- A
TCP sequence number mismatch due to packet reordering
Reordering can cause the firewall to drop packets as out-of-state.
- B
Firewall interface speed mismatch
Why wrong: Speed mismatch would cause physical layer issues, not session drops.
- C
Security policy change after session creation
Why wrong: Policy changes apply to new sessions, not existing ones.
- D
DNS resolution failure
Why wrong: DNS failure occurs before session creation.
- E
Asymmetric routing
Return traffic taking different path causes session lookup failure.
Question 302 of 516
Two Common Causes of Session Drops After TCP Handshake on Palo Alto Firewalls
This PCNSE practice question tests your understanding of troubleshoot. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which TWO are common causes of session drops after the initial handshake? (Choose two.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
TCP sequence number mismatch due to packet reordering
Option A is correct because TCP sequence number mismatches can occur when packets are reordered, causing the firewall's TCP state engine to see an unexpected sequence number and drop the session. The firewall tracks TCP sequence numbers to validate that packets belong to an established session; if a packet arrives with a sequence number that does not match the expected window, the firewall may interpret it as a spoofed or invalid packet and terminate the 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.
- ✓
TCP sequence number mismatch due to packet reordering
Why this is correct
Reordering can cause the firewall to drop packets as out-of-state.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Firewall interface speed mismatch
Why it's wrong here
Speed mismatch would cause physical layer issues, not session drops.
- ✗
Security policy change after session creation
Why it's wrong here
Policy changes apply to new sessions, not existing ones.
- ✗
DNS resolution failure
Why it's wrong here
DNS failure occurs before session creation.
- ✓
Asymmetric routing
Why this is correct
Return traffic taking different path causes session lookup failure.
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 session drops caused by policy changes (Option C) with stateful inspection issues, but Palo Alto firewalls do not retroactively apply policy changes to existing sessions unless explicitly configured to do so.
Detailed technical explanation
How to think about this question
Palo Alto Networks firewalls use a TCP state engine that validates sequence and acknowledgment numbers against RFC 793. When packets arrive out of order (e.g., due to asymmetric routing or load balancing), the firewall may see a sequence number outside the expected window and drop the packet, triggering a session teardown. Asymmetric routing (Option E) is another common cause because the firewall sees only one direction of traffic, breaking the stateful inspection; the firewall expects to see both SYN and SYN-ACK on the same interface, and if return traffic takes a different path, the session is dropped.
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 practitioner preparing for the PCNSE exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
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FAQ
Questions learners often ask
What does this PCNSE question test?
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: TCP sequence number mismatch due to packet reordering — Option A is correct because TCP sequence number mismatches can occur when packets are reordered, causing the firewall's TCP state engine to see an unexpected sequence number and drop the session. The firewall tracks TCP sequence numbers to validate that packets belong to an established session; if a packet arrives with a sequence number that does not match the expected window, the firewall may interpret it as a spoofed or invalid packet and terminate the 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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
This PCNSE practice question is part of Courseiva's free Palo Alto Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNSE exam.
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