ACX5448 needs to disable FEC (Forward Error Correction) mode to bring up 40G-links with QFX5100 because QFX5100 does not support FEC.
40G links between ACX5448 and QFX5100 switch stay down even with known good fibers and QSFP modules.
By default, ACX5448 40G-QSFP modules use FEC74 mode:
user@ACX5448# run show interfaces et-0/1/0 extensive | match fec Active defects : LINK, LOCAL-FAULT Ethernet FEC Mode : FEC74 <-- Default FEC settings. Ethernet FEC statistics Errors FEC Corrected Errors 0 FEC Uncorrected Errors 0 FEC Corrected Errors Rate 0 FEC Uncorrected Errors Rate 0
user@ACX5448> show chassis hardware Hardware inventory: Item Version Part number Description Chassis ACX5448 Midplane REV 07 650-077913 ACX5448
MIC 1 16x10/25GE 4x40/100GE PIC 1 BUILTIN 16x10/25GE 4x40/100GE Xcvr 0 REV 01 740-043308 QSFP+-40G-LR4
But QFX5100 does not support FEC on 1G/10G/40G interfaces, which results in Interface down:
user@QFX5100> show interfaces et-0/0/1 extensive | match "fec mode" | count Count: 0 lines <<- Means no FEC settings {master:0} user@QFX5100> show chassis hardware Hardware inventory: Item Version Part number Description Chassis QFX5100-24Q-2P Pseudo CB 0 Routing Engine 0 BUILTIN QFX Routing Engine FPC 0 REV 06 650-056265 QFX5100-24Q-2P CPU BUILTIN FPC CPU PIC 0 BUILTIN 24x 40G-QSFP Xcvr 1 REV 01 740-052009 QSFP+-40G-LR4
Commit the following configuration to disable FEC mode on ACX5448 side:
user@ACX5448# set interfaces et-0/1/0 gigether-options fec none user@ACX5448# commit user@ACX5448# run show interfaces et-0/1/0 | match fec Active defects : None Ethernet FEC Mode : NONE <<- FEC mode is disabled now. Ethernet FEC statistics Errors FEC Corrected Errors 0 FEC Uncorrected Errors 0 FEC Corrected Errors Rate 0 FEC Uncorrected Errors Rate 0
Then check interface status on ACX5448 and QFX5100 respectively.