On Juniper Networks EX3400 switches, when the interfaces are physically down, duplex is set as half by default.

You will see link-mode half duplex on extensive command.

master:0}

user@switch> show interfaces ge-0/0/0 extensive

Physical interface: ge-0/0/0, Enabled, Physical link is Down

 Interface index: 647, SNMP ifIndex: 512, Generation: 140

 Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Link-mode: Half-duplex

Once interfaces is physically up, it should change to full duplex.

user@host> show interfaces ge-0/0/0 extensive

Physical interface: ge-0/0/0, Enabled, Physical link is Up

  Interface index: 648, SNMP ifIndex: 514

  Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Link-mode: Full-duplex

09/14/2023 - KB Created