C9300-24T#
C9300-24T#conf t
C9300-24T(config)#int twentyFiveGigE 1/1/1
C9300-24T(config-if)#fec ?
 auto  Enable FEC Auto-Neg
cl108  Enable clause108with 25G
 c174  Enable clause74 with 25G
 cl91  Enable clause91 with 100G
  off  Turn FEC off

C9300-24T(config-if)#fec cl108
C9300-24T(config-if)#exit