- A
The ARP cache on the firewalls is stale
Why wrong: ARP would cause drops, not connection failures.
- B
Flow-based routing is misconfigured
Why wrong: Flow-based routing is not a feature; policy-based routing is.
- C
Session synchronization is not functioning for TCP
Why wrong: Session sync is enabled and TCP is synced.
- D
Asymmetric routing is causing the SYN packet to be processed by one firewall and the SYN-ACK by the other
Active/active requires careful design to ensure symmetric traffic flows.
Question 247 of 516
Asymmetric Routing Causing TCP Failures After Active/Active Failover
This PCNSE practice question tests your understanding of managing troubleshooting and high availability. 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 HA pair is configured with active/active mode and session sync enabled. After a failover, a network administrator notices that some new TCP connections fail. The firewall logs show no drops. What is the most likely issue?
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 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 the SYN packet to be processed by one firewall and the SYN-ACK by the other
In an active/active HA pair with session sync enabled, asymmetric routing can occur after a failover if the traffic flow changes such that the SYN packet is processed by one firewall and the SYN-ACK by the other. Since session sync only synchronizes established sessions, not half-open ones, the firewall receiving the SYN-ACK does not have a session entry for the SYN, causing it to drop the SYN-ACK as a non-SYN packet without a session. The firewall logs show no drops because the drop occurs at the session lookup stage and may not be logged by default, or the drop counter is not incremented for this specific scenario.
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 ARP cache on the firewalls is stale
Why it's wrong here
ARP would cause drops, not connection failures.
- ✗
Flow-based routing is misconfigured
Why it's wrong here
Flow-based routing is not a feature; policy-based routing is.
- ✗
Session synchronization is not functioning for TCP
Why it's wrong here
Session sync is enabled and TCP is synced.
- ✓
Asymmetric routing is causing the SYN packet to be processed by one firewall and the SYN-ACK by the other
Why this is correct
Active/active requires careful design to ensure symmetric traffic flows.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume session sync covers all TCP packets, including the initial handshake, but in reality, session sync only replicates established sessions, not half-open ones, making asymmetric routing during the handshake a critical failure point.
Detailed technical explanation
How to think about this question
In Palo Alto Networks active/active HA, session sync uses a proprietary protocol to replicate session state between peers, but it only synchronizes sessions that have completed the TCP three-way handshake (state ESTABLISHED). The SYN packet creates a half-open session (state SYN-SENT) that is not synced; if the return path changes due to asymmetric routing, the other firewall receives the SYN-ACK with no session and drops it per the default security policy, which treats non-SYN packets without a session as invalid. This behavior is documented in the PAN-OS High Availability Administration Guide, which notes that asymmetric routing in active/active requires careful design, such as using symmetric return or policy-based forwarding.
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.
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 |
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCNSE question test?
Managing Troubleshooting and High Availability — This question tests Managing Troubleshooting and High Availability — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Asymmetric routing is causing the SYN packet to be processed by one firewall and the SYN-ACK by the other — In an active/active HA pair with session sync enabled, asymmetric routing can occur after a failover if the traffic flow changes such that the SYN packet is processed by one firewall and the SYN-ACK by the other. Since session sync only synchronizes established sessions, not half-open ones, the firewall receiving the SYN-ACK does not have a session entry for the SYN, causing it to drop the SYN-ACK as a non-SYN packet without a session. The firewall logs show no drops because the drop occurs at the session lookup stage and may not be logged by default, or the drop counter is not incremented for this specific scenario.
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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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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