When using 1G optic on the uplink port for the Juniper Networks EX4100 switch the interface is down on the peer device and it can show up locally as up.

On Juniper Networks EX4100, when using 1G Optic for the uplink port the interface shows locally up:

{master:0}
root@EX4100> show interfaces terse | match 0/2/2
ge-0/2/2               up   up
ge-0/2/2.0             up   up  eth-switch

However, the peer device (EX2300-MP for this test) shows the interface down:

{master:0}
root@EX2300-MP> show interfaces terse | match 0/1/0
ge-0/1/0               up   down
ge-0/1/0.0             up   down eth-switch

Please take in consideration that there may also be cases in which local interface will also show down. For example, in somes cases between EX4100 to EX4600).

EX4100 series and EX4100-F series: VC (PIC1) 4x1G ports in network-mode will need auto-negotiation disabled on the peer device for the port to come up.

{master:0}[edit]
root@EX2300-MP# set interfaces ge-0/1/0 ether-options no-auto-negotiation
 
{master:0}[edit]
root@EX2300-MP# commit and-quit
configuration check succeeds
commit complete
Exiting configuration mode
 
{master:0}
root@EX2300-MP> show interfaces terse | match 0/1/0
ge-0/1/0               up   up
ge-0/1/0.0             up   up  eth-switch

2023-05-12 : KB created

2024-10-22: KB updated to reflect the interface might be down.