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You are here: Home / Cisco Routers / How to Configure a Loopback Interface on Cisco Router & Switch

How to Configure a Loopback Interface on Cisco Router & Switch

Written By Lazaros Agapidis

A loopback interface is a virtual interface that can be created within a network device and behaves much the same as a physical interface. 

network equipment

Loopback interfaces can be configured on a Cisco router or a switch and have many uses.  In this article, we’ll examine what they are, how they can be configured, and how they are helpful in a modern IP network.

Table of Contents

  • History
    • Loopback address
    • Loopback interface
  • What is a Loopback Interface?
  • How to Configure a Loopback Interface
    • Configuring a loopback interface on a Cisco router
    • Configuring a loopback interface on a Cisco switch
  • Difference between an SVI and a loopback interface
  • What makes loopback interfaces special?
  • Loopback interface use cases
  • Conclusion
    • Related Posts

History

The term loopback goes back to the days of analog telephony.  The concept involves setting up a connection so that any sent signals from a device are immediately and only received by that same device. 

The signal is essentially “looped back” to the sender.  This methodology was used to perform transmission tests of telephone access lines.

Loopback address

This term has been adopted in IP networking in several ways.  One is the loopback address block of 127.0.0.0/8, as defined in RFC 5735. 

IP addresses within this range, such as 127.0.0.1 for example, can be used by a host to refer to itself.  This is also known as a localhost address.  Communicating with such an address essentially causes the sending device to communicate with itself.

Loopback interface

Another way the term has been used, not to be confused with the loopback address, is with the concept of the loopback interface. 

This is a virtual Layer 3 network interface that can be created and used just like any other interface.  In essence, this is not a genuinely proper loopback since this is not used by a host to communicate with itself but can be used to communicate with other network devices.

What is a Loopback Interface?

A loopback interface is a logical entity that can be created within a router or a switch.  When created, it has many of the same characteristics as a Layer 3 port on a router or a Switched Virtual Interface (SVI) on a switch. 

It can be assigned an IP address, it obtains a virtual MAC address, and it can be both the source and destination of data transmissions with other network entities. 

The only thing it lacks, compared to a physical Layer 3 port on a router, is the ability to be connected to a subnet with additional hosts.

How to Configure a Loopback Interface

Loopback interfaces can be configured quite easily on both Cisco routers and switches.  Below you will find the configuration involved in each case.

Configuring a loopback interface on a Cisco router

The first thing to do is to issue the interface loopback command in global configuration mode to create a loopback interface.  Notice the following command line output:

R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface loopback ?
  <0-2147483647>  Loopback interface number
R1(config)#interface loopback

Each loopback interface is assigned a loopback interface number.  This number can be between 0 and 2147483647. 

MORE READING:  Cisco Show IP Route Command (Routing Table)-Example and Explanation

Theoretically, you can create that many loopback interfaces on a router. However, you will quickly run out of memory and CPU resources on the device before you get even close to creating so many. 

Let’s create a loopback interface with an interface number of 1:

R1(config)#interface loopback 1
R1(config-if)#
*Jan  4 09:39:01.352: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up

A Syslog message appears, informing us that the Loopback1 interface has come up.  That’s a good sign.  Now we are in interface configuration mode, the same mode we would enter when configuring a physical interface. 

Let’s configure an IP address on this loopback interface:

R1(config-if)#ip address 192.168.100.1 255.255.255.0
R1(config-if)#

Let’s take a look at all of the interfaces on this particular router:

R1#show ip interface brief

Interface                  IP-Address      OK? Method Status                Protocol

GigabitEthernet0/0         10.255.1.235    YES NVRAM  administratively down down   

GigabitEthernet0/1         192.168.12.1    YES NVRAM  up                    up     

Loopback0                  1.1.1.1         YES NVRAM  up                    up     

Loopback1                  192.168.100.1   YES manual up                    up     

R1#

We see all the interfaces on the router, including physical and loopback interfaces, as well as their assigned IP addresses.

