Use the aaa authentication command to create a series of authentication methods, or method list. You can specify up to four methods in the method list. To create a method list for authentication, use the aaa authentication command in global configuration mode. To disable this authentication method, use the no form of this command.

aaa authentication {login | ppp} {default | remote | list-name} {local | line | group {tacacs+ | radius | group-name}}

no aaa authentication {login | ppp} {default | remote | list-name}

To create a method list for authorization, use the aaa authorization command in global configuration mode. To disable authorization for a function, use the no form of this command.

aaa authorization {commands | exec | network} {default | list-name} {none | local | group {tacacs+ | radius | group-name}}

no aaa authorization {commands | exec | network} {default | list-name}

For further information click this link.

http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.2/security/command/reference/sr32aaa.html