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 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?
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
✓
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.
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.
✓
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.
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 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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates 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.
Detailed technical explanation
How to think about this question
STP, defined in IEEE 802.1D, uses Bridge Protocol Data Units (BPDUs) to elect a root bridge and determine port roles (root, designated, alternate, backup). On a two-port bridge, if both ports are connected to the same Layer 2 domain (e.g., both to the same switch), STP will block one port to prevent a loop. Even if the ports are on separate segments, STP may still block one port during the initial convergence (listening/learning states, each 15 seconds by default) or if the bridge incorrectly assumes a loop due to BPDU misconfiguration. The `brctl showstp <bridge>` command can reveal the port state (e.g., 'blocking') and role.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: 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.
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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