Configuring a loopback interface on a Cisco switch

A loopback interface can be configured on a Cisco switch in a very similar manner.  The commands are almost identical.  The following commands create a loopback with an interface number of 1, and assign an IP address to it:

SW1(config)#interface loopback 1
SW1(config-if)#

*Jan  4 09:50:33.620: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up

SW1(config-if)#ip address 10.10.10.1 255.255.255.0
SW1(config-if)#exit
SW1(config)#exit
SW1#

We can verify that the loopback has been configured correctly using the following command:

SW1#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

GigabitEthernet0/0     unassigned      YES unset  up                    up     

GigabitEthernet0/1     unassigned      YES unset  up                    up     

GigabitEthernet0/2     unassigned      YES unset  up                    up     

GigabitEthernet0/3     unassigned      YES unset  up                    up     

GigabitEthernet1/0     unassigned      YES unset  up                    up     

GigabitEthernet1/1     unassigned      YES unset  up                    up     

Loopback1              10.10.10.1      YES manual up                    up     

Vlan500                192.168.1.1     YES manual down                  down   

SW1#

At this point the switch is configured with a loopback interface and an SVI that are configured with IP addresses.  The rest of the interfaces are operating as switchports, that is, only at Layer 2.

We can verify that the loopback interface is operating correctly by pinging it like so:

SW1# ping 10.10.10.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

Difference between an SVI and a loopback interface

Note that even though an SVI and a loopback are both virtual Layer 3 interfaces, they differ in the way they operate. 

Firstly, The SVI exists on a particular VLAN and typically acts as the default gateway for any hosts connected to that VLAN.  A loopback interface on a switch does not belong to any VLAN and must have an IP address in a subnet that is distinct from subnets associated with any particular VLAN.

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Note also that an SVI will go down if there is not at least one active physical interface assigned to the VLAN to which the SVI belongs.

Note in the output in the previous section, the configured Vlan500 SVI has a status of down and a protocol of down. 

This is because there are no active access ports assigned to VLAN 500.  A loopback interface is not affected by such restraints, and is always active as long as the switch has power.

What makes loopback interfaces special?

Several characteristics of loopback interfaces make them especially useful for particular applications:

  • Loopback interfaces will not go down if a cable is unplugged or a physical interface fails.
  • Traffic sent from and destined to the network device itself can be sent and received using the IP address of the loopback interface instead of the address of a physical interface through which other traffic passes.
  • A loopback interface can be used to communicate directly with the device on which it is configured using Telnet, SSH, or other communication protocols, as long as at least one of the physical interfaces on the device is operational.

Loopback interface use cases

The above characteristics make loopback interfaces very useful for control plane operations and network maintenance.  Some of the most useful applications of loopback interfaces include:

  • Network device management – Administrators can obtain communication with network devices using CLI or web management interfaces by connecting directly to the IP addresses assigned to a loopback interface of a device.
  • Routing protocols – Many routing protocols, such as OSPF, EIGRP, and BGP, will use loopback addresses of routers and L3 switches to assign a router ID, which uniquely identifies that device within the routing domain.
  • Source and destination address for network monitoring – Many network monitoring systems that use SNMP, Netflow, or IPSLAs can use the loopback interfaces to send and receive monitoring data. The loopbacks can also be used to benchmark pings and traceroutes within a network for network auditing, testing, and observation.

Note that most uses of loopback interfaces have to do with network maintenance, monitoring, and upkeep.  End users will rarely come across any use for a loopback interface on a network device, nor will they ever get any direct benefit from their implementation.

Conclusion

Loopback interfaces are constructs that are useful in many respects but are nevertheless leveraged by other protocols, programs, and applications. 

As entities themselves, they do little more than create an interface through which communication can take place.  Knowing how other entities use them is an important part of further understanding their significance in networking.

