PCNSE Core Concepts and Architecture Practice Question
This PCNSE practice question tests your understanding of core concepts and architecture. 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 security engineer is troubleshooting a connectivity issue where traffic from a specific internal host is allowed by security policy but fails to establish a connection to an external server. The firewall logs show the session was created, but no response packets are seen. What is the most likely cause?
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 source NAT is not configured.
When traffic from an internal host is allowed by security policy and the session is created but no response packets are seen, the most likely cause is that source NAT (also known as outbound NAT or PAT) is not configured. Without source NAT, the firewall forwards the packet with the internal private IP address as the source, and the external server sends responses back to that private address, which is not routable over the public internet. The firewall sees the session as created because it matched the security policy and forwarded the initial packet, but the return traffic never reaches the firewall, so no response packets are logged.
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 destination NAT is configured incorrectly.
Why it's wrong here
Destination NAT would not affect outbound sessions; it changes the destination on inbound.
✗
The security policy is missing the return traffic rule.
Why it's wrong here
Security policy is stateful, so return traffic is allowed automatically if the outbound session is allowed.
✗
The firewall is in FIPS mode.
Why it's wrong here
FIPS mode affects encryption, not basic connectivity.
✓
The source NAT is not configured.
Why this is correct
Without source NAT, the packet's source IP remains private, and the server replies to that private IP, which may not return to the firewall.
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 often assume a session being 'created' means the connection is fully established, but in Palo Alto Networks, a session is created as soon as the first packet matches a security rule, even if NAT is not configured, leading to the misconception that the issue must be a missing return traffic rule or a routing problem.
Detailed technical explanation
How to think about this question
In a Palo Alto Networks firewall, source NAT (or outbound NAT) is typically configured using a NAT rule that translates the source IP address of packets from the internal zone to the external zone, often using the firewall's egress interface IP (dynamic IP and port, or DIPP). Without this translation, the firewall forwards the packet with the original source IP (e.g., 192.168.1.10), and the external server's response is sent to that non-routable address, causing it to be dropped by the internet router. The session is created because the firewall processes the initial packet through the security policy and NAT rules, but the absence of a NAT rule means no translation occurs, and the session remains in a half-open state until it times out.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Core Concepts and Architecture — This question tests Core Concepts and Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The source NAT is not configured. — When traffic from an internal host is allowed by security policy and the session is created but no response packets are seen, the most likely cause is that source NAT (also known as outbound NAT or PAT) is not configured. Without source NAT, the firewall forwards the packet with the internal private IP address as the source, and the external server sends responses back to that private address, which is not routable over the public internet. The firewall sees the session as created because it matched the security policy and forwarded the initial packet, but the return traffic never reaches the firewall, so no response packets are logged.
What should I do if I get this PCNSE 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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Question Discussion
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