The priority to packets can be set at layer 3 by changing the DSCP values. This document describes the steps to configure DSCP on the Palo Alto Networks device.
On the webGUI, open/add a security rule.
Navigate to the Other Settings section under the Actions tab:
Select IP Dscp or IP Precedence based on the need and select the underlying options.
In the following example, the "ef" (Expedited Forwarding or Express Forward) is selected:
The "ef" marking is applied if the first 3 bits out of the 6 bits allocated to DSCP in the Layer 3 packet header is 101 (EF). This is generally used for critical or RTP based voice traffic and similar applications.
The following is an example packet capture taken on the Palo Alto Networks firewall at the transmit stage: