Juniper's Intrusion Prevention System (IPS), a component of the Intrusion Detection and Prevention (IDP) feature on SRX Series devices, evaluates its rule base sequentially. The rules within an IDP policy are processed in the order they are configured, from top to bottom. If a specific traffic flow matches the criteria of multiple rules, the IPS does not stop at the first match. Instead, it evaluates all potential matches and then applies the action associated with the rule that has the highest severity. This "most severe action" logic ensures that the strongest security posture is enforced for any given threat.

A. IPS evaluates rules concurrently. This is incorrect. Juniper IPS policies are processed sequentially, in order, which is a fundamental aspect of their design to ensure predictable behavior.

D. IPS applies the least severe action to traffic matching multiple rules. This is incorrect and would represent a significant security weakness. The system is designed to apply the most severe action to provide maximum protection.

1. Juniper Networks TechLibrary

SRX Series

IDP Policy Overview: "IDP rules are processed in order

from first to last... When a packet matches the criteria in multiple rules

IDP applies the action of the most severe rule to the traffic."

Source: Juniper Networks TechLibrary

"IDP Policy Overview" for Junos OS.

2. Juniper Networks TechLibrary

SRX Series

Understanding IDP Policies: "The IDP policy is made up of rulebases

and each rulebase consists of a set of rules. IDP rules are processed in order

from first to last." This document explicitly confirms the sequential nature of rule evaluation.

Source: Juniper Networks TechLibrary

"Understanding IDP Policies" for Junos OS.

3. Juniper Networks TechLibrary

SRX Series

Example: Configuring an IDP Policy with Dynamic Application Awareness: This configuration example reiterates the operational logic: "If traffic matches multiple rules

the action of the most severe rule is applied."

Source: Juniper Networks TechLibrary

"Example: Configuring an IDP Policy with Dynamic Application Awareness" for Junos OS.