Hello Vinoth,
AAA stands for authentication, authorization and accounting
AAA is a framework that allows a group of network devices to be administered by using an interaction with AAA servers:
for example for authentication:
without AAA you need to configure a username password pair on every device manually with
username name password password
With AAA accounts are defined on the server once and can be used on all devices (if there are the rights see authorization)
accounting means the device reports to the servers events (including configuration changes on devices)
authorization: profiles with different rights can be implemented:
with AAA authorization a user logged in a device if tries to execute a command the device checks with the server if that user has sufficient privileges to execute it
A good starting point can be the following
http://www.cisco.com/en/US/docs/ios/security/configuration/guide/sec_aaa_overview_ps6350_TSD_Products_Configuration_Guide_Chapter.html
note:
be aware that during AAA testing you can lock out of your own device so some safety measures during testing.
Hope to help
Giuseppe
