When enabling automatic signature updates, it is recommended that you ensure the following configuration guidelines have been met:
•The router's clock is set up with the proper relative time.
•The frequency for Cisco IOS IPS to obtain updated signature information has been defined.
•The URL in which to retrieve the Cisco IOS IPS signature configuration files has been specified.
•Optionally, the username and password for which to access the files from the server have been specified.
http://www.cisco.com/en/US/docs/ios/12_4t/12_4t11/ips_v5.html#wp1079125
