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Solved: Differentiated Services Code Point (DSCP) replicate feature

This article describes how to copy the original DSCP marking when return traffic arrives as untagged on the FortiGate.


Currently, there are two main ways on how to mark traffic with DSCP:

Step 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>

Step 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:

This article describes how to copy the original DSCP marking when return traffic arrives as untagged on the FortiGate.

  • 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}

When the particular command is being enabled on a firewall policy, the following commands will become hidden:

# config firewall policy
edit <X>

Alex Lim is a certified IT Technical Support Architect with over 15 years of experience in designing, implementing, and troubleshooting complex IT systems and networks. He has worked for leading IT companies, such as Microsoft, IBM, and Cisco, providing technical support and solutions to clients across various industries and sectors. Alex has a bachelor’s degree in computer science from the National University of Singapore and a master’s degree in information security from the Massachusetts Institute of Technology. He is also the author of several best-selling books on IT technical support, such as The IT Technical Support Handbook and Troubleshooting IT Systems and Networks. Alex lives in Bandar, Johore, Malaysia with his wife and two chilrdren. You can reach him at [email protected] or follow him on Website | Twitter | Facebook

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