Here you go -
If you have a valid CCO account, you can download CNA 3.1
at:
http://www.cisco.com/cgi-bin/tablebuild.pl/NetworkAssistant
Please note this is a crypto image which requires you to fill some forms per Homeland
Security requirements.
If you have problems with your CCO account, you need to check your CCO profile to make
sure it is tied to a valid contract.