The answer is asymmetric routing leading to session mismatch. This is correct because Palo Alto Networks firewalls are stateful, meaning they track session state for bidirectional traffic; when forward traffic traverses one firewall but return traffic takes a different path, the firewall that initiated the session never sees the reply, causing a session mismatch and dropped packets. On the PCNSA exam, this scenario tests your understanding of stateful inspection fundamentals and how routing asymmetry breaks session consistency—a common trap is assuming any security policy allowing traffic is sufficient, when in fact the firewall must see both directions. Remember the memory tip: “One path in, same path out—if it splits, the session quits.”
PCNSA Palo Alto Networks Platforms and Architecture Practice Question
This PCNSA practice question tests your understanding of palo alto networks platforms 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 internal users cannot reach a public web server hosted on the internet. The firewall is configured with a security policy that allows traffic from the internal zone to the external zone on port 80. The engineer notices that traffic is being dropped. Upon checking the session table, the engineer sees that the session is initiated correctly but the return traffic is not matching the existing session. 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.
Why wrong: Application override would not cause session mismatch; it would force the application identification regardless of port.
B
Asymmetric routing
Asymmetric routing causes return traffic to take a different path, so the firewall does not see the return packets and cannot match them to the existing session.
C
DNS resolution failure
Why wrong: DNS resolution failure would prevent the user from resolving the server's IP address, but would not cause session mismatch for return traffic.
D
NAT policy mismatch
Why wrong: NAT policy mismatch would affect address translation but not the session lookup for return traffic.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Asymmetric routing
The session is initiated correctly, but return traffic is not matching the existing session. This is a classic symptom of asymmetric routing, where the forward traffic traverses one firewall and the return traffic takes a different path, bypassing the firewall that holds the session state. Palo Alto Networks firewalls are stateful and require both directions of traffic to pass through the same firewall to maintain session consistency.
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.
✗
Application override configured for HTTP
Why it's wrong here
Application override would not cause session mismatch; it would force the application identification regardless of port.
✓
Asymmetric routing
Why this is correct
Asymmetric routing causes return traffic to take a different path, so the firewall does not see the return packets and cannot match them to the existing session.
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.
✗
DNS resolution failure
Why it's wrong here
DNS resolution failure would prevent the user from resolving the server's IP address, but would not cause session mismatch for return traffic.
✗
NAT policy mismatch
Why it's wrong here
NAT policy mismatch would affect address translation but not the session lookup for return traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse a NAT policy mismatch (which affects address translation) with a session state issue, but the key clue is that the session is initiated correctly—pointing to a routing asymmetry rather than a translation or policy problem.
Detailed technical explanation
How to think about this question
Asymmetric routing occurs when Layer 3 routing decisions cause packets to take different paths in a multi-path or multi-firewall topology. Palo Alto firewalls use a session table that stores a 5-tuple (source IP, destination IP, source port, destination port, protocol) for each flow; if the return packet arrives on a different interface or firewall, the session lookup fails and the packet is dropped. In real-world deployments, this is often mitigated by using symmetric routing policies, virtual wire mode, or enabling asymmetric routing support features like 'allow asymmetric routing' under device settings.
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.
Palo Alto Networks Platforms and Architecture — This question tests Palo Alto Networks Platforms 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 — The session is initiated correctly, but return traffic is not matching the existing session. This is a classic symptom of asymmetric routing, where the forward traffic traverses one firewall and the return traffic takes a different path, bypassing the firewall that holds the session state. Palo Alto Networks firewalls are stateful and require both directions of traffic to pass through the same firewall to maintain session consistency.
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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