The answer is that three conditions must be met: each network interface must have an IP address in the respective subnet, IP forwarding must be enabled in the kernel, and a valid routing table must exist to direct traffic between the subnets. This is correct because a Linux router bridges two separate broadcast domains by assigning each interface a unique IP on its connected network, while the kernel parameter net.ipv4.ip_forward = 1 allows packet forwarding between those interfaces based on the routing table. On the LPIC-2 exam, this topic falls under Objective 212.1, testing your ability to configure a Linux system as a router; a common trap is forgetting that enabling IP forwarding alone is insufficient without proper interface addressing and routes. A reliable memory tip is the "three-legged stool" analogy: addressing, forwarding, and routing must all be present for the system to function as a router between two networks.
LPIC-2 Advanced Networking Configuration Practice Question
This LPIC-2 practice question tests your understanding of advanced networking configuration. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which THREE conditions must be met for a Linux system to act as a router between two networks?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IP forwarding must be enabled in the kernel (net.ipv4.ip_forward = 1).
Option A is correct because the Linux kernel must have IP forwarding enabled to forward packets between network interfaces. This is controlled by the sysctl parameter net.ipv4.ip_forward = 1, which allows the kernel to act as a router by forwarding IP packets from one interface to another based on the routing table.
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 forwarding must be enabled in the kernel (net.ipv4.ip_forward = 1).
Why this is correct
Required to forward packets.
Related concept
Read the scenario before looking for a memorised answer.
✗
The system must have a default gateway configured.
Why it's wrong here
Not required; routing can be done without a default gateway.
✓
The system must have routes to the networks it will forward traffic to.
Why this is correct
Routes are needed to know where to send packets.
Related concept
Read the scenario before looking for a memorised answer.
✓
Each interface must have an IP address in the respective subnet.
Why this is correct
Needed for the router to be on each network.
Related concept
Read the scenario before looking for a memorised answer.
✗
The firewall must allow forwarding (FORWARD chain policy ACCEPT).
Why it's wrong here
Not a condition; forwarding can be allowed with rules, but it's not a condition for routing at the kernel level.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the requirement for a default gateway (option B) with the need for specific routes to the networks being forwarded, but a router only needs routes to the destination networks, not a default gateway, unless it must forward traffic to networks beyond its directly connected ones.
Detailed technical explanation
How to think about this question
Under the hood, when net.ipv4.ip_forward is enabled, the kernel invokes the IP forwarding code path in the netfilter framework, which checks the routing table for each incoming packet to determine the outgoing interface. In real-world scenarios, a router often has multiple interfaces with IP addresses in different subnets, and without routes to those subnets (option C) or correct interface IPs (option D), the kernel cannot determine where to forward packets, even if IP forwarding is enabled.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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: IP forwarding must be enabled in the kernel (net.ipv4.ip_forward = 1). — Option A is correct because the Linux kernel must have IP forwarding enabled to forward packets between network interfaces. This is controlled by the sysctl parameter net.ipv4.ip_forward = 1, which allows the kernel to act as a router by forwarding IP packets from one interface to another based on the routing table.
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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Question Discussion
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