Customer can try to connect a SFP 1g but the interface will remains down , this can work with copper but no with fiber
The 1g SFP will be recognized with no issues, however the interface will be down root@EX2300> show chassis hardware detail no-forwarding Hardware inventory: Item Version Part number Serial number Description Chassis HV2222222222 EX2300-C-12P Pseudo CB 0 Routing Engine 0 BUILTIN BUILTIN RE-EX2300C-12P da0 1910 MB USB 65009849BB256399 Virtio Block Disk usb0 (addr 0.1) EHCI root HUB 0 Broadcom uhub0 usb1 (addr 0.2) USB2.0 Hub 1544 vendor 0x05e3 uhub1 usb2 (addr 0.3) USB DISK 2.0 16640 vendor 0x13fe umass0 FPC 0 REV 18 650-059984 HV3619340405 EX2300-C-12P CPU BUILTIN BUILTIN FPC CPU PIC 0 REV 18 BUILTIN BUILTIN 12x10/100/1000 Base-T PIC 1 REV 18 650-059984 HV3619340405 2x10G SFP/SFP+ Xcvr 0 REV 01 740-011613 ADDBF8AD1376 SFP-SX >>>>>>>>>>>>>>>>>>>>>>>>>>>>> sfp being recognized Power Supply 0 JPSU-170W-AC
You can use the following configuration to make the 1g SFP works
set interfaces ge-0/1/0 speed 1g
set interfaces ge-0/1/0 link-mode full-duplex
set interfaces ge-0/1/0 ether-options no-auto-negotiation
Reference link https://www.juniper.net/documentation/us/en/hardware/ex2300/topics/topic-map/ex2300-system-overview.html#id-ex2300-switches-hardware-overview__d21428e79