Technical Tip: Configuring 100GBASE-DR QSFP28 Transceiver Support with 'dr' Mediatype

Description

This article describes how to configure FortiGate interfaces to support 100GBASE-DR QSFP28 transceivers using the 'dr' mediatype setting, and explains the relationship between mediatype and Forward Error Correction (FEC) in this context.

Scope

FortiGate platforms with 100G QSFP28 ports. FortiOS 7.6.1 GA and later.

Solution

100GBASE-DR transceiver characteristics.

100GBASE-DR QSFP28 transceivers have the following key features:

• 500m range using a single-mode fiber in each direction.

• 53.125 GBd PAM4 line-side modulation using a single lambda.

• Line-side FEC implemented by the transceiver itself - no FEC is required on the host interface.

These transceivers use an internal gearbox to convert between the 4-lane NRZ host interface and the 1-lane PAM4 line side.

Important: Because of this gearbox architecture, 100GBASE-DR transceivers do not support 40G operation.

Starting from FortiOS 7.6.1 GA, the dr value has been added as a valid option for the mediatype property on 100G interfaces.

Key behavior when mediatype is set to dr:

• forward-error-correction is set to disable by default - this is mandatory, as the transceiver handles FEC internally.

• The interface uses an SR4 hardware configuration internally for the connection to the DR transceiver.

Test #1 - Setting mediatype to DR.

The following steps show how to configure mediatype dr on a 100G interface and verify the resulting configuration.

Step 1: Set mediatype to dr.

FG1K1F-3 (interface) # edit port34

FG1K1F-3 (port34) # set mediatype
  sr4    Use Short Range transceiver(4 lane)
  lr4    Use Long Range transceiver(4 lane)
  cr4    Use Copper transceiver(4 lane)
  dr     Use Depot Reach transceiver

FG1K1F-3 (port34) # set mediatype dr

FG1K1F-3 (port34) # end
Interface port34: changing FEC to disable because of a mediatype change to dr

Step 2: Verify the resulting configuration.

FG1K1F-3 # sh sys int port34
config system interface
    edit "port34"
        set vdom "root"
        set type physical
        set mediatype dr
        set snmp-index 34
        set forward-error-correction disable
        set speed 100Gfull
    next
end

Step 3: Confirm link is up.

FG1K1F-3 # diagnose hard dev nic port34 | grep -B 2 link
==== Link Status ===============
Admin      :Up
link_status:Up

FEC restriction in DR mode:

All 100GBASE-DR transceivers have embedded FEC. As a result, it is not allowed to enable FEC at the FortiGate interface level when mediatype is set to dr.

The only available option for forward-error-correction is disable:

FG1K1F-3 # config sys int

FG1K1F-3 (interface) # edit port34

FG1K1F-3 (port34) # set forward-error-correction
  disable    Disable forward error correction (FEC).

Note: The disable option is the only selectable value when mediatype dr is active.

Test #2 - Setting mediatype to SR4 or LR4 (comparison).

For reference, this test shows the behavior when mediatype is set to sr4 with a DR transceiver, illustrating why the correct mediatype selection is critical.

Step 1: Set mediatype to sr4.

FG1K1F-3 # config sys int

FG1K1F-3 (interface) # edit port34

FG1K1F-3 (port34) # set mediatype sr4

FG1K1F-3 (port34) # end
Interface port34: changing FEC to cl91-rs-fec because of a mediatype change to sr4

When mediatype sr4 is set, the interface automatically enables FEC cl91-rs by default.

Step 2: Verify the resulting configuration.

FG1K1F-3 # sh sys int port34
config system interface
    edit "port34"
        set vdom "root"
        set type physical
        set mediatype sr4
        set snmp-index 34
        set forward-error-correction cl91-rs-fec
        set speed 100Gfull
    next
end

Step 3: Check link status - the link is DOWN.

FG1K1F-3 # diagnose hard dev nic port34 | grep -B 2 link
==== Link Status ===============
Admin      :Up
link_status:Down

Root Cause: The link is down because FEC is enabled at the interface level (cl91-rs-fec) and the DR transceiver also has embedded FEC. This double-FEC conflict causes the link to fail.

If mediatype sr4 must be used with a DR transceiver (not recommended), manually disable FEC at the interface level:

FG1K1F-3 # sh sys int port34
config system interface
    edit "port34"
        set vdom "root"
        set type physical
        set mediatype sr4
        set snmp-index 34
        set forward-error-correction disable
        set speed 100Gfull
    next
end

Verify the link is now up:

FG1K1F-3 # diagnose hard dev nic port34 | grep -B 2 link
==== Link Status ===============
Admin      :Up
link_status:Up

Summary: Mediatype and FEC behavior.

Best practice: Always use 'set mediatype dr' when connecting 100GBASE-DR QSFP28 transceivers on FortiOS 7.6.1 GA or later. This ensures FEC is automatically disabled, matching the transceiver's embedded FEC behavior and maintaining a stable link.