The answer is asymmetric routing causing packets to arrive at a firewall that did not see the initial SYN. When a Palo Alto firewall receives a non-SYN TCP packet without a corresponding session entry, it cannot validate the three-way handshake state, so it drops the packet with the reason 'tcp-non-syn'. This occurs because asymmetric routing sends the SYN through one firewall, which builds the session state, but subsequent packets take a different path and hit a second firewall that never saw the handshake. On the PCNSA exam, this question tests your understanding of session-based stateful inspection and how routing asymmetry breaks it. A common trap is assuming a security policy issue, but the policy is correct—the problem is the missing session state. Remember the mnemonic: "SYN sets the state, asymmetry seals the fate."
PCNSA Securing Traffic Practice Question
This PCNSA practice question tests your understanding of securing traffic. 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 network engineer is troubleshooting a drop in traffic from a critical application. The traffic is allowed by the security policy, but the firewall is dropping the packets. The engineer views the session log and sees that the session is being terminated due to 'tcp-non-syn'. 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 is causing packets to arrive at a firewall that did not see the initial SYN.
When a firewall sees a non-SYN TCP packet without having seen the initial SYN, it cannot validate the TCP three-way handshake state. This typically occurs with asymmetric routing, where the SYN traverses one firewall and subsequent packets arrive at a different firewall that lacks the session state. The firewall drops these packets with the 'tcp-non-syn' reason because it has no corresponding session entry to associate them with.
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 TCP sequence numbers are out of order, causing the packets to be out of the expected window.
Why it's wrong here
Out-of-order sequence numbers cause tcp-out-of-window drops, not tcp-non-syn.
✗
The NAT policy is misconfigured, causing the source IP to not be translated correctly.
Why it's wrong here
NAT misconfiguration leads to NAT-related drops, not tcp-non-syn.
✗
The security policy uses an incorrect service object that doesn't match the application.
Why it's wrong here
Incorrect service would cause application-mismatch, not tcp-non-syn.
✓
Asymmetric routing is causing packets to arrive at a firewall that did not see the initial SYN.
Why this is correct
Asymmetric routing leads to tcp-non-syn drops because the firewall has no session for the non-SYN packet.
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 confuse 'tcp-non-syn' with TCP sequence number issues or NAT problems, but the key is recognizing that this drop occurs only when the firewall has no session state, which points directly to asymmetric routing.
Detailed technical explanation
How to think about this question
Palo Alto Networks firewalls use a stateful TCP proxy that validates the three-way handshake; if a packet arrives with the SYN flag cleared and no session exists, the firewall generates a 'tcp-non-syn' drop. Asymmetric routing is common in environments with multiple firewalls or load balancers, and can be mitigated by using symmetric return routing, policy-based forwarding, or enabling 'allow asymmetric routing' under device settings. The session log entry for 'tcp-non-syn' will show the action as 'drop' and the reason as 'tcp-non-syn'.
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.
Securing Traffic — This question tests Securing Traffic — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Asymmetric routing is causing packets to arrive at a firewall that did not see the initial SYN. — When a firewall sees a non-SYN TCP packet without having seen the initial SYN, it cannot validate the TCP three-way handshake state. This typically occurs with asymmetric routing, where the SYN traverses one firewall and subsequent packets arrive at a different firewall that lacks the session state. The firewall drops these packets with the 'tcp-non-syn' reason because it has no corresponding session entry to associate them with.
What should I do if I get this PCNSA 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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