Symptom
The LR4 transceiver such as QSFP-100G-LR4-S does not support fec and is internally set to fec off.
However, fec rs-fec can be configured, which is misleading. Change is to disallow the CLI fec rs-fec
for this transceiver type.
Conditions
When user configures fec rs-fec on a physical interface that has the LR4 optics,
the command is rejected. When an ascii startup-config already has this CLI,
the command, the command is rejected. When a saved config file has this CLI,
the operation of "configure replace " can fail at this command.
In this case, user has to manually edit and remove this CLI from the saved config.
This command exists when LR4 optics was deployed. Only in a H image, the CLI is
rejected. It is applicable to N9K and N3K switches that use the LR4 optics.
