Configuring Loopback Interface

Lab Objectives

•   Create interface loopback 1 and assign the IP address 10.233.21.251 255.255.255.0 to the interface.
•   Remove interface Loopback 1.

Lab Instruction

Loopback interfaces are software based logical interfaces that are “always up”. They are not tied to any phsyical interface therefore they cannot go down unless they are administratively shut down.

To create a loopback interface you need to navigate to global configuration mode and execute the command interface loopback # The interface is created automagically. Configuration shown below;

Router con0 is now available Press RETURN to get started. Router>enable Password: Router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)#interface loopback ? <0-2147483647> Loopback interface number Router(config)#interface loopback 1 %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up

After the interface is created you’ll notice the interface changes to state up instantly. As per the objective you’re required to configure the IP address 10.233.21.251/24 on this interface as shown below;

Router(config)#ip address 10.233.21.251 255.255.255.0

Objective 2 request that we remove the Loopback 1 interface, you can remove loopback interfaces by negating the interface command using the no interface loopback # command as shown below;

Router(config-if)#no interface loopback 1 % Not all config may be removed and may reappear after reactivating the logical-interface/sub-interfaces Router(config)# %LINK-5-CHANGED: Interface Loopback1, changed state to administratively down %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to down

Once you’ve removed the interface you’ll notice a warning that not all config may have been removed and may reappear after reactivating the logical-interface/sub-interface.

This has been an issue with Cisco IOS for years and it has yet to be fixed. Basically this warning is letting you know that once you remove a logical or sub-interface using the no interface command there is the potential for some of the previous configuration to return when re-creating the same interface. If you experience this issue, a router reload will resolve this problem.

Configuring IP address on Physical interface

Lab Objectives

•   Configure the Primary IP Address of 10.234.51.254/24 on interface FastEthernet0/0
•   Configure a Secondary IP Address of 172.27.48.254/24 on Interface FastEthernet0/0
•   Activate Interface FastEthernet0/0
•   Verify your configuration changes in the running-configuration.

Lab Instruction

Configuring an IP address on a Cisco router and/or switch is a very common task and you’re required know how to do it without referencing any documentation as a CCNA.

To get started, navigate your way into Global configuration mode as shown below;

Router con0 is now available Press RETURN to get started. Router>enable Password: Router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)#

The first objective of this lab is to configure the IP address 10.234.51.254 on interface FastEthernet0/0. In order to to this configuration you’ll need to place yourself into FastEthernet0/0 interface configuration mode as shown below;

NOTE: To view the available interfaces for configuration execute the interface ? from global configuration mode.

Router(config)#interface FastEthernet 0/0 Router(config-if)#

Once in interface configuration mode you use the command syntax ip address n.n.n.h s.s.s.m; to complete the first lab objective we’ll need to execute the ip address 10.234.51.254 255.255.255.0 from the FastEthernet0/0 interface configuration mode as shown below;

Router(config-if)#ip address 10.234.51.254 255.255.255.0 Router(config-if)#

The second objective of this lab is to configure a secondary address.

As noted in the beginning of the lab, secondary IP addresses are commonly used during network re-ip migrations. Using secondary IP Addresses in production as a architectural design is not recommended by Cisco. To complete the second objective of this lab you’ll need to issue the ip adddress command again with the correct address and just add the word secondary to the end of the command syntax.

Router(config-if)#ip address 172.27.48.254 255.255.255.0 secondary Router(config-if)#

Now for the last configuration ojective of this lab, you need to enable this interface because by default, all interfaces on a Cisco router are placed in “Administratively Down”. To bring an interface up, issue the no shutdown command.

Router(config-if)#no shutdown Router(config-if)# %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up Router(config-if)#

To verify your configuration changes, exit the interface configuration mode to privileged mode by pressing CTRL + Z and executing the command show run interface FastEthernet 0/0

Router(config-if)#^Z %SYS-5-CONFIG_I: Configured from console by console Router#show run interface FastEthernet 0/0 Building configuration... Current configuration : 148 bytes ! interface FastEthernet0/0 ip address 172.27.48.254 255.255.255.0 secondary ip address 10.234.51.254 255.255.255.0 duplex auto speed auto end Router#