13.3 Interfaces, up and down
This section revisits the process of configuring a router terminal and serves as a reminder of the commands to use. It then delves more deeply into the information returned after a show ip interface brief command. Watch the video below (which is just over a minute and a half long) and then test your understanding by answering the questions that follow.
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.
Activity 3 Test yourself
1. After the command Router#configure terminal, which one of the following options shows what the prompt will change to?
The correct answer is b.
2. The Router(config)# prompt shows that the CLI is at the level where configuration changes can be made. What is the name of this level?
The correct answer is d.
3. At the Router(config-if)# prompt, which of the following could be entered to have a useful effect?
Name of the router
IP address and subnet mask
The correct answer is c.
4. The following output shows part of a response to the command show ip interface brief. From the options below, choose the best explanation of the term ‘administratively down’ in the status column.
Router#show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 192.168.1.2 YES manual up up
GigabitEthernet0/1 unassigned YES unset administratively down down
GigabitEthernet0/2 unassigned YES unset administratively down down
Vlan1 unassigned YES unset administratively down down
The interface is faulty
The interface is switched off
The interface is not yet configured with an IP address
The interface is not connected to any devices
The correct answer is b.