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?