Quick Answer
The answer is the `traceroute` command. This is correct because the Junos `traceroute` command traces the Layer 3 path packets take to a remote destination by sending UDP probes with incrementing Time-To-Live (TTL) values; each router along the path decrements the TTL, and when it reaches zero, the router sends back an ICMP Time Exceeded message, allowing the source to map every hop. On the JNCIA-Junos exam, this question tests your ability to differentiate between operational commands for connectivity troubleshooting, often contrasting `traceroute` with `ping` (which only tests reachability, not the path) or `show route` (which displays the routing table but not the actual forwarded path). A common trap is confusing `traceroute` with `traceoptions`, which is a configuration tool for debugging routing protocols, not for live path tracing. Remember the memory tip: “Traceroute traces the TTL trail” to recall that it relies on TTL expiration to reveal each hop.
This JNCIA-JUNOS practice question tests your understanding of junos os fundamentals. 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 junior engineer is troubleshooting connectivity issues and wants to trace the path packets take to a remote destination. Which Junos command should be used?
monitor traffic
Why wrong: Captures packets for analysis, does not trace the path.
show route
Why wrong: Shows the routing table, not the actual packet path.
traceroute
Displays the route packets take to a destination, hop by hop.
ping
Why wrong: Tests connectivity but does not show the path taken.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
traceroute
Option C is correct because the 'traceroute' command in Junos is specifically designed to trace the path packets take to a remote destination by sending UDP probes with increasing TTL values and analyzing ICMP Time Exceeded messages from intermediate routers. This directly addresses the junior engineer's need to map the Layer 3 path and identify where connectivity failures occur.
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.
monitor traffic
Why it's wrong here
Captures packets for analysis, does not trace the path.
show route
Why it's wrong here
Shows the routing table, not the actual packet path.
traceroute
Why this is correct
Displays the route packets take to a destination, hop by hop.
Related concept
Read the scenario before looking for a memorised answer.
ping
Why it's wrong here
Tests connectivity but does not show the path taken.
Common exam traps
The trap here is that candidates often confuse 'ping' (which tests reachability) with 'traceroute' (which traces the path), leading them to select Option D when the question explicitly asks for path tracing rather than simple connectivity testing.
Trap categories for this question
Command / output trap
Shows the routing table, not the actual packet path.
Detailed technical explanation
The Junos 'traceroute' command sends UDP datagrams with a destination port in the range 33434-33534 by default, incrementing the Time-to-Live (TTL) field starting from 1. Each router that decrements the TTL to 0 sends back an ICMP Time Exceeded message (Type 11, Code 0), allowing the source to identify each hop. If the destination is reached, it returns an ICMP Port Unreachable (Type 3, Code 3) because the high UDP port is typically unused, which terminates the trace. This behavior is defined in RFC 1393 and is distinct from the ICMP Echo-based traceroute used in some other implementations.
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
Junos OS Fundamentals — This question tests Junos OS Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: traceroute — Option C is correct because the 'traceroute' command in Junos is specifically designed to trace the path packets take to a remote destination by sending UDP probes with increasing TTL values and analyzing ICMP Time Exceeded messages from intermediate routers. This directly addresses the junior engineer's need to map the Layer 3 path and identify where connectivity failures occur.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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