow, not the underlying configuration issue preventing that flow. It does not provide details about trunk link configuration or the allowed VLANs, which is the likely culprit after a switch replacement.
When this WOULD be correct
This command would be correct when troubleshooting connectivity issues within the same VLAN, such as verifying which switch port a device is connected to or checking for MAC address flapping. For example, after replacing a switch, if users in the same VLAN cannot communicate with each other, 'show mac address-table' helps confirm MAC learning on the new switch.
- ✗
show ip route
Why it's wrong here
The `show ip route` command displays the IP routing table, which is primarily relevant for Layer 3 communication between different IP subnets or VLANs. Since the problem describes users in the *same* VLAN experiencing issues, their communication within that VLAN is a Layer 2 function. While a trunking issue could indirectly impact inter-VLAN routing by preventing a VLAN's Switched Virtual Interface (SVI) from being reachable, this command provides no information about the Layer 2 trunk configuration itself, making it unsuitable for diagnosing a direct trunking problem.
When this WOULD be correct
This command would be correct if the question asked about verifying that the router has a route to the server's subnet, for example: 'After replacing a faulty switch, users cannot reach a server on a different subnet. Which command verifies the router has a route to that subnet?'
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.
✓show interfaces trunkCorrect answer▾
Why this is correct
The `show interfaces trunk` command is indispensable for diagnosing inter-switch connectivity issues affecting specific VLANs. It provides a comprehensive overview of all trunking interfaces on the switch, detailing their operational status, encapsulation type (e.g., IEEE 802.1Q), and, critically, the list of VLANs explicitly allowed to traverse each trunk link. If the VLAN where users are experiencing problems is not included in the allowed list on the trunk connecting the new switch to the network, traffic for that VLAN will be dropped, preventing communication.
✗show vlanWrong answer — click to see why▾
Why this is wrong here
The 'show vlan' command displays VLAN information on the switch, not the router. The question specifies verifying VLAN allowed on a trunk from the router, which requires 'show interfaces trunk' on the router or switch.
★ When this WOULD be the correct answer
If the question asked 'Which command on a switch displays the VLANs configured and their port assignments?' then 'show vlan' would be correct, as it lists VLANs and associated ports on the switch.
Why candidates choose this
Candidates may confuse VLAN configuration commands with trunk verification commands, or assume 'show vlan' shows trunk information because it relates to VLANs.
✗show mac address-tableWrong answer — click to see why▾
Why this is wrong here
The 'show mac address-table' command displays MAC address forwarding table entries, not VLAN trunking information. It cannot verify whether a specific VLAN is allowed on a trunk port.
★ When this WOULD be the correct answer
This command would be correct when troubleshooting connectivity issues within the same VLAN, such as verifying which switch port a device is connected to or checking for MAC address flapping. For example, after replacing a switch, if users in the same VLAN cannot communicate with each other, 'show mac address-table' helps confirm MAC learning on the new switch.
Why candidates choose this
Candidates may confuse MAC address table with VLAN trunking, thinking that checking MAC addresses on the trunk will reveal VLAN membership, or they may mistakenly believe the trunk's allowed VLAN list is derived from MAC addresses.
✗show ip routeWrong answer — click to see why▾
Why this is wrong here
The 'show ip route' command displays the routing table, which is used for determining the path to reach different subnets. It does not show VLAN information or trunk port configurations, so it cannot verify whether a specific VLAN is allowed on a trunk.
★ When this WOULD be the correct answer
This command would be correct if the question asked about verifying that the router has a route to the server's subnet, for example: 'After replacing a faulty switch, users cannot reach a server on a different subnet. Which command verifies the router has a route to that subnet?'
Why candidates choose this
Candidates may think that since the server is on a different subnet, the issue must be related to routing, so they choose a routing verification command without considering that the problem might be at Layer 2 on the trunk link.
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?”
Visual reference
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Related to this question
Learn chapter
Interface Error Types: CRC, Runts, Giants
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
Trunk port
A trunk port is a switch port configured to carry traffic for multiple VLANs, using a tagging protocol to identify which VLAN each frame belongs to.
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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.