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 router is experiencing packet loss for connections that should be forwarded. The router's IP forwarding is enabled, and the routing table is correct. Which kernel parameter is most likely causing the issue?
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.
When set to 1 (strict) or 2 (loose), rp_filter can drop packets if the source IP is not reachable via the incoming interface, causing loss in complex routing scenarios.
B
net.ipv4.conf.all.log_martians
Why wrong: log_martians logs suspicious packets but does not drop them.
C
net.ipv4.conf.all.accept_redirects
Why wrong: Accepting redirects is unrelated to packet loss for forwarded traffic.
D
net.ipv4.ip_forward
Why wrong: ip_forward is already enabled as stated.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
net.ipv4.conf.all.rp_filter
The `net.ipv4.conf.all.rp_filter` parameter enables Reverse Path Filtering (RPF), which drops packets arriving on an interface if the kernel does not have a route back to the source IP via that same interface. On a router forwarding traffic between networks, strict RPF can cause packet loss when asymmetric routing is present, even though IP forwarding and the routing table are correct.
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.
✓
net.ipv4.conf.all.rp_filter
Why this is correct
When set to 1 (strict) or 2 (loose), rp_filter can drop packets if the source IP is not reachable via the incoming interface, causing loss in complex routing scenarios.
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.
✗
net.ipv4.conf.all.log_martians
Why it's wrong here
log_martians logs suspicious packets but does not drop them.
✗
net.ipv4.conf.all.accept_redirects
Why it's wrong here
Accepting redirects is unrelated to packet loss for forwarded traffic.
✗
net.ipv4.ip_forward
Why it's wrong here
ip_forward is already enabled as stated.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume packet loss must be due to IP forwarding being disabled or a routing table error, overlooking that Reverse Path Filtering can drop packets even when forwarding and routes are correct.
Detailed technical explanation
How to think about this question
Reverse Path Filtering operates by checking the source address of incoming packets against the FIB (Forwarding Information Base). If `rp_filter` is set to 1 (strict mode), the kernel verifies that the best route back to the source uses the same interface the packet arrived on; if not, the packet is dropped. In a multi-homed router with asymmetric routing (e.g., BGP or OSPF paths), this can silently discard legitimate traffic, a common issue in complex network topologies.
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: net.ipv4.conf.all.rp_filter — The `net.ipv4.conf.all.rp_filter` parameter enables Reverse Path Filtering (RPF), which drops packets arriving on an interface if the kernel does not have a route back to the source IP via that same interface. On a router forwarding traffic between networks, strict RPF can cause packet loss when asymmetric routing is present, even though IP forwarding and the routing table are correct.
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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