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.
An administrator notices that traffic from zone A to zone B is being dropped silently. Security rules are in place. Troubleshooting shows that the session does not appear in the session table. 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 traffic is taking an asymmetric path and the firewall sees only one direction.
When traffic is silently dropped and the session does not appear in the session table, it indicates that the firewall never saw the complete three-way TCP handshake or the first packet of the flow. Asymmetric routing causes the firewall to see only one direction of traffic (e.g., only the SYN from zone A to zone B but not the SYN-ACK return), so the firewall cannot create a session entry because it requires both directions to validate the state. This results in a silent drop without any session table entry or log entry.
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 traffic is being decrypted by an SSL Forward Proxy rule.
Why it's wrong here
Decryption does not cause drops; it may affect identification but not silent drops.
✓
The traffic is taking an asymmetric path and the firewall sees only one direction.
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The traffic is matched by a rule with action 'deny' and logging is disabled.
Why it's wrong here
Even with logging disabled, a denied session appears briefly in the session table.
✗
The interzone default rule is set to deny.
Why it's wrong here
Default deny rules log drops, so they would not be silent.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Palo Alto Networks often tests the misconception that a deny rule or default rule would cause the session to be absent from the session table, but in Palo Alto firewalls, even denied sessions appear in the session table (with a deny action) — the absence of any session entry points specifically to asymmetric routing or a packet that never reached the firewall.
Detailed technical explanation
How to think about this question
Under the hood, Palo Alto Networks firewalls use a stateful inspection engine that requires both the forward and reverse traffic to pass through the same firewall to build a session. When traffic takes an asymmetric path, the firewall sees only one side of the conversation (e.g., SYN from A to B but not SYN-ACK from B to A), so it cannot complete the TCP three-way handshake and never creates a session entry. This is often seen in environments with multiple firewalls or load balancers that route traffic asymmetrically, and can be diagnosed by checking the packet buffer (show counter global filter packet-filter) or using the 'show session all' command to see if any partial sessions exist.
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 traffic is taking an asymmetric path and the firewall sees only one direction. — When traffic is silently dropped and the session does not appear in the session table, it indicates that the firewall never saw the complete three-way TCP handshake or the first packet of the flow. Asymmetric routing causes the firewall to see only one direction of traffic (e.g., only the SYN from zone A to zone B but not the SYN-ACK return), so the firewall cannot create a session entry because it requires both directions to validate the state. This results in a silent drop without any session table entry or log entry.
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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