Technical Tip: Differentiated Services Code Point (DSCP) replicate feature

[sgiannogloudis]

Description	This articles describes how to copy the original DSCP marking when return traffic arrives as untagged on the FortiGate.
Scope	FortiOS 7.2.1+ versions.
Solution	Initially, in order to have an understanding regarding the DSCP feature on FortiOS, read this related KB article: https://community.fortinet.com/t5/FortiGate/Technical-Tip-Differentiated-Services-Code-Point-DSCP-ma... Currently, there are two main ways on how to mark traffic with DSCP: 1) Directly via the firewall policy: # config firewall policy     edit <X>         set diffserv-forward enable         set diffservcode-forward <binary_integer>         set diffserv-reverse enable         set diffservcode-rev <binary_integer>  2) Or via as SD-WAN rule: # config system sdwan # config service     edit <X>         set dscp-forward enable         set dscp-reverse enable         set dscp-forward-tag <binary_integer>         set dscp-reverse-tag <binary_integer> Let's assume, there is the below topology:  - End hosts generate traffic towards the servers which are located behind Hub FortiGate. - Spoke FortiGate is marking the traffic with the configured DSCP values for specific type of traffic. - Servers usually do not have the capability to mark the right QoS values, so reply traffic is arriving as untagged on Hub FortiGate and traverses without the correct DSCP values on its way back to branch. - In order to have bidirectional end-to-end QoS markings on the reply traffic as well, it is necessary to configure the firewall policy on the Hub with the below commands: # config firewall policy     edit <X>         set diffserv-copy {enable | disable} end When the particular command is being enabled on a firewall policy, the following commands will become hidden: # config firewall policy     edit <X>     diffserv-forward     diffserv-reverse     diffservcode-forward     diffservcode-rev