LPIC-2 Advanced Networking Configuration Practice Question
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?
⚠ Common exam trap
Many 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.
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
✓
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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