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2 different LAGs can be built on the same QSFP port after being split (say port17.1 & 17.2 on LAG1 and port 17.3 & 17.4 on LAG2). These subports behave like regular physical interfaces and can be aggregated into separate LAGs without issues.
FortiGate QSFP28 breakouts are standards-based IEEE 802.3 and compatible with any vendor respecting the same standard.
FortiGate (QSFP28 port17) -> [QSFP28-to-4×SFP28 DAC].
├─ SFP28 #1 → SW1 (25G port A) → LAG1.
├─ SFP28 #2 → SW1 (25G port B) → LAG1.
├─ SFP28 #3 → SW2 (25G port C) → LAG2.
└─ SFP28 #4 → SW2 (25G port D) → LAG2.
- Enable QSFP28 Port Splitting:
config system global
set split-port port17
end
The system will reboot to make the setting take effect.
After the reboot: port17 → port17.1, port17.2, port17.3, port17.4 (all default 10G).
-
Set Subport Speed to 25G (All 4 at once).
config system interface
edit "port17.1"
set speed 25000full
next
end
Speed change applies to all four subports simultaneously. No reboot needed.
-
Create LAG1 (port17.1 + 17.2 → SW1).
config system interface
edit "LAG1"
set vdom "root"
set type aggregate
set member "port17.1" "port17.2"
set lacp-mode active
set device-identification enable
set lldp-transmission enable
set role lan
set allowaccess ping https ssh
set ip 10.1.1.1 255.255.255.0
next
end
-
Create LAG2 (port17.3 + 17.4 → SW2).
config system interface
edit "LAG2"
set vdom "root"
set type aggregate
set member "port17.3" "port17.4"
set lacp-mode active
set device-identification enable
set lldp-transmission enable
set role lan
set allowaccess ping https ssh
set ip 10.2.2.1 255.255.255.0
next
end
-
Verification commands:
Check split status: get system interface physical.
Confirm 25G speed: diagnose hardware deviceinfo nic port17.1.
LAG status: diagnose netlink aggregate name LAG1.
Traffic stats: diagnose netlink interface list LAG1.
Related documents:
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