Interface is full
This occurs most commonly when you are labbing or testing something and an interface ends up with a lot of configuration, possibly something like the following:
Router(config-if)#do sh run int fa0/0
Building configuration…Current configuration : 320 bytes
!
interface FastEthernet0/0
description Show the Default command
ip dhcp relay information trusted
ip address 198.18.56.1 255.255.255.0
no ip route-cache cef
no ip route-cache
ip ospf authentication-key MYOHMY
ip ospf network point-to-point
ip ospf hello-interval 20
duplex half
standby 1 ip 198.18.54.90
endRouter(config-if)#
You want to clear this configuration and reuse the interface for some other task. Normally you would need to issue a the “no” command for each line.
The ‘default’ command
The default command has many uses, and this is only one of them, but possibly it is the most useful.
Router(config-if)#default interface fa0/0
Building configuration…Interface FastEthernet0/0 set to default configuration
Router(config)#do sh run int fa0/0
Building configuration…Current configuration : 61 bytes
!
interface FastEthernet0/0
no ip address
duplex half
endRouter(config)#
That’s it. Simple and elegant.

(4 votes, average: 9.00 out of 10)


Beware of it on production switches, though! By default, switchports are not shutdown and defaulting will make them come up, possibly in the wrong VLAN. Of course, STP should not make this too dangerous, but it’s a good thing to be aware of. What I usually do is:
default interface X
interface X
shutdown
! whatever else I need to do
I had to teach that one to some of our new boys this past week.
You can also use the range directive to reset configs on a whole mess of ports.
Router(config)#default int range f0/0/0 – 4
Or something like that.
I also like using it with the individual interface commands, if I only want to “default” something on an interface:
conf t > int f0/0 > default speed > default duplex
Marko, I just tested default interface command on my 6500, and it *does* retain the shutdown/noshutdown state of the interface. Everything else was “defaulted”.