Update:
It looks like the "untrusted" option is unsupported on the EX3400 platform.
VLAN-CLIENT-TEST { vlan-id 100; forwarding-options { dhcp-security { arp-inspection; group GROUP-TEST { overrides { ## ## Warning: statement ignored: unsupported platform (ex3400-48p) ## untrusted; } } } } }
According to this documentation, it works on the EX9200.
Does anyone know if/when this feature will be supported on the EX3400?