5

By default, why the Cisco Router's loopback interface can not be showed up?

Router#show ip interface brief 
Interface              IP-Address      OK? Method Status                Protocol 
GigabitEthernet0/1     unassigned      YES NVRAM  administratively down down 
FastEthernet2/3        unassigned      YES unset  down                  down 
FastEthernet2/4        unassigned      YES unset  down                  down 
FastEthernet2/5        unassigned      YES unset  down                  down 
FastEthernet2/6        unassigned      YES unset  down                  down 
GigabitEthernet2/1     12.12.12.2      YES manual up                    up 
GigabitEthernet2/2     23.23.23.2      YES manual up                    up 
Dot11Radio2/1          unassigned      YES NVRAM  administratively down down 
Vlan1                  unassigned      YES NVRAM  up                    up

you see the upper display, there is no loopback interface in the list.

but if I enter the loopback interface 1, then end and show interfaces, it will shows:

Router#show ip interface brief 
Interface              IP-Address      OK? Method Status                Protocol 
GigabitEthernet0/1     unassigned      YES NVRAM  administratively down down 
FastEthernet2/3        unassigned      YES unset  down                  down 
FastEthernet2/4        unassigned      YES unset  down                  down 
FastEthernet2/5        unassigned      YES unset  down                  down 
FastEthernet2/6        unassigned      YES unset  down                  down 
GigabitEthernet2/1     12.12.12.2      YES manual up                    up 
GigabitEthernet2/2     23.23.23.2      YES manual up                    up 
Dot11Radio2/1          unassigned      YES NVRAM  administratively down down 
Loopback1              unassigned      YES unset  up                    up 
Vlan1                  unassigned      YES NVRAM  up                    up

EDIT-01

this is the logs when I enter the loopback interface:

Router(config)#interface loopback 1
%LINK-5-CHANGED: Interface Loopback1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback1, changed state to up

there shows the loopback interface status change, but I have a question, the other interface such as this item:

FastEthernet2/3        unassigned      YES unset  down                  down 

there shows the Physical and Protocol all are down there even shows up in the command, if loopback interface is both down, why there do not shows it?

  • 1
    A loopback interface is a virtual interface, that you create. It is always online and will never go down, no matter what. It's only shown if you create it. It's normally used as a management interface, since it's always available. – user36472 Nov 1 '18 at 10:07
  • Your edit is really a different question that should be asked in a different question, but the beauty of loopback interfaces is that they can never go down unless you configure them that way. – Ron Maupin Nov 1 '18 at 14:54
6

A loopback interface is a logical, virtual interface in a Cisco Router. A loopback interface is not a physical interface like Fast Ethernet interface or Gigabit Ethernet interface.

A loopback interface has many uses. The loopback interface IP Address can determine a routers OSPF Router ID. A loopback interface is always up and allows Border Gateway Protocol (BGP) neighborship between two routers to stay up even if one of the outbound physical interface connected between the routers is down.

Loopback interfaces can be used as the termination points for Remote Source-Route Bridging (RSRB), and Data-Link Switching Plus (DLSW+). Loopback interfaces interfaces are always up and running and always available, even if other physical interfaces in the router are down.

A loop back interface is a software interface which can be used to emulate a physical interface. By default, router doesn’t have any loopback interfaces (loopback interfaces are not enabled by default), but they can easily be created.

Loopback interfaces are treated similar to physical interfaces in a router and we can assign IP addresses to them. The command syntax to create a loopback interface is shown below.

Router(Config)#int loopback <loopback_interface_number>
Router(Config-if)#ip address <ip_address> <subnet_mask>

To create a loopback interface, use the following command in a Cisco Router.

Router(Config)#int loopback 0
Router(Config-if)#ip address 10.0.0.10 255.255.255.255

Router(Config)#int loopback 1
Router(Config-if)#ip address 10.0.0.11 255.255.255.255

Router(Config)#int loopback 2
Router(Config-if)#ip address 10.0.0.12 255.255.255.255

Etc.

Copy source: http://www.omnisecu.com/cisco-certified-network-associate-ccna/what-is-loopback-interface-in-a-router.php

  • 2
    ... or more briefly, virtual interfaces don't exist until you create them with, eg, interface loopback 99; and they are destroyed with no interface loopback99 – jonathanjo Nov 1 '18 at 11:31
1

There are potentially dozens of loopback interfaces you can create in a router. We might encounter only one interface typically, but many companies get creative with their use of loopbacks and can have several for different purposes. For example, one company I worked for used to name their loopbacks after their OSPF router zones/IDs. So you might initially think that you're supposed to number your loopbacks starting at zero, but nope you can have a loopback58 or loopback34.

So now re-imagine your question as to asking why a loopback doesn't automatically appear by default. Can you imagine having dozens of unused loopbacks appearing that you'll never use. The programmer that make the feature had to consider that people didn't want virtual interfaces and therefore left them off by default.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.