This LFCS practice question tests your understanding of networking. 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.
A server on a corporate network is intermittently losing connectivity. The administrator runs 'tcpdump -i eth0 icmp' and sees 'ICMP time exceeded in-transit' messages from a router. 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.
There is a mismatch in MTU size.
Why wrong: MTU issues cause 'fragmentation needed' messages.
The destination host is down.
Why wrong: That would produce 'destination unreachable' messages.
A firewall is blocking the packets.
Why wrong: Firewalls typically drop packets silently or send 'administratively prohibited'.
There is a routing loop or the TTL is too low.
'Time exceeded' indicates TTL expired, common in loops.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
There is a routing loop or the TTL is too low.
The 'ICMP time exceeded in-transit' message indicates that a packet's TTL (Time to Live) has reached zero before reaching its destination. This is most commonly caused by a routing loop, where packets circulate endlessly between routers, or by an initial TTL value that is too low for the number of hops required. The router that decrements the TTL to zero sends this ICMP Type 11 Code 0 message back to the source, as defined in RFC 792.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
There is a mismatch in MTU size.
Why it's wrong here
MTU issues cause 'fragmentation needed' messages.
The destination host is down.
Why it's wrong here
That would produce 'destination unreachable' messages.
A firewall is blocking the packets.
Why it's wrong here
Firewalls typically drop packets silently or send 'administratively prohibited'.
There is a routing loop or the TTL is too low.
Why this is correct
'Time exceeded' indicates TTL expired, common in loops.
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.
Common exam traps
Linux Foundation often tests the distinction between ICMP error types, and the trap here is that candidates confuse 'time exceeded' with 'destination unreachable' or assume any connectivity loss is due to a firewall or MTU issue, rather than recognizing the specific TTL exhaustion symptom.
Detailed technical explanation
Under the hood, each router decrements the TTL field in the IP header by at least 1; when it reaches 0, the router discards the packet and generates an ICMP time exceeded message. In a routing loop, packets may traverse the same routers repeatedly, rapidly exhausting the TTL. A common real-world scenario is a misconfigured static route or a redistribution issue causing a loop, which can be diagnosed by observing increasing TTL values in successive 'traceroute' hops or by using 'tcpdump' to see multiple identical 'time exceeded' messages from the same router.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: There is a routing loop or the TTL is too low. — The 'ICMP time exceeded in-transit' message indicates that a packet's TTL (Time to Live) has reached zero before reaching its destination. This is most commonly caused by a routing loop, where packets circulate endlessly between routers, or by an initial TTL value that is too low for the number of hops required. The router that decrements the TTL to zero sends this ICMP Type 11 Code 0 message back to the source, as defined in RFC 792.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
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