A QFX5130-48C interface may come up briefly and then stay down when the port is configured for 40G but uses a 100G QSFP28 DAC. In this case, the switch showed interface et-0/0/54 configured for speed 40g, while the inserted cable was detected as a third-party QSFP28-100G-CU5M DAC.
The interface remains admin up but link down. The switch may log errors similar to these:
Set AN Advert Failed. Invalid parameter
Enabling AN+LT Failed. Operation failed
This usually points to a negotiation or link training problem caused by a mismatch between the configured speed and the media type, rather than a hardware failure. In this case, the port itself supports 40G, but the installed DAC is identified as 100G media.
The recommended fix is to match the configured speed with the media type on both ends of the link.
As a workaround, you can also test with FEC disabled on both ends while keeping both sides hard-set to 40G:
set interfaces et-0/0/54 ether-options fec none
If the link still does not come up, replace the cable with a supported 40G QSFP+ DAC for 40G use, or keep the current DAC and run the link at 100G. Third-party DAC compatibility may also be a contributing factor.
Reference for port speed specifications on QFX5130-48C devices: https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet-switches/topics/topic-map/port-speed-qfx-switches.html#xd_c3b24b0f5e29504d--1234b8b4-189d35e37cb--7ee8