PCNSE Securing Traffic and App-ID Practice Question
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 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'.
Option C is correct: App-ID may correctly identify most sessions as 'trading-app', but if some sessions have slightly different characteristics (e.g., variations in the protocol), the firewall may fail to identify them and fall back to 'unknown-tcp'. The prior deny rule then blocks those sessions. Option A is wrong because App-ID does not assign multiple identities to the same session. Option B is wrong because the traffic uses the default port. Option D is wrong because no decryption policy is mentioned.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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
Standard ACLs match source addresses.
✗
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: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PCNSE ACL questions on filtering logic and placement.
Securing Traffic and App-ID — This question tests Securing Traffic and App-ID — Standard ACLs match source addresses..
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'. — Option C is correct: App-ID may correctly identify most sessions as 'trading-app', but if some sessions have slightly different characteristics (e.g., variations in the protocol), the firewall may fail to identify them and fall back to 'unknown-tcp'. The prior deny rule then blocks those sessions. Option A is wrong because App-ID does not assign multiple identities to the same session. Option B is wrong because the traffic uses the default port. Option D is wrong because no decryption policy is mentioned.
What should I do if I get this PCNSE question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PCNSE ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Question Discussion
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