Try this step
configure terminal
Enter global configuration mode.
Step 2
interface fastethernet0
Enter configuration interface mode.
Step 3
speed {10 | 100 | auto}
Configure the Ethernet speed. Cisco recommends that you use auto, the default setting.
Step 4
duplex {auto | full | half}
Configure the duplex setting. Cisco recommends that you use auto, the default setting.
Step 5
end
Return to privileged EXEC mode.
Step 6
show running-config
Verify your entries.
Step 7
copy running-config startup-config