http://www.openwall.com/lists/john-users/2006/05/14/8
http://www.osix.net/modules/article/?id=455
In a nutshell:
I. Config file => passwd file (for John)
sed -n 's/[ :]/_/g; s/^(.{1,})_5_($1$[$./0-9A-Za-z]{27,31})_*$/1:2/p' < cisco-device-config > passwd
II. Start the process (nice 19 -> low priority, you can work on your computer..):
nice -n 19 john -incremental:lanman -session:cisco passwd