- A
It captures packets from the Routing Engine only.
Why wrong: Incorrect. 'monitor traffic' captures packets on the network interface, not only from the Routing Engine. It can capture both control and forwarding plane traffic.
- B
It can filter packets using the 'matching' option to capture only specific traffic.
Correct. The 'matching' option allows filtering packets using BPF syntax, similar to tcpdump.
- C
It captures packets on a specific interface and displays them in real-time.
Correct. The 'interface' option captures packets on a specified interface and displays them in real-time.
- D
It requires a reboot to start capturing packets.
Why wrong: Incorrect. 'monitor traffic' starts capturing immediately without requiring a reboot.
- E
It reads packets from a previously saved pcap file.
Correct. The 'file' option reads packets from a previously saved pcap file, enabling offline analysis.
Monitor Traffic Command in Junos: Filtering and Real-Time Capture
This JNCIA-JUNOS practice question tests your understanding of operational monitoring and maintenance. 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 TWO statements correctly describe the use of the 'monitor traffic' command in Junos?
Quick Answer
The correct answer identifies that the monitor traffic command captures packets on a specific interface and displays them in real-time, and that it supports a matching option to filter traffic by criteria such as protocol. This is because the command operates as a built-in packet sniffer in Junos, using the interface keyword to bind the capture to a single port and the matching keyword to apply a Berkeley Packet Filter (BPF) expression, like matching "icmp", which reduces noise and focuses only on relevant packets for live troubleshooting. On the JNCIA-Junos exam, this question tests your ability to distinguish real-time capture from log-based monitoring, with a common trap being confusion over whether monitor traffic can write to a file (it can, but only with additional options like write-file). Remember the memory tip: “Match to catch, interface to watch” — matching filters the stream, interface targets the port.
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
It can filter packets using the 'matching' option to capture only specific traffic.
Option B is correct because the 'monitor traffic' command supports a 'matching' option that allows you to specify a BPF filter to capture only specific traffic. Option C is correct because 'monitor traffic interface <interface-name>' captures packets on a specific interface and displays them in real-time. Option E is correct because the 'monitor traffic' command can read packets from a previously saved pcap file using the 'file' option (e.g., 'monitor traffic file <filename>'), which is useful for offline analysis.
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.
- ✗
It captures packets from the Routing Engine only.
Why it's wrong here
Incorrect. 'monitor traffic' captures packets on the network interface, not only from the Routing Engine. It can capture both control and forwarding plane traffic.
- ✓
It can filter packets using the 'matching' option to capture only specific traffic.
Why this is correct
Correct. The 'matching' option allows filtering packets using BPF syntax, similar to tcpdump.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
It captures packets on a specific interface and displays them in real-time.
Why this is correct
Correct. The 'interface' option captures packets on a specified interface and displays them in real-time.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
It requires a reboot to start capturing packets.
Why it's wrong here
Incorrect. 'monitor traffic' starts capturing immediately without requiring a reboot.
- ✓
It reads packets from a previously saved pcap file.
Why this is correct
Correct. The 'file' option reads packets from a previously saved pcap file, enabling offline analysis.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'monitor traffic' with 'monitor interface' or assume it requires a reboot, but Junos captures live traffic without any system restart, and the 'matching' option is a powerful BPF-based filter that works exactly like tcpdump's filter syntax.
Detailed technical explanation
How to think about this question
Under the hood, 'monitor traffic' leverages the Junos kernel's packet capture infrastructure, which uses a Berkeley Packet Filter (BPF) to apply the 'matching' filter at the interface level, minimizing CPU impact. A subtle behavior is that by default, it captures only the first 96 bytes of each packet (snap length), which can be adjusted with the 'c size' option. In real-world scenarios, this command is invaluable for verifying firewall filter behavior or diagnosing routing protocol issues without needing external tools.
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 practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Operational Monitoring and Maintenance — This question tests Operational Monitoring and Maintenance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: It can filter packets using the 'matching' option to capture only specific traffic. — Option B is correct because the 'monitor traffic' command supports a 'matching' option that allows you to specify a BPF filter to capture only specific traffic. Option C is correct because 'monitor traffic interface <interface-name>' captures packets on a specific interface and displays them in real-time. Option E is correct because the 'monitor traffic' command can read packets from a previously saved pcap file using the 'file' option (e.g., 'monitor traffic file <filename>'), which is useful for offline analysis.
What should I do if I get this JNCIA-JUNOS 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 11, 2026
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