This KB describes the issue which interface does not come up when using certain DAC cable on EX or QFX series switch and provides resolution.

On EX or QFX series switch, interface does not come up when using certain DAC cable.

You may observe the issue such as below:

• VCP does not come up and virtual chassis would be broken when upgraded to affected releases.
• Network port does not come up.
• The same issue would be remained even after rebooting the device.

This issue is seen with QSFP+-40G-CU50CM only. Other DAC cables doesn't hit this issue.
For example, here is the output when VCP does not come up on QFX5110. The issue is seen with QSFP+-40G-CU50CM but not with QSFP+-40G-CU1M.

{master:0}
root@QFX5110> show chassis hardware | match "FPC|QSFP"
FPC 0            REV 27   650-061152   WS3719400132      QFX5110-48S-4C
  CPU                     BUILTIN      BUILTIN           FPC CPU
    Xcvr 48      REV 01   740-038623   MOC15106231049    QSFP+-40G-CU1M
    Xcvr 50      REV 01   740-044512   MOC14125122561    QSFP+-40G-CU50CM
FPC 1            REV 31   650-061152   WS3721140296      QFX5110-48S-4C
  CPU                     BUILTIN      BUILTIN           FPC CPU
    Xcvr 48      REV 01   740-038623   MOC15106231049    QSFP+-40G-CU1M
    Xcvr 50      REV 01   740-044512   MOC14125122561    QSFP+-40G-CU50CM

root@QFX5110> show virtual-chassis vc-port
localre:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
0/48        Configured         -1    Up           40000        1   vcp-255/0/48
0/50        Configured         -1    Down         40000 

fpc1:
--------------------------------------------------------------------------
Interface   Type              Trunk  Status       Speed        Neighbor
or                             ID                 (mbps)       ID  Interface
PIC / Port
0/48        Configured         -1    Up           40000        0   vcp-255/0/48
0/50        Configured         -1    Down         40000   

The root cause was there in setting pre-emphasis value for QSFP+-40G-CU50CM.
This lead to avoid link up and issue was seen.

The fix has been implemented in PR1656540 for setting proper pre-emphasis value.
PR1656540: The interface might not come up on EX platforms

Here is the OS which has the issue:
20.4R3-S4 and later
21.1R3-S3 and later
21.2R3-S2 and later
21.3R3-S1 and later

2023-05-17 : Created a KB.