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Question 202 of 516

Core Concepts and Architecture →mediumMultiple ChoiceObjective-mapped

Troubleshooting Silent Drops: Asymmetric Routing and Missing Sessions

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.

An administrator notices that traffic from zone A to zone B is being dropped silently. Security rules are in place. Troubleshooting shows that the session does not appear in the session table. 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.

A
The traffic is being decrypted by an SSL Forward Proxy rule.
Why wrong: Decryption does not cause drops; it may affect identification but not silent drops.
B
The traffic is taking an asymmetric path and the firewall sees only one direction.
Asymmetric routing prevents session setup, causing silent drops.
C
The traffic is matched by a rule with action 'deny' and logging is disabled.
Why wrong: Even with logging disabled, a denied session appears briefly in the session table.
D
The interzone default rule is set to deny.
Why wrong: Default deny rules log drops, so they would not be silent.

Full breakdown with real-world context →

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

✓

The traffic is taking an asymmetric path and the firewall sees only one direction.

When traffic is silently dropped and the session does not appear in the session table, it indicates that the firewall never saw the complete three-way TCP handshake or the first packet of the flow. Asymmetric routing causes the firewall to see only one direction of traffic (e.g., only the SYN from zone A to zone B but not the SYN-ACK return), so the firewall cannot create a session entry because it requires both directions to validate the state. This results in a silent drop without any session table entry or log entry.

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 traffic is being decrypted by an SSL Forward Proxy rule.
Why it's wrong here
Decryption does not cause drops; it may affect identification but not silent drops.
✓
The traffic is taking an asymmetric path and the firewall sees only one direction.
Why this is correct
Asymmetric routing prevents session setup, causing silent drops.
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 traffic is matched by a rule with action 'deny' and logging is disabled.
Why it's wrong here
Even with logging disabled, a denied session appears briefly in the session table.
✗
The interzone default rule is set to deny.
Why it's wrong here
Default deny rules log drops, so they would not be silent.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Palo Alto Networks often tests the misconception that a deny rule or default rule would cause the session to be absent from the session table, but in Palo Alto firewalls, even denied sessions appear in the session table (with a deny action) — the absence of any session entry points specifically to asymmetric routing or a packet that never reached the firewall.

Detailed technical explanation

How to think about this question

Under the hood, Palo Alto Networks firewalls use a stateful inspection engine that requires both the forward and reverse traffic to pass through the same firewall to build a session. When traffic takes an asymmetric path, the firewall sees only one side of the conversation (e.g., SYN from A to B but not SYN-ACK from B to A), so it cannot complete the TCP three-way handshake and never creates a session entry. This is often seen in environments with multiple firewalls or load balancers that route traffic asymmetrically, and can be diagnosed by checking the packet buffer (show counter global filter packet-filter) or using the 'show session all' command to see if any partial sessions exist.

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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

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.

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?

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 traffic is taking an asymmetric path and the firewall sees only one direction. — When traffic is silently dropped and the session does not appear in the session table, it indicates that the firewall never saw the complete three-way TCP handshake or the first packet of the flow. Asymmetric routing causes the firewall to see only one direction of traffic (e.g., only the SYN from zone A to zone B but not the SYN-ACK return), so the firewall cannot create a session entry because it requires both directions to validate the state. This results in a silent drop without any session table entry or log entry.

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.

About these practice questions

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Same concept, more angles

1 more ways this is tested on PCNSE

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Refer to the exhibit. A firewall administrator is investigating why traffic from a source IP 10.1.1.100 to destination 192.168.1.50 is not establishing sessions. The firewall has been up for 45 days. Based on the counters shown, what is the most likely cause?

hard

A.Incorrect NAT rule configuration
B.Security policy denying the traffic
✓ C.Asymmetric routing causing out-of-state packets
D.Zone Protection Profile dropping SYN packets

Why C: The counters show 'flow_pkt_non_syn' and 'flow_pkt_non_syn_drop' incrementing, which indicates the firewall is receiving packets that do not have the SYN flag set for new session establishment. This is a classic symptom of asymmetric routing, where the firewall sees return or mid-stream packets before the initial SYN, causing it to drop them as out-of-state. Since the firewall has been up for 45 days, stale session table entries are not the issue; the traffic path is likely not symmetric, so the firewall never sees the SYN to create a session.

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Question Discussion

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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.