Learn Cisco CLI – Part 6 – Loopback Interfaces

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Table of Contents
Learn Cisco CLI - Part 6

Loopback Interfaces

These blog posts I created to help me as a reference tool, as well as a way of retaining the knowledge. If you find it useful that’s just a plus.

A loopback interface is a logical interface that can be assigned an IP address, and it remains up as long as the router itself is operational. This interface is not tied to any physical port, making it ideal for use in scenarios where you need a stable, always-up interface, such as:

  • Router ID Assignment: Used in OSPF, BGP, and other routing protocols to uniquely identify a router.
  • Testing and Management: Provides a reliable address for testing connectivity and managing the router.

Configuration Commands

Interface

Changes from global configuration mode to interface configuration mode, allowing you to configure a specific interface. For loopback interfaces, you specify Loopback followed by a number (e.g., Loopback0).

				
					Syntax:
interface loopback[number]

Example:

				
					Router1>enable
Router1#configure terminal
Router1(config)#interface loopback0

IP Address

Assigns an IP address to the loopback interface. This IP address will be used for router identification, testing, or management.

				
					Syntax:
ip address [ip-address] [subnet-mask]

Example:

				
					Router1>enable
Router1#configure terminal
Router1(config)#interface loopback0
Router1(config-if)#ip address 192.168.1.1 255.255.255.0

Hostname

Sets the device name, which helps in identifying the router in a network. Although not directly related to loopback interfaces, setting the hostname is a common practice when configuring routers.

				
					Syntax:
hostname [name]

Example:

				
					Router>enable
Router#configure terminal
Router(config)#hostname Router1
Router1(config)#

Verification and Management

Show IP Interface Brief

Displays a brief summary of the interface status and configuration, including loopback interfaces.

				
					Syntax:
show ip interface brief

Example:

				
					Router1>enable
Router1#show ip interface brief

Sample Output:

				
					Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        192.168.1.1     YES manual up                    up
Loopback0              192.168.1.1     YES manual up                    up

Show Running-Config

Displays the active configuration file, allowing you to verify the configuration of loopback interfaces and other settings.

				
					Syntax:
show running-config

Example:

				
					Router1>enable
Router1#show running-config

Ping

Sends an Internet Control Message Protocol (ICMP) echo request to the specified address to test connectivity. Useful for verifying that the loopback interface is reachable.

				
					Syntax:
ping [ip-address]

Example:

				
					Router1>enable
Router1#ping 192.168.1.1

Sample Example:

				
					Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms

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