- A
ip link add link eth0 name eth0.100 type vlan id 100; ip link add br0 type bridge; ip link set eth0.100 master br0; ip link set br0 up
Correct sequence: create VLAN subinterface, create bridge, attach VLAN to bridge.
- B
ip link add br0 type bridge; ip link set eth0 master br0; ip link set br0 up
Why wrong: This does not create a VLAN interface; it bridges the physical interface directly.
- C
ip link add eth0.100 link eth0 type vlan id 100; ip link set eth0.100 master br0; ip link add br0 type bridge
Why wrong: The order is fine, but the 'link' and 'name' keywords are reversed.
- D
ip link add name br0 type bridge; ip link add link br0 name vlan100 type vlan id 100; ip link set eth0 master br0
Why wrong: VLAN is created on the bridge, not on the physical link; traffic not tagged.
LFCS Networking Practice Question
This LFCS practice question tests your understanding of networking. 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 Linux administrator needs to configure VLAN tagging on a network bridge to isolate traffic from different virtual machines. The physical interface is eth0, and VLAN ID 100 should be accessible via the bridge br0. Which set of commands correctly creates this configuration using the ip command?
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
ip link add link eth0 name eth0.100 type vlan id 100; ip link add br0 type bridge; ip link set eth0.100 master br0; ip link set br0 up
Option A is correct because it first creates a VLAN interface (eth0.100) on top of physical interface eth0 with VLAN ID 100 using `ip link add link eth0 name eth0.100 type vlan id 100`. It then creates a bridge (br0) with `ip link add br0 type bridge`, attaches the VLAN interface to the bridge as a port with `ip link set eth0.100 master br0`, and finally brings the bridge up. This sequence ensures that traffic tagged with VLAN 100 on eth0 is properly forwarded through the bridge to virtual machines, while untagged or other VLAN traffic is isolated.
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.
- ✓
ip link add link eth0 name eth0.100 type vlan id 100; ip link add br0 type bridge; ip link set eth0.100 master br0; ip link set br0 up
Why this is correct
Correct sequence: create VLAN subinterface, create bridge, attach VLAN to bridge.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
ip link add br0 type bridge; ip link set eth0 master br0; ip link set br0 up
Why it's wrong here
This does not create a VLAN interface; it bridges the physical interface directly.
- ✗
ip link add eth0.100 link eth0 type vlan id 100; ip link set eth0.100 master br0; ip link add br0 type bridge
Why it's wrong here
The order is fine, but the 'link' and 'name' keywords are reversed.
- ✗
ip link add name br0 type bridge; ip link add link br0 name vlan100 type vlan id 100; ip link set eth0 master br0
Why it's wrong here
VLAN is created on the bridge, not on the physical link; traffic not tagged.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often forget the order of operations — the bridge must exist before enslaving a port, and the VLAN interface must be created on the physical NIC, not on the bridge itself.
Trap categories for this question
Keyword trap
The order is fine, but the 'link' and 'name' keywords are reversed.
Detailed technical explanation
How to think about this question
Under the hood, VLAN interfaces (e.g., eth0.100) are 802.1Q VLAN sub-interfaces that add or remove VLAN tags in the Ethernet header as frames pass through. When such a VLAN interface is enslaved to a Linux bridge, the bridge treats it as a regular port, allowing the bridge to forward only frames with the matching VLAN tag. In real-world scenarios, this setup is common in KVM or libvirt environments where each VM is assigned a specific VLAN for network segmentation, and the bridge acts as a virtual switch connecting VMs to the physical network.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this LFCS question test?
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ip link add link eth0 name eth0.100 type vlan id 100; ip link add br0 type bridge; ip link set eth0.100 master br0; ip link set br0 up — Option A is correct because it first creates a VLAN interface (eth0.100) on top of physical interface eth0 with VLAN ID 100 using `ip link add link eth0 name eth0.100 type vlan id 100`. It then creates a bridge (br0) with `ip link add br0 type bridge`, attaches the VLAN interface to the bridge as a port with `ip link set eth0.100 master br0`, and finally brings the bridge up. This sequence ensures that traffic tagged with VLAN 100 on eth0 is properly forwarded through the bridge to virtual machines, while untagged or other VLAN traffic is isolated.
What should I do if I get this LFCS 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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Last reviewed: Jun 25, 2026
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