- A
ip rule add from 10.0.0.10 to 0/0 table isp1; ip route add default via 1.1.1.1 table isp1; ip rule add to 10.0.0.10 table isp1
Why wrong: Extra rule for incoming traffic is unnecessary; rp_filter still may drop packets.
- B
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1 table isp1; sysctl -w net.ipv4.conf.all.rp_filter=0
Policy routing with table-specific default and disabled rp_filter allows asymmetric routing.
- C
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1 table isp1
Why wrong: Missing sysctl tuning; reverse path filtering will drop return packets arriving on a different interface.
- D
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1
Why wrong: Default route not added to custom table; also no rp_filter adjustment.
LPIC-2 Advanced Networking Configuration Practice Question
This LPIC-2 practice question tests your understanding of advanced networking configuration. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 asymmetric routing issues. The network has two internet connections (ISP1 and ISP2) with default routes. The administrator wants to ensure that traffic originating from a specific source IP uses ISP1 for both incoming and outgoing packets. Which ip rule configuration achieves this?
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
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1 table isp1; sysctl -w net.ipv4.conf.all.rp_filter=0
Option B is correct because it creates a policy routing rule that matches traffic from source IP 10.0.0.10 and directs it to a custom routing table named 'isp1', which contains a default route via ISP1's gateway (1.1.1.1). Additionally, disabling reverse path filtering (rp_filter=0) is essential to allow asymmetric routing — without it, the kernel would drop return packets arriving via a different interface than the one used for outgoing traffic, which is exactly the scenario in asymmetric routing.
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.
- ✗
ip rule add from 10.0.0.10 to 0/0 table isp1; ip route add default via 1.1.1.1 table isp1; ip rule add to 10.0.0.10 table isp1
Why it's wrong here
Extra rule for incoming traffic is unnecessary; rp_filter still may drop packets.
- ✓
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1 table isp1; sysctl -w net.ipv4.conf.all.rp_filter=0
Why this is correct
Policy routing with table-specific default and disabled rp_filter allows asymmetric routing.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1 table isp1
Why it's wrong here
Missing sysctl tuning; reverse path filtering will drop return packets arriving on a different interface.
- ✗
ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1
Why it's wrong here
Default route not added to custom table; also no rp_filter adjustment.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often forget to disable reverse path filtering (rp_filter) when implementing policy routing for asymmetric paths, assuming that adding the policy rule and custom table is sufficient, but the kernel's default strict rp_filter will silently drop return packets that arrive on a different interface.
Detailed technical explanation
How to think about this question
Reverse path filtering (rp_filter) is a kernel security feature that verifies the source IP of incoming packets against the routing table to ensure the return path uses the same interface; setting it to 0 disables this check, allowing asymmetric routing. Policy routing via 'ip rule' uses routing tables (e.g., 'isp1') that are separate from the main table, and the 'from' selector matches the source IP of the packet — this is essential for directing traffic from a specific host to a specific ISP. In real-world multi-homed networks, asymmetric routing is common, and disabling rp_filter on the relevant interfaces (or globally) is a standard practice to prevent packet drops.
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.
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FAQ
Questions learners often ask
What does this LPIC-2 question test?
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: ip rule add from 10.0.0.10 table isp1; ip route add default via 1.1.1.1 table isp1; sysctl -w net.ipv4.conf.all.rp_filter=0 — Option B is correct because it creates a policy routing rule that matches traffic from source IP 10.0.0.10 and directs it to a custom routing table named 'isp1', which contains a default route via ISP1's gateway (1.1.1.1). Additionally, disabling reverse path filtering (rp_filter=0) is essential to allow asymmetric routing — without it, the kernel would drop return packets arriving via a different interface than the one used for outgoing traffic, which is exactly the scenario in asymmetric routing.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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