In a recent post I talked about Using Config Replace For Managing Router & Switch Initial Configs.  This is a great feature that can be useful to quickly switch between initial configs and a default config while labbing on your routers without having to write erase and reload - which can be very time consuming.  However, I previously assumed that this required a staged "blank" config in the router's flash, because regular IOS and IOS-XE don't have a built in text editor such as vi/nano/pico that would let you create a blank file on the router's flash.

Thanks to my buddy xous in #cisco on irc.freenode.net, I found that you can use TCL to write a file to flash through the IOS CLI, the same as if it was a native text editor.  The end result is that you can use the following code to write a blank config file at any time to rollback to.

tclsh

puts [ open "flash:blank.cfg" w+] {

version 15.4

!

end

}

tclquit

An example implementation of this is as follows:

R1#show ip int brief | exclude unassign

Interface              IP-Address      OK? Method Status                Protocol

GigabitEthernet1.13    155.1.13.1      YES TFTP   up                    up

GigabitEthernet1.100   169.254.100.1   YES TFTP   up                    up

GigabitEthernet1.146   155.1.146.1     YES TFTP   up                    up

Loopback0              150.1.1.1       YES TFTP   up                    up

Tunnel0                155.1.0.1       YES TFTP   up                    up      
R1#tclsh

R1(tcl)#puts [ open "flash:blank.cfg" w+] {

+>(tcl)#version 1.1

+>(tcl)#!

+>(tcl)#end

+>(tcl)#}

R1(tcl)#tclquit

R1#config replace flash:blank.cfg

This will apply all necessary additions and deletions

to replace the current running configuration with the

contents of the specified configuration file, which is

assumed to be a complete configuration, not a partial

configuration. Enter Y if you are sure you want to proceed. ? [no]: yes

Overwriting with a file sized 50% or less than running config's. Proceed? [no]: yes

% Be sure to ask the CA administrator to revoke your certificates.
*snip*
Router#

Router#show ip int brief | ex unassign

Interface              IP-Address      OK? Method Status                Protocol
Router#

The end result is that you can at any time default the router's config without having to erase and reload. Note that there are certain caveats such as deleted subinterfaces on Ethernet or Serial links which will still exist as "deleted" in the running config, but in most cases for our applications this will not be an issue.

Also as a shameless plug for IRC feel free to join the discussion in the channel #cisco on irc.freenode.net and chat with us. You can find me there with nick bmcgahan.

Happy labbing!