When implementing an application override in a Palo Alto Networks firewall, the primary goal is to explicitly define how specific traffic is identified and processed by the firewall, bypassing the regular App-ID process. This is particularly useful for traffic that might be misidentified by App-ID or for applications that require special handling for performance reasons.
To successfully implement application override, the following items must be configured:
B). Application override policy rule:
This is a specialized policy rule that you create to specify the criteria for the traffic you want to override. In this rule, you define the source and destination zones, addresses, and ports. Instead of relying on the App-ID engine to identify the application, the firewall uses the criteria defined in the application override policy to classify the traffic.
C). Security policy rule:
After defining an application override policy, you must also configure a security policy rule to allow the overridden traffic through the firewall. This rule specifies the action (allow, deny, drop, etc.) for the traffic that matches the application override policy. It's essential to ensure that the security policy rule matches the traffic defined in the application override policy to ensure that the intended traffic is allowed through the firewall.
For detailed guidance on configuring application override and the necessary security policies, refer to the official Palo Alto Networks documentation. This resource provides step-by-step instructions and best practices for effectively managing traffic using application overrides.

https://docs.paloaltonetworks.com/globalprotect/9-1/globalprotect-admin/globalprotect-gateways/split-tunnel-traffic-on-globalprotect-gateways/configure-a-split-tunnel-based-on-the-domain-and-application

To address the issue of application performance degradation due to excessive VoIP traffic, the administrator should configure QoS on the egress interface for the traffic flows and a QoS profile defining traffic classes. QoS stands for Quality of Service, which is a feature that allows the firewall to manage bandwidth usage and prioritize traffic based on various criteria, such as application, user, service, etc. QoS can help improve the performance and quality of latency-sensitive applications, such as VoIP, by guaranteeing them sufficient bandwidth and priority over other traffic1.
To enable QoS on the firewall, the administrator needs to create a QoS profile and a QoS policy. A QoS profile defines the eight classes of service that traffic can receive, including priority, guaranteed bandwidth, maximum bandwidth, and weight. A QoS policy identifies the traffic that matches a specific class of service based on source and destination zones, addresses, users, applications, services, etc2. The administrator can also create a custom QoS profile or use the default one.
The administrator should apply QoS on the egress interface for the traffic flows, which is the interface where the traffic leaves the firewall. This is because QoS can only shape outbound traffic and not inbound traffic. The egress interface can be either internal or external, depending on the direction of the VoIP traffic. For example, if the VoIP traffic is from internal users to external servers, then the egress interface is the untrust interface facing the ISP. If the VoIP traffic is from external users to internal servers, then the egress interface is the trust interface facing the LAN3.
The administrator should assign a high priority and a sufficient guaranteed bandwidth to the VoIP traffic in the QoS profile. This will ensure that the VoIP packets are processed first by the firewall and are not dropped or delayed due to congestion. The administrator can also limit or block other applications that consume too much bandwidth or pose security risks in the same or different QoS classes4.
An Application Override policy for SIP traffic is not necessary to address this issue. An Application Override policy is used to change or customize the App-ID of certain traffic based on port and protocol criteria. This can be useful for optimizing performance or security for some applications that are difficult to identify or have non-standard behaviors. However, SIP is a predefined App-ID that identifies Session Initiation Protocol (SIP) traffic, which is commonly used for VoIP signaling. The firewall can recognize SIP traffic without an Application Override policy5.
QoS on the ingress interface for the traffic flows is not effective to address this issue. As mentioned earlier, QoS can only shape outbound traffic and not inbound traffic. Applying QoS on the ingress interface will not have any impact on how the firewall handles or prioritizes the incoming packets6.
A QoS policy for each application is not required to address this issue. A QoS policy can match multiple applications in a single rule by using application filters or application groups. This can simplify and consolidate the QoS policy configuration and management. The administrator does not need to create a separate QoS policy for each application unless there is a specific need to assign different classes of service or parameters to each application7.