The automatic training algorithm used in U-Boot revision 2.0 would end up with incorrect memory interface parameters that did not work in all cases. FPC might fail to boot or have runtime crash due to memory hang.
A new memory training algorithm was developed and implemented in U-Boot revision 3.0. QFX10000-36Q Assy # 750-051354 Rev 33 and above includes new U-Boot 3.0. Assy # 750-068822 Rev 4 and above includes new U-Boot 3.0. QFX10000-30C Assy # 750-051357 Rev 30 and above includes new U-Boot 3.0 Revisions before that would include U-Boot Rev 2.0, which might be susceptible to the problem. Customers are recommended to upgrade line card U-Boot firmware on these affected units.
See attached document for firmware upgrade procedure