When a Juniper QFX5120-48Y switch is connected to a peer device using a 1-Gbps fiber SFP transceiver, the interface may appear up/up on the Juniper side while remaining down/down on the peer device. This behavior is typically observed during device replacement or migration scenarios and is caused by auto-negotiation limitations on the QFX5120 platform when using 1-Gbps fiber SFPs.
Interface shows up/up on Juniper QFX5120
Corresponding interface shows down/down on the peer device
No alarms, errors, or CRCs observed on the Juniper interface
Hardcoding speed and duplex on the Juniper interface does not resolve the issue
Disabling auto-negotiation on the Juniper interface alone does not bring the link up
Juniper QFX5120-48Y switches do not support auto-negotiation when a 1-Gbps fiber SFP transceiver is used. In such cases, auto-negotiation must be disabled on the peer device interface.
Steps to resolve:
Disable auto-negotiation on the peer device interface connected to the QFX5120.
Configure the peer interface speed to 1 Gbps.
Bounce (disable/enable) the peer interface.
Verify that the interface comes up on both ends.
Once auto-negotiation is disabled on the peer device and the interface is bounced, the link should establish successfully.
https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet-switches/topics/topic-map/port-speed-qfx-switches.html