13.3 Interfaces, up and down

Although privilege exec mode lets you see all the interfaces by using the show ip interface briefcommand, it doesn’t let you change any configurations. To do that you have to go down a level to ‘global configuration’ mode by typing configure terminalor conf t. Notice the prompt changes again. And then we have to choose the interface we’re configuring. You don’t have to type the whole name: this abbreviation g0/0is enough. Once again the prompt changes.

You have already seen how to apply an IP address and a subnet mask to the terminal, so I won’t go over that again. You can’t actually change the configuration of an interface while it is ‘up’, or operating. So by default interfaces are initially down, both ‘administratively’ and in terms of the protocol, as shown here. ‘Administratively down’ means that the interface is switched off. Under the ‘Protocol’ heading, ‘down’ means that the interface is not able to communicate with an interface it is connected to. We switch on an interface that is administratively down by using the no shut command at the same prompt that we use for assigning an IP address and a subnet mask to an interface. If the interface is connected to another interface that is administratively up, the no shutcommand usually causes the status under the ‘Protocol’ heading to change to ‘up’ too. That’s because interconnected interfaces that are administratively ‘up’ can negotiate the details of the protocol between themselves, so you don’t have to configure that yourself.

Thank you for watching.