Creating and Configuring Loopback Interfaces

A loopback interface is a virtual interface that resides on a router. It is not connected to any other device. Loopback interfaces are very useful because they will never go down, unless the entire router goes down. This helps in managing routers because there will always be at least one active interface on the routers, the loopback interface.

To create a loopback interface, all you need to do is enter configuration mode for the interface:

Router(config)interface loopback {number}

The only option on this command is to specify a number between 0 and 2,147,483,647. Cisco IOS Software gives you plenty of loopback interfaces, if you want to use all of them. When entering this command, Cisco IOS Software automatically creates the loopback interface, places you into interface configuration mode, and removes the interface from shutdown mode. When that is complete, you only need to assign an IP address to the interface. The criteria for the IP addresses of the loopback interfaces is as follows:

  • Create loopback interfaces on all routers using IP address 192.169.X. X/24 (where X is the router number). So, R1 would have a loopback address of 192.169.1.1/24, R2 would be 192.169.2.2/24, and so on.

Because you are already on R6, create and configure the loopback interface on R6; then go to R1, R2, and so on, and create and configure all the loopback interfaces and assign appropriate IP addresses. Example 7-42 takes you through the process on R6.

Example 7-42. R6 Loopback Interfaces Configuration

R6(config)#interface loopback 0
R6(config-if)#
%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
R6(config-if)#ip address 192.169.6.6 255.255.255.0

Because the router automatically removes the loopback from shutdown state, you receive the console message indicating that the interface is up.

Now configure the rest of the routers, starting with R1. Don't forget to save the running-config to NVRAM (startup-config) before leaving the routers. See Example 7-43.

Example 7-43. Loopback Interface Configuration

R6#<ctrl-shft-6><x>
    Termserver#1
[Resuming connection 1 to r1 ... ]
[OK]
__________________________________________________________________

R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface loopback 0
%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
R1(config-if)#ip address 192.169.1.1 255.255.255.0
R1(config-if)#end
%SYS-5-CONFIG_I: Configured from console by console
R1#copy running-config startup-config
Building configuration...
[OK]
R1#<ctrl-shft-6><x>
Termserver#2
 [Resuming connection 2 to r2 ... ]
____________________________________________________________________

R2#
R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#interface loopback 0
R2(config-if)#ip address 192.169.2.2 255.255.255.0
R2(config-if)#end
R2#copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
[OK]
R2#<ctrl-shft-6><x>
    Termserver#3
[Resuming connection 3 to r3 ... ]
[OK]
____________________________________________________________________

R3#config t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#interface loopback 0
%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
R3(config-if)#ip address 192.169.3.3 255.255.255.0
R3(config-if)#end
R3#copy running-config startup-config
Building configuration...
[OK]
R3#<ctrl-shft-6><x>
    Termserver#4
[Resuming connection 4 to r4 ... ]
____________________________________________________________________

R4#config t
R4(config)#interface loopback 0
%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
R4(config-if)#ip address 192.169.4.4 255.255.255.0
R4(config-if)#end
R4#copy running-config startup-config
Building configuration...
[OK]
R4#<ctrl-shft-6><x>
Termserver#5
[Resuming connection 5 to r5 ... ]
____________________________________________________________________

R5#config t
Enter configuration commands, one per line. End with CNTL/Z.
R5(config)#interface loopback 0
%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up
R5(config-if)#ip address 192.169.5.5 255.255.255.0
R5(config-if)#end
R5#copy running-config startup-config
Building configuration...
[OK]
R5#

Everything looks okay. A router interface description is not necessary here because no other type of device can connect to a loopback interface. Now that you have configured all the interfaces and have IP connectivity, you should familiarize yourself with the Cisco Discovery Protocol (CDP). CDP is an information-gathering tool that enables you to discover directly connected Cisco devices and their network layer addresses.

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