The answer is asymmetric routing caused by a misconfiguration in the active/passive HA setup. In an active/passive pair, only the active firewall handles traffic; when internal traffic exits via one ISP but return traffic enters through the other ISP, the active firewall sees the return path as a new session, creating many sessions in the 'active' state with minimal data transfer. This mismatched flow forces the control plane to process these orphaned sessions, spiking CPU on the management interface, while the passive firewall remains idle. On the PCNSA exam, this scenario tests your understanding of HA session ownership and the importance of symmetric path enforcement—a common trap is blaming OSPF or ISP links when the root cause is the firewall’s inability to match return traffic to an existing session. Remember the mnemonic: “One way out, two ways back? CPU attack.”
PCNSA Core Concepts Practice Question
This PCNSA practice question tests your understanding of core concepts. 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.
Your organization has deployed a Palo Alto Networks PA-5250 firewall in a high-availability active/passive configuration. The firewall is connected to two ISPs for redundancy. The internal network uses OSPF with the firewall as an ASBR redistributing a default route. Recently, users reported intermittent connectivity to external resources. During troubleshooting, you notice that the active firewall's management interface has high CPU usage, and the show session all command displays many sessions in the 'active' state but with minimal data transfer. The passive firewall shows no such issues. The OSPF neighbor relationships are stable. What is the most likely cause of the intermittent connectivity?
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
✓
Asymmetric routing is occurring due to a misconfiguration in the active/passive HA setup.
In an active/passive HA configuration, only the active firewall processes traffic. If asymmetric routing occurs—where traffic from the internal network to the internet uses one ISP link on the active firewall, but return traffic arrives via the other ISP link—the active firewall may see the return traffic as a new session or a non-symmetric flow. This causes the firewall to create sessions that remain in 'active' state with minimal data transfer, as the firewall attempts to match return packets to existing sessions but fails due to path asymmetry. The high CPU on the management interface results from the control plane processing these mismatched sessions, while the passive firewall is unaffected because it does not handle traffic.
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.
✗
OSPF is flapping and causing route instability.
Why it's wrong here
OSPF neighbors are stable, so flapping is not present.
✓
Asymmetric routing is occurring due to a misconfiguration in the active/passive HA setup.
Why this is correct
Asymmetric routing can cause sessions to be stuck and high management CPU as the firewall tries to process out-of-state packets.
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.
✗
A DDoS attack is overwhelming the management plane.
Why it's wrong here
DDoS would affect dataplane more, and management CPU would not be sole symptom.
✗
The firewall's licenses have expired, causing feature degradation.
Why it's wrong here
License expiry would not cause the described symptoms.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often attribute high CPU on the management interface to a DDoS attack or license issues, but the key clue is the 'active' sessions with minimal data transfer, which points to asymmetric routing in an HA environment rather than a control-plane attack or feature degradation.
Detailed technical explanation
How to think about this question
Asymmetric routing in an active/passive HA setup occurs when the firewall's default route points to one ISP, but return traffic from the internet enters via the other ISP due to BGP or policy-based routing on the ISP side. The firewall's session table expects symmetric flows; when return packets arrive on a different interface than the original flow, the firewall treats them as new sessions, leading to session churn and increased CPU usage on the management plane (dataplane CPU may also spike). This is often mitigated by configuring policy-based forwarding (PBF) or using symmetric routing with BGP communities to ensure return traffic uses the same ISP link.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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 — This question tests Core Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Asymmetric routing is occurring due to a misconfiguration in the active/passive HA setup. — In an active/passive HA configuration, only the active firewall processes traffic. If asymmetric routing occurs—where traffic from the internal network to the internet uses one ISP link on the active firewall, but return traffic arrives via the other ISP link—the active firewall may see the return traffic as a new session or a non-symmetric flow. This causes the firewall to create sessions that remain in 'active' state with minimal data transfer, as the firewall attempts to match return packets to existing sessions but fails due to path asymmetry. The high CPU on the management interface results from the control plane processing these mismatched sessions, while the passive firewall is unaffected because it does not handle traffic.
What should I do if I get this PCNSA 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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