What does no shutdown command do? Cisco

[Teresa895]

Newbie here. I'm studying networking and struggling to understand the no shutdown command. I don't understand what no shutdown command actually does. I've read that it enables the interface but I don't even understand what that means. What does enabling the interface do? Does it allow you to configure its settings?

[Martin L]

if interface is UP UP , aka operational, you can shut it down; like if u want to make some changes. you issue shutdown. once you do those changes, u can bring interface back to up up, operational, by issuing no shutdown command.

"no" is use a lot in ios.  basically, if u want to undo command, meaning remove command, you can and sometimes must use no.  i.e. to add default gateway you do:  ip route 0.0.0.0. 0.0.0.0.0 1.2.3.4, to remove it you must add no in front of it, aka no ip route 0.0.0.0. 0.0.0.0.0 1.2.3.4

Regards, ML
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[Martin L]

another example, you have just configured and made mistake:

interface fastEthernet0/0

ip address 192.168.4.1 255.255.255.0

arp timeout 100

no shut

opps, i made mistake, you can fix it with no command or by typing correct numbers.

interface fastEthernet0/0

shutdown

no ip address 192.168.4.1 255.255.255.0

ip address 192.168.40.1 255.255.255.192

no arp timeout 100

no shutdown

Technically, in this case You could jsut  type correct ip without shutdown command or type default values for arp time out (if u can remember ). 

Regards, ML
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[Shaffer_388]

The no shutdown command enables an interface (brings it up). This command must be used in interface configuration mode. It is useful for new interfaces and for troubleshooting. When you're having trouble with an interface, you may want to try a shut and no shut.