On an SRX Series device, for a new traffic session (first-path processing), the packet flow logic requires determining the destination zone before a security policy can be evaluated. The destination zone is derived from the egress interface. To identify the correct egress interface for the packet's destination IP address, the device must first perform a longest-match lookup in the Layer 3 routing table. Only after the route lookup is complete and the destination zone is known can the device proceed with the security policy search to determine if the traffic is permitted between the source and destination zones.

A: This is incorrect. A security policy search requires both a source and destination zone. The destination zone is determined by the egress interface, which is found via the route lookup.

B: This is incorrect for the same reason as A. The policy search follows the route lookup. Additionally, Application Layer Gateway (ALG) processing occurs after a policy permits the session.

D: This is incorrect. ALG checks are applied to traffic that has already been permitted by a security policy, not before the policy search. The sequence is route lookup, then policy search.

1. Juniper Networks TechLibrary

Understanding Packet Flow in SRX Series Devices. This document outlines the first-path processing sequence. It explicitly shows that after initial packet sanity checks and NAT processing

the device performs a "Route lookup" to find the egress interface and zone. This is followed by the "Policy lookup" to check for a matching policy that allows the traffic.

2. Juniper Networks TechLibrary

Security Policies Processing. This documentation explains that a security policy match is based on traffic direction

defined by a from-zone and a to-zone. It states

"The device derives the from-zone from the interface the packet enters and the to-zone from the interface the packet exits." The exit interface is determined by the route lookup

confirming the route lookup must precede the policy lookup.