LPIC-2 Advanced Networking Configuration Practice Question
This LPIC-2 practice question tests your understanding of advanced networking configuration. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 notices that VLAN tagging is not working on a Linux bridge. The bridge interface br0 has member ports eth0 and eth1. The administrator runs 'bridge vlan show' and sees that only the default VLAN 1 is present. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The bridge is not enabled for VLAN filtering
The 'bridge vlan show' command only displays VLAN filtering information when VLAN filtering is enabled on the bridge. By default, Linux bridges operate in a simple switching mode without VLAN awareness, meaning all frames are forwarded regardless of VLAN tags. The administrator must explicitly enable VLAN filtering using 'ip link set dev br0 type bridge vlan_filtering 1' to activate per-VLAN forwarding and filtering, which is why only the default untagged VLAN 1 appears.
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.
✓
The bridge is not enabled for VLAN filtering
Why this is correct
Without enabling VLAN filtering on the bridge, only the default VLAN (VLAN 1) is allowed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The kernel module for VLAN is not loaded
Why it's wrong here
The bridge VLAN filtering is built into the bridge module, not a separate VLAN module.
✗
The VLAN protocol is not set to 802.1Q
Why it's wrong here
Linux bridges default to 802.1Q VLAN tagging; this is unlikely the issue.
✗
Ports eth0 and eth1 are not set to promiscuous mode
Why it's wrong here
Promiscuous mode is not required for VLAN tagging to work on a bridge.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume VLAN tagging works automatically on a Linux bridge because they are familiar with VLAN interfaces or managed switches, but Linux bridges require explicit VLAN filtering configuration to process 802.1Q tags.
Detailed technical explanation
How to think about this question
When VLAN filtering is enabled on a Linux bridge, the bridge uses the VLAN ID in the 802.1Q header to make forwarding decisions based on the per-port VLAN membership table managed via 'bridge vlan' commands. Without vlan_filtering=1, the bridge behaves like a simple hub or switch that forwards all frames without inspecting VLAN tags, effectively ignoring any VLAN information. In real-world scenarios, this is a common pitfall when migrating from traditional VLAN interfaces to bridge-based VLAN filtering, as administrators expect VLAN tags to be processed automatically.
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.
Advanced Networking Configuration — This question tests Advanced Networking Configuration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The bridge is not enabled for VLAN filtering — The 'bridge vlan show' command only displays VLAN filtering information when VLAN filtering is enabled on the bridge. By default, Linux bridges operate in a simple switching mode without VLAN awareness, meaning all frames are forwarded regardless of VLAN tags. The administrator must explicitly enable VLAN filtering using 'ip link set dev br0 type bridge vlan_filtering 1' to activate per-VLAN forwarding and filtering, which is why only the default untagged VLAN 1 appears.
What should I do if I get this LPIC-2 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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