FEC91 (Forward Error Correction) is a method used in networking to improve the reliability of data transmission over a network. It is often used in high-speed data links, such as those used in data centers and other mission-critical environments.

FEC mode in JunOS is a feature that allows the router to detect and correct errors in the data transmission. It is enabled by default, but in some cases, it may be necessary to disable it.

To disable FEC mode in JunOS, you will need to access the command-line interface (CLI) of the router. You can do this by connecting to the router via SSH or through the console port.

Once you are logged in, enter the following command to disable FEC mode:

set interfaces et-0/0/0 gigether-options fec none
commit

This command disables FEC mode on the et-0/0/0 interface, and the commit command saves the configuration change.

It’s important to note that disabling FEC mode may result in a decrease in the overall performance and reliability of the data transmission on the network, so it should be done only when necessary and after a thorough analysis of the network’s requirements.