Eliminating dropped packets on LAG interfaces

In some network and traffic configurations and for some FortiGate models with NP6 processors, traffic passing through a LAG may experience excessive amounts of dropped packets. This can happen if the FortiGate switch fabric and NP6 processor select different ingress and egress XAUI interfaces for the same traffic flow through a LAG interface, resulting in possible collisions and dropped packets.

Some FortiGate models allow you to resolve this problem by using the following command to cause both the switch fabric and the NP6 processor to use the same XAUI port mapping:

config system npu

set lag-out-port-select {disable | enable}

end

This option is disabled by default, causing the FortiGate to use a different method for selecting ingress and egress XAUI interfaces for a LAG than for a single interface. Normally the default setting is recommended.

If you enable lag-out-port-select, the FortiGate uses the same method for selecting the ingress and egress XAUI interfaces for LAGs as is used for standalone interfaces; which should eliminate the dropped packets. This option is supported on some FortiGate models with NP6 processors including the FortiGate-3800D family, 3900E family, 5001E, 6000F family and 7000E family.