The answer is a missing or incorrect default gateway in the routing table. Even with IP forwarding enabled, a Linux router relies on its routing table to determine the next hop for outbound packets; without a correct default gateway or a specific route for the destination network, the kernel has no path to forward traffic and silently drops the packets. This scenario directly tests your understanding of the Linux kernel’s packet forwarding logic on the LPIC-2 exam, where a common trap is assuming that enabling net.ipv4.ip_forward alone is sufficient. The administrator already verified that firewall rules are not blocking traffic, so the root cause must be a routing table misconfiguration. A useful memory tip: think of IP forwarding as the engine, but the routing table as the steering wheel—without a route, the car has no direction.
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 network administrator notices that a Linux router with two network interfaces is not forwarding packets between them, despite having IP forwarding enabled in the kernel. The administrator has verified that the firewall rules are not blocking the traffic. What is the most likely cause of 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.
Why wrong: ARP cache staleness would cause connectivity issues but not prevent forwarding entirely.
B
The default gateway on the router is not set correctly.
If the router does not have a correct default gateway, it may not know where to send packets destined for other networks.
C
The iptables FORWARD chain policy is set to DROP.
Why wrong: The administrator verified firewall rules are not blocking traffic.
D
The routing table does not contain a route for the destination network.
Why wrong: The router can still forward packets if it has a default route or specific routes; missing routes would cause unreachability, not lack of forwarding.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The default gateway on the router is not set correctly.
Option B is correct because even with IP forwarding enabled, a Linux router must have a route in its routing table for the destination network to know which interface to forward packets out of. Without a correct default gateway or a specific route, the router will drop packets for unknown destinations, as it cannot determine the next hop. The administrator verified that firewall rules are not blocking traffic, so the issue is a missing or incorrect route.
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.
✗
The ARP cache on the router is stale.
Why it's wrong here
ARP cache staleness would cause connectivity issues but not prevent forwarding entirely.
✓
The default gateway on the router is not set correctly.
Why this is correct
If the router does not have a correct default gateway, it may not know where to send packets destined for other networks.
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 iptables FORWARD chain policy is set to DROP.
Why it's wrong here
The administrator verified firewall rules are not blocking traffic.
✗
The routing table does not contain a route for the destination network.
Why it's wrong here
The router can still forward packets if it has a default route or specific routes; missing routes would cause unreachability, not lack of forwarding.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume IP forwarding alone is sufficient for routing, overlooking that the kernel still needs a routing table entry to determine the next hop, and they may confuse a missing default gateway with a firewall or ARP issue.
Detailed technical explanation
How to think about this question
Under the hood, Linux uses the FIB (Forwarding Information Base) to make routing decisions; when a packet arrives on one interface, the kernel performs a longest-prefix match lookup in the routing table. If no match is found, the packet is dropped and an ICMP Destination Unreachable (Network Unreachable) message may be sent to the source. In real-world scenarios, a missing default route is a common misconfiguration when setting up a router, especially when multiple subnets are involved, as the router needs explicit routes for each network or a default gateway to reach external destinations.
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: The default gateway on the router is not set correctly. — Option B is correct because even with IP forwarding enabled, a Linux router must have a route in its routing table for the destination network to know which interface to forward packets out of. Without a correct default gateway or a specific route, the router will drop packets for unknown destinations, as it cannot determine the next hop. The administrator verified that firewall rules are not blocking traffic, so the issue is a missing or incorrect route.
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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