FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 189603
This article describes how to fix 25 Gigabit Ethernet connection between FortiGate and Cisco Nexus.

- FortiGate with 25 Gigabit Ethernet interface.
- FortiGate with Fortinet : FN-TRAN-SFP28-SR model to connect to Cisco Nexus via fiber optic cable.
- Cisco Nexus with 25 Gigabit Ether net interface.
- Cisco Nexus with Cisco : SFP-10/25G-CSR-S model to connect to FortiGate via fiber optic cable.
- The connection between FortiGate and Cisco Nexus can be only the single interface or a link aggregation interface.

- When a FortiGate unit with 25Gigabit Ethernet interface is connected to a Cisco Nexus Ethernet interface with the default setting. But the interface is still not up for both sites.
- At Cisco Nexus site, when a user try to correct the setting at 25 Gigabit Ethernet interface with:
# fec rs-ieee
It can show the error message as below.
"ERROR: FEC validation failed on ethX/Y due to incompatible configured speed or transceiver type"
(X/Y is your Cisco Nexus interface.)
-Consider then to disable FEC (Forward error correction) for both site.

Solution to fix the issue:

At FortiGate.
# config system interface
    edit <interface name>
        set forward-error-corrrection disable           <----- (Enabled by default).
At FortiGate with link aggregation interface:
# config system interface
    edit <portX>
        set forward-error-corrrection disable           <----- (Enabled by default).
        edit <portY>
            set forward-error-corrrection disable       <----- (Enabled by default).
(portX and portY are the member of the link aggregation interface.)

At Cisco Nexus:
# conf terminal
# interface ethernet  X/Y
# set fec off