This article refers to a scenario experienced where a link between EX4650 and QFX5120 using 740-158998 QSFP28-100GBASE-SR1.2 was not coming up.

FEC mode configured the same on both sides:

root@EX4650# set interfaces et-0/0/0 gigether-options fec fec91

root@QFX5120# set interfaces et-0/0/0 gigether-options fec fec91

Link down:

root@EX4650> show interfaces et-0/0/0 extensive

Physical interface: et-0/0/0, Enabled, Physical link is down

After checking the fec mode on EX4650 side, it was working as none. I changed the fec to fec74 and checked the vty output and fec was not changing.

> request pfe execute target fpc0 command "show dcb ifd all"

 ifd name   global-dev local-dev  port-num  port-name

 ----------------------------------------------------------

 et-0/0/0       0     0     57    xe0

 et-0/0/1       0     0     58    xe1

 et-0/0/2       0     0     59    xe2

-Checking the output for et-0/0/0 ( xe0 ), the output shows FEC clause as false which means is not configured. This value is not changing.

>request pfe execute target fpc0 command "set dcb bc \"phy control xe0\""

CL74           = False

CL91           = False

CL108          = False

Possibly fec not supported for this optic on EX4650, after setting the QFX side to none, link came up fine.