The QFX5120-48Y interface was not coming up when connected to an Array Networks device. During troubleshooting, a speed mismatch was identified between the QFX5120-48Y interface and the transceiver being used on the remote device.

The QFX5120-48Y port was configured to operate at 25G, while the remote Array device was using a 10G transceiver.

• Port 32 on the QFX5120-48Y remained Down and the link was not established.
• The QFX5120-48Y port was configured for 25G, while the remote side was using a 10G transceiver.
• A third-party 10G transceiver was initially used on the QFX5120-48Y. Since it is a non-Juniper optic, its compatibility and behaviour could not be confirmed.

• By default, in QFX5120-48Y, ports 0–47 are set to 10G. These ports are hardware-bound in quads (groups of four), starting at port 0 (e.g., 0–3, 4–7, 8–11, etc.). You cannot change the speed of a single port; you must change the entire quad group at the chassis level

set chassis fpc <fpc-slot> pic <pic-number> port <port-number> speed <speed>

• The existing 25G configuration was removed from the QFX5120-48Y chassis.

delete chassis fpc <fpc-slot> pic <pic-number> port <port-number> speed <speed>

• A supported Juniper 10G transceiver was installed on port 32 and the interface was restored to its default 10G operation.
• After the port was configured for 10G operation, the link came up successfully.
• The functionality was further verified by connecting a DAC cable, and the port operated as expected.

The issue was caused by a speed mismatch between the QFX5120-48Y interface configured for 25G and the 10G transceiver used on the remote side. Configuring the port for the appropriate 10G operation and using a supported Juniper transceiver resolved the issue.

https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet-switches/topics/topic-map/port-speed-qfx-switches.html#xd_4c6d88350d41a1cd-2baf0cb8-18e0e3b28c6--7d83