The answer is that the destination IP is not reachable from the firewall. When a SYN packet is received but no SYN-ACK is sent, it indicates the firewall is unable to complete the TCP three-way handshake because it cannot route the packet to the next hop or the destination host is down. In this scenario, the firewall silently drops the SYN packet without generating a SYN-ACK, as it cannot establish a session without a valid route. On the PCNSE exam, this concept tests your understanding of how Palo Alto firewalls handle session setup and routing—a common trap is assuming the issue lies with security policies or NAT, but the packet capture’s silence on the SYN-ACK points directly to a reachability failure. Remember the mnemonic: “No SYN-ACK? Check the track”—meaning verify the route and destination availability before blaming the rulebase.
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.
A network engineer is configuring a new PA-220 firewall. They need to allow HTTP traffic from the 'trust' zone to the 'untrust' zone. However, the traffic is being dropped. A packet capture shows that the SYN packet is received but no SYN-ACK is sent. 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 destination IP is not reachable from the firewall.
The packet capture shows the SYN packet is received by the firewall but no SYN-ACK is sent. This indicates the firewall is not completing the TCP three-way handshake. The most common cause is that the destination IP is not reachable from the firewall, meaning the firewall cannot route the SYN packet to the next hop or the destination host is down. In this scenario, the firewall drops the SYN packet silently without generating a SYN-ACK because it cannot establish 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.
✗
There is no NAT policy to translate the source IP.
The destination IP is not reachable from the firewall.
Why this is correct
If the firewall cannot route to the destination, it will drop the SYN.
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.
✗
The firewall is not configured to inspect HTTP traffic.
Why it's wrong here
Inspection occurs after the session is established.
✗
The security policy does not have an allow rule for HTTP.
Why it's wrong here
If no allow rule, the SYN would be dropped with a deny reason.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a missing security policy or NAT rule is the cause when a SYN packet is received but no SYN-ACK is sent, but the correct diagnostic is to check routing and destination reachability first.
Detailed technical explanation
How to think about this question
When a Palo Alto Networks firewall receives a SYN packet for a new session, it performs a route lookup to determine the egress interface and next-hop. If the route is missing or the next-hop is unreachable (e.g., ARP resolution fails), the firewall drops the SYN packet and does not send a SYN-ACK. This behavior is defined by RFC 1122, which requires a host to silently drop packets when the destination is unreachable. In a real-world scenario, this often occurs when a static route is misconfigured or the default gateway is down.
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 destination IP is not reachable from the firewall. — The packet capture shows the SYN packet is received by the firewall but no SYN-ACK is sent. This indicates the firewall is not completing the TCP three-way handshake. The most common cause is that the destination IP is not reachable from the firewall, meaning the firewall cannot route the SYN packet to the next hop or the destination host is down. In this scenario, the firewall drops the SYN packet silently without generating a SYN-ACK because it cannot establish 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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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.
Question Discussion
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