hardMultiple ChoiceObjective-mapped
IPS Signature Tuning — Reducing False Positives
A Cisco Firepower sensor is generating a high number of false positives from a rule that triggers on large ICMP packets. The analyst suspects the rule threshold is too low. Which tuning action most effectively reduces false positives while maintaining detection of actual attacks?
Quick Answer
The answer is to increase the packet size threshold. This tuning action directly addresses the root cause of false positives by raising the minimum packet size that triggers the signature, ensuring that legitimate large ICMP traffic is ignored while still capturing true oversized attack packets, such as those exceeding 65,535 bytes in a fragmented ICMP echo attack. On the Cisco CyberOps Associate 200-201 exam, this scenario tests your understanding of IPS signature tuning to reduce false positives without sacrificing detection—a common trap is choosing to disable the rule entirely or lower the threshold, which either eliminates detection or worsens noise. Remember the memory tip: “Raise the size, lower the noise” to keep detection sharp while filtering out benign traffic.
⚠ Common exam trap
Cisco often tests the misconception that changing the rule action (e.g., to drop) or adding exceptions is the best way to reduce false positives, when in fact the most precise and effective method is to adjust the detection threshold parameter that is causing the false positives.
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
✓
Increase the packet size threshold.
Increasing the packet size threshold (Option D) directly addresses the root cause of the false positives: the rule is triggering on legitimate large ICMP packets that are below the actual attack size. By raising the threshold to a value that still captures known attack vectors (e.g., ICMP echo requests exceeding 65,535 bytes in a fragmented attack), the sensor reduces noise while preserving detection of true malicious oversized packets. This is the most effective tuning action because it adjusts the detection parameter without disabling or bypassing the rule.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Change the rule action from alert to drop.
Why it's wrong here
Changing action does not reduce false positives; it still triggers.
- ✗
Disable the rule entirely.
Why it's wrong here
Disabling the rule removes detection capability.
- ✗
Add an exception for trusted source IPs.
Why it's wrong here
Exceptions help but are less systematic than adjusting the threshold.
- ✓
Increase the packet size threshold.
Why this is correct
This directly addresses the cause of false positives without disabling detection.
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Same concept, more angles
2 more ways this is tested on 200-201
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. An IPS sensor is configured inline and drops traffic that triggers the signature 'OVERFLOW-ICMP-ECHO', which triggers on ICMP packets with size > 1024 bytes. A network administrator reports that legitimate network monitoring tools using large ICMP packets are being blocked. What is the best course of action?
hard- A.Increase the threshold to 2048
- ✓ B.Create a whitelist for the monitoring tool's source IP
- C.Disable the signature entirely
- D.Change the sensor mode to IDS for that signature
Why B: Creating a whitelist for the monitoring tool's source IP allows the IPS to continue dropping malicious oversized ICMP packets while permitting legitimate traffic from known, trusted sources. This maintains security for the rest of the network without disabling the signature or changing its mode, which would reduce protection.
Variation 2. Which THREE factors should be considered when tuning an IPS signature? (Choose three.)
hard- A.Time of day
- ✓ B.Application protocol
- ✓ C.Source and destination IPs
- ✓ D.Signature severity
- E.Packet length
Why B: Application protocol (B) is critical because IPS signatures are protocol-aware and must match the specific protocol context (e.g., HTTP, SMTP, DNS) to avoid false positives. Tuning based on the protocol ensures the signature only inspects traffic where the vulnerability or exploit is relevant, such as applying a SQL injection signature only to HTTP traffic.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-201 practice question is part of Courseiva's free Cisco 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 200-201 exam.