The answer is that the filter was never applied to an interface. In Junos, configuring a firewall filter under the `edit firewall` hierarchy only defines the filter’s rules and terms; it does not activate traffic inspection. For the filter to process packets, it must be explicitly applied to an interface using a `family inet filter input` or `output` statement, such as `set interfaces ge-0/0/0 unit 0 family inet filter input BLOCK-ICMP`. Without this binding, the filter exists in the configuration as a dormant object, which explains why the packet count remains zero. On the JNCIA-Junos exam, this is a classic trap: candidates often remember to define the filter but forget the critical application step, which is tested under the “Firewall Filters” section. A reliable memory tip is “Define it, then bind it”—the filter is just a template until you attach it to an interface.
JNCIA-JUNOS User Interfaces Practice Question
This JNCIA-JUNOS practice question tests your understanding of user interfaces. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 is trying to configure a firewall filter on a Juniper device. They enter configuration mode and type 'set firewall family inet filter BLOCK-ICMP term 1 from protocol icmp'. They then type 'set firewall family inet filter BLOCK-ICMP term 1 then reject'. After committing, they notice that ICMP traffic is not being blocked. They run 'show configuration firewall' and see the filter is present. They run 'show firewall filter BLOCK-ICMP' to see the counters and notice the packet count is zero. What is the most likely reason?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The filter is not applied to any interface.
Option B is correct because a firewall filter in Junos must be applied to an interface to take effect. Simply configuring the filter under the 'edit firewall' hierarchy does not activate it; the filter must be referenced with a 'family inet' statement under the interface configuration (e.g., 'set interfaces ge-0/0/0 unit 0 family inet filter input BLOCK-ICMP'). Without this application, the filter exists in the configuration but never processes traffic, resulting in zero packet counts.
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 protocol icmp is not correct; it should be 'icmp6'.
Why it's wrong here
ICMP is the correct protocol for IPv4 ICMP; ICMP6 is for IPv6.
✓
The filter is not applied to any interface.
Why this is correct
A firewall filter must be applied to an interface to take effect.
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 term 'then reject' should be 'then discard'.
Why it's wrong here
Both 'reject' and 'discard' drop packets; the difference is in ICMP replies, but the filter would still block traffic.
✗
The filter must be applied under the 'edit firewall' hierarchy.
Why it's wrong here
The configuration is correct; the filter does not need to be applied at the edit firewall level.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume configuring a firewall filter under the 'edit firewall' hierarchy automatically activates it, similar to Cisco IOS where ACLs are applied globally or to interfaces with separate commands, but Junos requires explicit interface application for the filter to process traffic.
Detailed technical explanation
How to think about this question
Junos firewall filters are stateless access control lists that operate on a per-interface basis. The filter configuration under 'edit firewall' defines the rules, but the filter only processes traffic when it is bound to an interface using the 'filter input' or 'filter output' statement under the interface's protocol family. This design allows the same filter to be reused across multiple interfaces without duplication. A common real-world scenario is forgetting to apply the filter after creating it, leading to no traffic being filtered despite a correct filter definition.
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.
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.
User Interfaces — This question tests User Interfaces — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The filter is not applied to any interface. — Option B is correct because a firewall filter in Junos must be applied to an interface to take effect. Simply configuring the filter under the 'edit firewall' hierarchy does not activate it; the filter must be referenced with a 'family inet' statement under the interface configuration (e.g., 'set interfaces ge-0/0/0 unit 0 family inet filter input BLOCK-ICMP'). Without this application, the filter exists in the configuration but never processes traffic, resulting in zero packet counts.
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.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This JNCIA-JUNOS practice question is part of Courseiva's free Juniper Networks 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 JNCIA-JUNOS exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.