This 200-901 practice question tests your understanding of network fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 engineer is configuring a new switch and needs to ensure that frames from VLAN 10 and VLAN 20 are isolated on the same trunk link to another switch. Which IEEE standard should be configured on the trunk interfaces?
802.3
Why wrong: 802.3 is the Ethernet standard, not for VLAN trunking.
802.11
Why wrong: 802.11 is for wireless LANs, not trunking.
802.1Q
802.1Q is the IEEE standard for VLAN tagging on trunk links.
802.1X
Why wrong: 802.1X is for network access control, not trunking.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
802.1Q
C is correct because 802.1Q is the IEEE standard that defines VLAN tagging, allowing multiple VLANs (such as VLAN 10 and VLAN 20) to be carried over a single trunk link while maintaining isolation between them. By inserting a 4-byte VLAN tag into the Ethernet frame, 802.1Q enables the receiving switch to identify which VLAN a frame belongs to, ensuring traffic from different VLANs remains separate.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Common exam traps
Cisco often tests the distinction between 802.1Q (VLAN tagging) and 802.1X (authentication), so the trap here is confusing a trunking protocol with a security protocol, leading candidates to pick 802.1X when the question is about VLAN isolation on a trunk.
Detailed technical explanation
802.1Q works by inserting a 4-byte tag (including a 12-bit VLAN ID) between the source MAC address and the EtherType/Length field, recalculating the FCS. On a trunk link, the native VLAN (usually VLAN 1) is sent untagged, while all other VLANs are tagged; this subtle behavior can cause unexpected connectivity if the native VLAN mismatches between switches. In real-world scenarios, misconfigured 802.1Q trunks can lead to VLAN hopping attacks or broadcast storms if the native VLAN is not pruned or secured.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: 802.1Q — C is correct because 802.1Q is the IEEE standard that defines VLAN tagging, allowing multiple VLANs (such as VLAN 10 and VLAN 20) to be carried over a single trunk link while maintaining isolation between them. By inserting a 4-byte VLAN tag into the Ethernet frame, 802.1Q enables the receiving switch to identify which VLAN a frame belongs to, ensuring traffic from different VLANs remains separate.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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