LPIC-2 Advanced Networking Configuration Practice Question
A Linux server with two Ethernet interfaces (eth0 and eth1) has been set up as a transparent bridge using brctl. The bridge is up and shows both interfaces as members. However, hosts connected to eth0 cannot communicate with hosts on eth1. What is the most likely cause?
⚠ Common exam trap
Watch out — candidates often assume a transparent bridge with only two ports will always forward traffic immediately, forgetting that STP is enabled by default in brctl and can block ports even in simple topologies, leading them to incorrectly suspect missing IP addresses or promiscuous mode.
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
✓
Spanning Tree Protocol (STP) is enabled and is blocking one of the ports.
When Spanning Tree Protocol (STP) is enabled on a Linux bridge using brctl, it will automatically transition ports into the blocking state to prevent loops. If the bridge has only two ports and no redundant paths, STP may still keep one port in the blocking state (e.g., during the listening/learning phase or due to a misconfiguration), which prevents frames from being forwarded between eth0 and eth1. This is the most likely cause because STP is enabled by default in brctl and can block ports even in a simple two-port bridge.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Spanning Tree Protocol (STP) is enabled and is blocking one of the ports.
Why this is correct
STP prevents network loops by placing redundant ports in blocking state, which can inadvertently block traffic until a port transitions to forwarding.
- ✗
The bridge filtering rules in ebtables are dropping frames.
Why it's wrong here
While possible, it is less likely than STP unless ebtables rules were explicitly configured.
- ✗
The bridge has no IP address assigned.
Why it's wrong here
A transparent bridge operates at L2 and does not require an IP address for forwarding traffic.
- ✗
The interfaces are not in promiscuous mode.
Why it's wrong here
When interfaces are added to a bridge, they are automatically placed in promiscuous mode.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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