- A
The application 'trading-app' is not fully recognized for some sessions, causing fallback to 'unknown-tcp'.
Inconsistent identification can occur if the application signature does not match all variations of the traffic.
- B
The application is identified as both 'trading-app' and 'unknown-tcp' due to a software bug.
Why wrong: App-ID does not assign multiple identities to a single session.
- C
The traffic is using a non-standard port, so the standard rule does not match.
Why wrong: The traffic uses default port 5000, so port is not the issue.
- D
There is a decryption policy causing the application to be misidentified.
Why wrong: No decryption policy is mentioned; the traffic is likely not encrypted.
Question 421 of 516
Why Low-Latency Apps Drop Sporadically Due to App-ID Fallback
This PCNSE practice question tests your understanding of securing traffic and app-id. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 financial trading firm has a low-latency network. The firewall administrator notices that some trading application traffic is being dropped sporadically. The security policy allows the application 'trading-app' over default port 5000. The logs show the application is identified correctly as 'trading-app', but the action is deny. The administrator checks the security policy and finds that there is a prior rule that denies all traffic with application 'unknown-tcp'. What could be causing the trading application traffic to match the deny rule?
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 application 'trading-app' is not fully recognized for some sessions, causing fallback to 'unknown-tcp'.
The correct answer is A because App-ID uses a multi-pass approach: the first packet is classified based on IP/port, but the application may not be fully identified until several packets are inspected. If the trading application uses a dynamic or non-standard handshake, some sessions may not be recognized as 'trading-app' before the security policy is evaluated, causing them to fall back to 'unknown-tcp' and match the deny rule. This sporadic behavior explains why only some sessions are dropped even though the application is eventually identified correctly.
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 application 'trading-app' is not fully recognized for some sessions, causing fallback to 'unknown-tcp'.
Why this is correct
Inconsistent identification can occur if the application signature does not match all variations of the traffic.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The application is identified as both 'trading-app' and 'unknown-tcp' due to a software bug.
Why it's wrong here
App-ID does not assign multiple identities to a single session.
- ✗
The traffic is using a non-standard port, so the standard rule does not match.
Why it's wrong here
The traffic uses default port 5000, so port is not the issue.
- ✗
There is a decryption policy causing the application to be misidentified.
Why it's wrong here
No decryption policy is mentioned; the traffic is likely not encrypted.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume App-ID is instant and always correct on the first packet, but PCNSE tests the understanding that App-ID may require multiple packets to fully identify an application, leading to a temporary 'unknown-tcp' classification that can match a deny rule before the correct application is recognized.
Detailed technical explanation
How to think about this question
App-ID uses a three-phase process: first, it identifies the application by IP/port (protocol decoding), then by signature (pattern matching), and finally by behavioral analysis (e.g., SIP with ALG). The 'unknown-tcp' application is a catch-all for TCP traffic that has not been matched to any known application signature. In low-latency environments, the firewall may enforce policy on the first few packets before App-ID completes, causing 'unknown-tcp' to match if the application signature requires multiple packets or a specific exchange (e.g., a custom protocol handshake). This is a common issue with proprietary or non-standard applications that do not have a well-known signature.
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
What to study next
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FAQ
Questions learners often ask
What does this PCNSE question test?
Securing Traffic and App-ID — This question tests Securing Traffic and App-ID — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The application 'trading-app' is not fully recognized for some sessions, causing fallback to 'unknown-tcp'. — The correct answer is A because App-ID uses a multi-pass approach: the first packet is classified based on IP/port, but the application may not be fully identified until several packets are inspected. If the trading application uses a dynamic or non-standard handshake, some sessions may not be recognized as 'trading-app' before the security policy is evaluated, causing them to fall back to 'unknown-tcp' and match the deny rule. This sporadic behavior explains why only some sessions are dropped even though the application is eventually identified correctly.
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.
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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