Source: http://www.cqure.net/wp/2011/02/pulling-cisco-configs-with-nmap/
A few hours ago I committed a new script created by Vikas Singhal to Nmap. It implements the functionality to initiate a tftp transfer of a Cisco configuration through SNMP. In order to do so, the device obviously needs to support this functionality, and you need to know the private SNMP-community string.
The script can either save the configuration to a file in a directory specified as a script argument or displays it on screen. In order to achieve this, I’ve contributed with a minimal tftp server, implemented as a Nmap NSE library. This eliminates the need to setup and configure a separate tftp-server as it’s all being taken care of transparently by Nmap.
In order to try it out you can either update from subversion or download the tftp library and the snmp-ios-config script and place them in nselib and the scripts directory. For more information on how to run the script, check out the documentation.