This article explains why when setting up "FEC91", the interface goes down when using QSFP-100GBase-DR.

Symptoms

Interface goes down when setting up "FEC91" under the following hierarchy:

[edit interfaces interface-name gigether-options]

FEC is integrated into the 100G DR optic itself. Per IEEE clause 91RS(544,514) FEC on XCVR module is enabled during normal operations and the electrical interface between the pluggable and the PFE runs without FEC. So manual configuration of "FEC91" is not needed for 100G-DR optical interface.

In conclusion, you do not have to configure "FEC91" on this optic as it is already enabled, even if on the CLI output shows up as "FEC NONE", the interface will come up if is connected to a peer that has "FEC91" enabled.