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)#

That’s it. Simple and elegant.

Other Posts in A Series On The Same Topic

  1. Cisco IOS CLI Regex: sh ip bgp in (2nd May 2012)
  2. IOS CLI Tip: More accurate pipe commands (1st May 2012)
  3. Cisco Nexus NXOS and Fixing broken “switchto” syntax with alias (18th December 2011)
  4. show ip eigrp topology all (22nd May 2011)
  5. Cisco IOS CLI Shortcuts (6th February 2011)
  6. The poor man's IOS Traffic Generator (19th September 2009)
  7. IOS: "terminal monitor" on, off - logging to your terminal (17th September 2009)
  8. IOS: Console, Terminal, Monitor, VTY - what is what ? (16th September 2009)
  9. IOS: Clearing an interface configuration (13th September 2009)
  10. IOS: Setting Terminal Window Length (10th September 2009)
  11. IOS CLI: show run linenum (9th September 2009)
  12. IOS: Setting the TCP timeout on IOS (14th August 2008)
  13. IOS: enable and .... disable ? (20th July 2008)
  14. IOS: Reverse SSH console access - Part 2 (25th June 2008)
  15. IOS:Open Source Lab DNS and IP addressing (2nd June 2008)
  16. IOS: Reverse SSH console access (29th May 2008)
  17. ip tcp timestamp (14th April 2008)
  18. Cisco ASA and IOS command tip - test aaa-server (18th February 2008)
About Greg Ferro

Greg Ferro is a Network Engineer/Architect, mostly focussed on Data Centre, Security Infrastructure, and recently Virtualization. He has over 20 years in IT, in wide range of employers working as a freelance consultant including Finance, Service Providers and Online Companies. He is CCIE#6920 and has a few ideas about the world, but not enough to really count.

He is a host on the Packet Pushers Podcast, blogger at EtherealMind.com and on Twitter @etherealmind and Google Plus

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  • http://cisco.markom.info/ Marko Milivojevic

    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

  • http://aconaway.com Aaron

    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.

  • Oldpaul

    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

  • Oldpaul

    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”.

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