The answer is IEEE 802.1Q, the standard that defines VLAN tagging on Ethernet trunks. This standard inserts a 4-byte tag into the Ethernet frame header to identify VLAN membership, allowing multiple VLANs to traverse a single link between a switch and a router without needing separate physical interfaces for each VLAN. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of trunking and inter-VLAN routing, often appearing in scenario-based questions where you must choose the correct protocol for carrying multiple VLANs over one link. A common trap is confusing 802.1Q with ISL, a Cisco proprietary alternative that is no longer on the exam; remember that 802.1Q is the open, industry-standard method. For a quick memory tip, think of the “Q” in 802.1Q as standing for “Queue” or “Quad-byte tag,” since it adds exactly four bytes to the frame.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network administrator is configuring a new switch to carry traffic for multiple VLANs on a single link to a router. Which IEEE standard is used for VLAN tagging on Ethernet trunks?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
802.1Q
802.1Q is the IEEE standard that defines VLAN tagging on Ethernet trunks, inserting a 4-byte tag into the Ethernet frame to identify VLAN membership. This allows multiple VLANs to traverse a single link between a switch and a router, enabling inter-VLAN routing without separate physical interfaces.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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802.3af
Why it's wrong here
802.3af is the Power over Ethernet (PoE) standard, not related to VLAN tagging.
802.1Q is the standard for VLAN tagging, allowing switches to identify which VLAN a frame belongs to across a trunk link.
Related concept
Read the scenario before looking for a memorised answer.
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802.11ac
Why it's wrong here
802.11ac is a wireless networking standard, not used for Ethernet VLAN tagging.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between 802.1Q (tagging) and 802.1D (STP), leading candidates to confuse VLAN trunking with loop prevention protocols.
Detailed technical explanation
How to think about this question
The 802.1Q tag is inserted between the Source MAC and EtherType fields, increasing the frame size from 1518 to 1522 bytes. The tag contains a 12-bit VLAN ID (VID) allowing up to 4094 VLANs, and a 3-bit Priority Code Point (PCP) for Class of Service (CoS). In a router-on-a-stick configuration, the router's subinterfaces must match the native VLAN (untagged) and the allowed VLANs (tagged) to correctly process traffic.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 802.1Q — 802.1Q is the IEEE standard that defines VLAN tagging on Ethernet trunks, inserting a 4-byte tag into the Ethernet frame to identify VLAN membership. This allows multiple VLANs to traverse a single link between a switch and a router, enabling inter-VLAN routing without separate physical interfaces.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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