The answer is asymmetric routing causing out-of-state packets, as indicated by the non-SYN drop counters. When the firewall increments `flow_pkt_non_syn` and `flow_pkt_non_syn_drop`, it means it is receiving packets without the SYN flag for new sessions, which occurs when return or mid-stream traffic arrives before the initial handshake—a classic symptom of asymmetric routing. On the PCNSE exam, this scenario tests your understanding of how Palo Alto firewalls enforce stateful inspection; a common trap is to blame stale session entries, but the 45-day uptime rules that out. The key insight is that the firewall must see the three-way handshake to build a session table entry, so non-SYN drops directly point to a path asymmetry where the SYN takes a different route. Memory tip: “No SYN, no session—asymmetric routing is the confession.”
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.
Exhibit
Refer to the exhibit.
admin@PA-5050> show system info | match uptime
Uptime: 45 days 3 hours 22 mins
admin@PA-5050> show session all filter source 10.1.1.100 destination 192.168.1.50
Session filter returned 0 sessions
admin@PA-5050> show counter global | match flow_tcp_non_syn
flow_tcp_non_syn: 15
admin@PA-5050> show counter global | match flow_tcp_handshake_fail
flow_tcp_handshake_fail: 8
Refer to the exhibit. A firewall administrator is investigating why traffic from a source IP 10.1.1.100 to destination 192.168.1.50 is not establishing sessions. The firewall has been up for 45 days. Based on the counters shown, 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.
Refer to the exhibit.
admin@PA-5050> show system info | match uptime
Uptime: 45 days 3 hours 22 mins
admin@PA-5050> show session all filter source 10.1.1.100 destination 192.168.1.50
Session filter returned 0 sessions
admin@PA-5050> show counter global | match flow_tcp_non_syn
flow_tcp_non_syn: 15
admin@PA-5050> show counter global | match flow_tcp_handshake_fail
flow_tcp_handshake_fail: 8
A
Incorrect NAT rule configuration
Why wrong: NAT issues would not cause TCP non-SYN counters.
B
Security policy denying the traffic
Why wrong: Denied traffic would show in deny counters, not TCP handshake failures.
C
Asymmetric routing causing out-of-state packets
Non-SYN packets without a session indicate asymmetric routing.
D
Zone Protection Profile dropping SYN packets
Why wrong: Zone protection would block SYN packets, not cause non-SYN packets.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Asymmetric routing causing out-of-state packets
The counters show 'flow_pkt_non_syn' and 'flow_pkt_non_syn_drop' incrementing, which indicates the firewall is receiving packets that do not have the SYN flag set for new session establishment. This is a classic symptom of asymmetric routing, where the firewall sees return or mid-stream packets before the initial SYN, causing it to drop them as out-of-state. Since the firewall has been up for 45 days, stale session table entries are not the issue; the traffic path is likely not symmetric, so the firewall never sees the SYN to create a session.
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.
Denied traffic would show in deny counters, not TCP handshake failures.
✓
Asymmetric routing causing out-of-state packets
Why this is correct
Non-SYN packets without a session indicate asymmetric routing.
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.
✗
Zone Protection Profile dropping SYN packets
Why it's wrong here
Zone protection would block SYN packets, not cause non-SYN packets.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a security policy or NAT issue when traffic fails, but the specific counter 'flow_pkt_non_syn_drop' directly points to asymmetric routing, not policy or NAT misconfiguration.
Trap categories for this question
Command / output trap
Denied traffic would show in deny counters, not TCP handshake failures.
Detailed technical explanation
How to think about this question
In PAN-OS, the firewall maintains a session table and expects to see the three-way handshake (SYN, SYN-ACK, ACK) to establish state. When asymmetric routing occurs, the firewall may receive a SYN-ACK or data packet without having seen the initial SYN; the packet flow engine identifies these as non-SYN packets and drops them by default, incrementing 'flow_pkt_non_syn_drop'. This behavior is defined in RFC 793 and enforced by the firewall's stateful inspection engine, which can be tuned with 'tcp-drop-non-syn' settings but is enabled by default.
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: Asymmetric routing causing out-of-state packets — The counters show 'flow_pkt_non_syn' and 'flow_pkt_non_syn_drop' incrementing, which indicates the firewall is receiving packets that do not have the SYN flag set for new session establishment. This is a classic symptom of asymmetric routing, where the firewall sees return or mid-stream packets before the initial SYN, causing it to drop them as out-of-state. Since the firewall has been up for 45 days, stale session table entries are not the issue; the traffic path is likely not symmetric, so the firewall never sees the SYN to create a session.
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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