- A
The management instance needs a firewall filter that allows SSH; add a filter to the management instance's loopback.
Why wrong: The management instance currently has no filter, but the filter from internet instance is applied globally to loopback.
- B
The firewall filter in the 'internet' instance incorrectly blocks SSH; modify the filter to allow SSH from any source.
Why wrong: The filter is correct for the intended policy; the problem is instance interaction.
- C
The BGP configuration is causing the loopback to be unreachable; disable BGP on the management instance.
Why wrong: BGP does not affect SSH access.
- D
The firewall filter applied to the loopback interface is shared across all routing instances; use a separate interface for management traffic or apply the filter only to the 'internet' instance's logical system.
The loopback interface is shared; the filter affects all traffic to the loopback.
Quick Answer
The answer is that the firewall filter applied to the loopback interface is shared across all routing instances, so the solution is to use a separate interface for management traffic or apply the filter only to the internet instance’s logical system. This happens because in Junos, the loopback interface (lo0) is a single physical interface, and any firewall filter bound to it is automatically inherited by all routing instances, even if the filter was configured within a specific instance like 'internet'. On the JNCIA-Junos exam, this tests your understanding of how routing instances interact with the loopback and the default behavior of firewall filters—a common trap is assuming filters are instance-scoped. Remember that the loopback is a shared resource; a filter on lo0 in one instance affects traffic from all instances, including management. Memory tip: “One loopback, one filter—instance names don’t isolate it.”
JNCIA-JUNOS Junos Configuration Basics Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos configuration basics. 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 company has a Juniper MX router acting as a BGP route reflector. The router has two routing instances: 'internet' and 'management'. The 'internet' instance uses a firewall filter to block traffic to the router's loopback address from external sources. After a recent configuration change, the engineer notices that SSH access to the router's loopback IP (10.0.0.1) from the internal management network (192.168.0.0/16) is being dropped. The firewall filter for the loopback interface in the 'internet' instance is correctly configured to allow traffic from 192.168.0.0/16. The management instance has no firewall filter applied. The engineer suspects that the issue is related to how the routing instances interact with the loopback interface. Which of the following is the most likely cause and solution?
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 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
The firewall filter applied to the loopback interface is shared across all routing instances; use a separate interface for management traffic or apply the filter only to the 'internet' instance's logical system.
In Junos, firewall filters applied to the loopback interface (lo0) are shared across all routing instances by default because the loopback is a single physical interface. Even though the filter is configured in the 'internet' instance, it affects traffic destined to the loopback IP from any instance, including the 'management' instance. To isolate management traffic, you must either use a separate physical interface or apply the filter only to the 'internet' instance's logical system (e.g., using a firewall filter with a routing-instance match condition).
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 management instance needs a firewall filter that allows SSH; add a filter to the management instance's loopback.
Why it's wrong here
The management instance currently has no filter, but the filter from internet instance is applied globally to loopback.
- ✗
The firewall filter in the 'internet' instance incorrectly blocks SSH; modify the filter to allow SSH from any source.
Why it's wrong here
The filter is correct for the intended policy; the problem is instance interaction.
- ✗
The BGP configuration is causing the loopback to be unreachable; disable BGP on the management instance.
- ✓
The firewall filter applied to the loopback interface is shared across all routing instances; use a separate interface for management traffic or apply the filter only to the 'internet' instance's logical system.
Why this is correct
The loopback interface is shared; the filter affects all traffic to the loopback.
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
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume firewall filters are instance-specific on the loopback interface, but Junos applies them globally by default, leading to the misconception that adding a filter to the management instance or modifying the existing filter will solve the problem.
Detailed technical explanation
How to think about this question
Junos applies firewall filters to the loopback interface (lo0) globally by default, meaning a filter configured under one routing instance's lo0 interface will affect all traffic destined to the router's loopback addresses, regardless of the source routing instance. This behavior is due to the loopback being a shared interface in the default routing instance; to achieve per-instance filtering, you must use logical systems or apply the filter with a routing-instance match condition in the firewall term. In real-world scenarios, this often causes unexpected drops of management traffic when a restrictive filter is applied in a production routing instance.
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.
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Junos Configuration Basics — This question tests Junos Configuration Basics — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall filter applied to the loopback interface is shared across all routing instances; use a separate interface for management traffic or apply the filter only to the 'internet' instance's logical system. — In Junos, firewall filters applied to the loopback interface (lo0) are shared across all routing instances by default because the loopback is a single physical interface. Even though the filter is configured in the 'internet' instance, it affects traffic destined to the loopback IP from any instance, including the 'management' instance. To isolate management traffic, you must either use a separate physical interface or apply the filter only to the 'internet' instance's logical system (e.g., using a firewall filter with a routing-instance match condition).
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.
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Last reviewed: Jun 11, 2026
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.
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