- A
The SRX's source IP for outbound traffic is incorrect; it should use the untrust interface IP but is using the trust IP.
Why wrong: By default, source NAT is typically applied; if ping works, source IP is likely correct.
- B
The return traffic from the website is not reaching the SRX, possibly due to asymmetric routing or the website's firewall blocking the SRX's source IP.
Since SRX can ping, the forward path works; return path or the destination server's security may be blocking traffic.
- C
The default route is missing; the ISP must be configured as a static route.
Why wrong: The default route is configured and works for other traffic.
- D
There is a more specific static route to 198.51.100.0/24 with a different next-hop causing traffic to be sent elsewhere.
Why wrong: No evidence of such route; if it existed, SRX might not reach the IP via ping.
Quick Answer
The answer is asymmetric routing or a return path black hole, because the default route on the SRX correctly forwards outbound traffic to 203.0.113.1, but the public website at 198.51.100.10 may be sending its return traffic through a different path that never reaches the SRX, or the website’s firewall is dropping packets from the SRX’s source IP. This scenario tests your understanding of default route troubleshooting in Junos, specifically how a working forward path does not guarantee a working return path—a common trap on the JNCIA-Junos exam where candidates assume a successful ping from the SRX proves end-to-end connectivity. The key insight is that the SRX’s own ping uses its management or egress interface, while user traffic from the trust zone must traverse security policies and NAT, and the return traffic must follow the reverse path symmetrically. Remember the memory tip: “Forward works, return lurks”—always verify both directions when troubleshooting asymmetric routing or black holes.
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing 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.
You are managing a small branch office with a Juniper SRX firewall that connects to the internet via a single ISP. The internal network uses 192.168.1.0/24. You need to configure a default route so that all internet-bound traffic goes to the ISP's next-hop 203.0.113.1. The SRX has two interfaces: ge-0/0/0 (untrust) with IP 203.0.113.2/30 and ge-0/0/1 (trust) with IP 192.168.1.1/24. You add the following configuration: 'set routing-options static route 0.0.0.0/0 next-hop 203.0.113.1'. After committing, devices on the trust network can ping the internet (e.g., 8.8.8.8) successfully. However, users report that they cannot access a specific public website hosted at 198.51.100.10. You verify that the SRX can reach that IP via ping and traceroute. What is the most likely cause of this issue?
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 return traffic from the website is not reaching the SRX, possibly due to asymmetric routing or the website's firewall blocking the SRX's source IP.
Option C is correct because the default route is working, but the specific path may have a problem like a routing loop or black hole. Since the SRX can reach the IP, the issue is likely on the return path or the destination is filtering traffic. Option A is incorrect because the default route is present and working for other destinations. Option B is incorrect because there is no evidence of a more specific route. Option D is incorrect because the SRX can ping, so source IP is not the issue.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 SRX's source IP for outbound traffic is incorrect; it should use the untrust interface IP but is using the trust IP.
Why it's wrong here
By default, source NAT is typically applied; if ping works, source IP is likely correct.
- ✓
The return traffic from the website is not reaching the SRX, possibly due to asymmetric routing or the website's firewall blocking the SRX's source IP.
Why this is correct
Since SRX can ping, the forward path works; return path or the destination server's security may be blocking traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The default route is missing; the ISP must be configured as a static route.
Why it's wrong here
The default route is configured and works for other traffic.
- ✗
There is a more specific static route to 198.51.100.0/24 with a different next-hop causing traffic to be sent elsewhere.
Why it's wrong here
No evidence of such route; if it existed, SRX might not reach the IP via ping.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related JNCIA-JUNOS NAT questions on configuration and troubleshooting.
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Routing Fundamentals — study guide chapter
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Routing Fundamentals — This question tests Routing Fundamentals — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The return traffic from the website is not reaching the SRX, possibly due to asymmetric routing or the website's firewall blocking the SRX's source IP. — Option C is correct because the default route is working, but the specific path may have a problem like a routing loop or black hole. Since the SRX can reach the IP, the issue is likely on the return path or the destination is filtering traffic. Option A is incorrect because the default route is present and working for other destinations. Option B is incorrect because there is no evidence of a more specific route. Option D is incorrect because the SRX can ping, so source IP is not the issue.
What should I do if I get this JNCIA-JUNOS question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related JNCIA-JUNOS NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 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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