Related Posts

  • The Most Important Cisco Show Commands You Must Know (Cheat Sheet)
  • Cisco Show Interface Command on Routers and Switches Explained
  • Cisco Switch Layer2 Layer3 Design and Configuration
  • Description of Switchport Mode Access vs Trunk Modes on Cisco Switches
  • What is an SFP Port-Module in Network Switches and Devices

Filed Under: Cisco Routers, Cisco Switches

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About Lazaros Agapidis

Lazaros Agapidis is a Telecommunications and Networking Specialist with over twenty years of experience.
He works primarily with IP networks, VoIP, Wi-Fi, and 5G, has extensive experience in training professionals for Cisco certifications, and his expertise extends into telecommunications services and infrastructure from both an enterprise and a service provider perspective.
In addition to his numerous vendor certifications, Lazaros has a solid online presence as an expert in his field, having worked in both public and private sectors within North America and in Europe.
He has enjoyed sharing his practical experiences in writing as well as through engaging online training.
LinkedIn: Lazaros Agapides

Comments

  1. Edgar Wahl says

    February 27, 2023 at 8:02 am

    Hi,
    thank you for that very good cisco documentation.
    I have here a Cisco 881g-4g LTE Router.

    service timestamps debug datetime msec
    service timestamps log datetime msec
    no service password-encryption
    service internal
    !
    hostname yourname
    !
    boot-start-marker
    boot-end-marker
    !
    !
    logging buffered 51200 warnings
    no logging console
    !
    no aaa new-model
    !
    !
    p dhcp excluded-address 10.10.10.1
    !
    ip dhcp pool ccp-pool
    import all
    network 10.10.10.0 255.255.255.128
    default-router 10.10.10.1
    lease 0 2
    o ip domain lookup
    ip domain name yourdomain.com
    ip inspect WAAS flush-timeout 10
    ip cef
    no ipv6 cef
    !
    !
    !
    !
    !
    multilink bundle-name authenticated
    !
    !
    chat-script lte “” “AT!CALL” TIMEOUT 20 “OK”
    !
    !
    !
    !
    !
    !
    cts logging verbose
    license udi pid C881G-4G-GA-K9 sn xxxxxxx
    controller Cellular 0
    lte modem link-recovery rssi onset-threshold -110
    lte modem link-recovery monitor-timer 20
    lte modem link-recovery wait-timer 10
    lte modem link-recovery debounce-count 6
    no cdp run
    !
    interface Cellular0
    ip address negotiated
    ip nat outside
    ip virtual-reassembly in
    encapsulation slip
    dialer in-band
    dialer idle-timeout 0
    dialer string lte
    dialer string gsm
    dialer-group 1
    cwmp wan default
    !
    interface FastEthernet0
    no ip address
    !
    interface FastEthernet1
    no ip address
    !
    interface FastEthernet2
    no ip address
    !
    interface FastEthernet3
    no ip address
    !
    interface FastEthernet4
    no ip address
    shutdown
    duplex auto
    speed auto
    !
    !
    interface Vlan1
    description $ETH_LAN$
    ip address 10.10.10.1 255.255.255.128
    ip nat inside
    ip virtual-reassembly in
    ip tcp adjust-mss 1452
    !
    ip forward-protocol nd
    ip http server
    ip http access-class 23
    ip http authentication local
    ip http secure-server
    ip http timeout-policy idle 60 life 86400 requests 10000
    !
    i will ping to the name server after connection. But he don´t give traffic of the Cellular 0 modul
    The LTE LED is blinking also bilnking Fastethernet 0 Port. Also i have IPAdress correct on my Laptop 10.10.10..2
    all is correct.
    vlan1 going nat inside and cellular going nat outside.

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We Provide Technical Tutorials and Configuration Examples about TCP/IP Networks with focus on Cisco Products and Technologies. This blog entails my own thoughts and ideas, which may not represent the thoughts of Cisco Systems Inc. This blog is NOT affiliated or endorsed by Cisco Systems Inc. All product names, logos and artwork are copyrights/trademarks of their respective owners.

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