The answer is asymmetric routing causing the firewall to drop the SYN packet. This occurs because Palo Alto Networks firewalls are stateful; they must see both the SYN and the SYN-ACK traverse the same firewall to build a valid session table entry. When the SYN takes one path and the SYN-ACK returns via a different interface or a different firewall, the return packet is treated as non-session traffic and dropped, even though the security policy permits the initial SYN. On the PCNSE exam, this scenario tests your understanding of stateful inspection and the critical requirement for symmetric traffic flow. A common trap is assuming a permit rule alone guarantees connectivity, but the firewall’s state table must be populated by both directions. Memory tip: “SYN goes left, SYN-ACK goes right—state table says goodnight.”
PCNSE Troubleshoot Practice Question
This PCNSE practice question tests your understanding of troubleshoot. 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.
An engineer is troubleshooting a case where users on a specific subnet cannot reach a web server behind a Palo Alto Networks firewall. The security policy allows the traffic, and the firewall sees the session hit the rule. However, the server does not receive the request. 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
✓
Asymmetric routing causes the firewall to drop the SYN packet
The most likely cause is asymmetric routing, where the SYN packet traverses one firewall path but the SYN-ACK returns via a different path that does not go through the same firewall. Since Palo Alto Networks firewalls are stateful and require both directions of a TCP handshake to pass through the same device to build the session table entry, the SYN-ACK arriving on a different interface or firewall is treated as a non-session packet and dropped, even though the security policy permits the initial SYN. This explains why the firewall sees the session hit the rule but the server never receives the request.
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.
✗
Session offload is causing the packet to bypass security checks
Why it's wrong here
Session offload is for performance and would not prevent the server from receiving the request.
✗
The firewall is unable to resolve the destination MAC address
Why it's wrong here
This would cause no traffic, but the firewall sees the session; MAC resolution is usually fine.
✓
Asymmetric routing causes the firewall to drop the SYN packet
Why this is correct
The firewall might receive the SYN but if the return path is different, it can drop the packet or not forward it properly.
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 destination NAT is misconfigured
Why it's wrong here
If policy allows and session is seen, NAT is likely working; the server not receiving suggests a routing issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a security policy hit means the packet is fully allowed, but they forget that stateful inspection requires symmetric traffic flow for the TCP handshake to complete, and the firewall will drop the SYN-ACK if it arrives on a different interface or firewall.
Detailed technical explanation
How to think about this question
In a Palo Alto Networks firewall, the session table is built only when both the SYN and SYN-ACK are seen on the same firewall interface pair. Asymmetric routing occurs when Layer 3 devices (e.g., routers or load balancers) send the SYN-ACK back through a different path, causing the firewall to see a TCP packet with no matching session and drop it as a non-SYN non-established packet. This is a common issue in multi-homed or load-balanced environments where return traffic does not follow the same path as the original request, and it can be diagnosed by checking the session table for a half-open state or using the `show session all` command to see if the session is in a 'SYN_SENT' state without progressing to 'ESTABLISHED'.
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.
Troubleshoot — This question tests Troubleshoot — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Asymmetric routing causes the firewall to drop the SYN packet — The most likely cause is asymmetric routing, where the SYN packet traverses one firewall path but the SYN-ACK returns via a different path that does not go through the same firewall. Since Palo Alto Networks firewalls are stateful and require both directions of a TCP handshake to pass through the same device to build the session table entry, the SYN-ACK arriving on a different interface or firewall is treated as a non-session packet and dropped, even though the security policy permits the initial SYN. This explains why the firewall sees the session hit the rule but the server never receives the request.
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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