PCNSA Securing Traffic Practice Question
A network engineer is troubleshooting a drop in traffic from a critical application. The traffic is allowed by the security policy, but the firewall is dropping the packets. The engineer views the session log and sees that the session is being terminated due to 'tcp-non-syn'. What is the most likely cause?
⚠ Common exam trap
Many candidates confuse 'tcp-non-syn' with TCP sequence number issues or NAT problems, but the key is recognizing that this drop occurs only when the firewall has no session state, which points directly to asymmetric routing.
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
✓
Asymmetric routing is causing packets to arrive at a firewall that did not see the initial SYN.
When a firewall sees a non-SYN TCP packet without having seen the initial SYN, it cannot validate the TCP three-way handshake state. This typically occurs with asymmetric routing, where the SYN traverses one firewall and subsequent packets arrive at a different firewall that lacks the session state. The firewall drops these packets with the 'tcp-non-syn' reason because it has no corresponding session entry to associate them with.
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 TCP sequence numbers are out of order, causing the packets to be out of the expected window.
Why it's wrong here
Out-of-order sequence numbers trigger a different drop reason, such as 'tcp-out-of-window' or 'tcp-replay', because the firewall tracks the expected sequence range separately from the initial handshake state. Sequence checking is the relevant mechanism when diagnosing retransmission or asymmetric-routing issues, not a missing SYN.
- ✗
The NAT policy is misconfigured, causing the source IP to not be translated correctly.
Why it's wrong here
A NAT misconfiguration alters translated addresses and typically yields routing or session-setup failures, not a 'tcp-non-syn' termination, since the firewall still observes the initial SYN. NAT policy troubleshooting applies when return traffic fails to match an existing session or source translation is absent.
- ✗
The security policy uses an incorrect service object that doesn't match the application.
Why it's wrong here
An incorrect service object would cause the session to be denied outright by policy, producing a deny log rather than an allow-then-terminate 'tcp-non-syn' entry. Service objects are the right focus when traffic on unexpected ports is silently blocked despite an apparently permissive rule.
- ✓
Asymmetric routing is causing packets to arrive at a firewall that did not see the initial SYN.
Why this is correct
The firewall drops the session because it never observed the initial SYN handshake. With asymmetric routing, return or subsequent packets traverse a different firewall that lacks session state, so it treats them as non-SYN and terminates the session.
Visual reference
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNSA 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 PCNSA exam.