IOS: Clearing an Interface Configuration

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
end

Router(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
end

Router(config)#