Explanation
http://www.cisco.com/en/US/tech/tk59/technologies_tech_note09186a0080094e99.shtml
Packet Encryption
RADIUS encrypts only the password in the access-request packet, from the client to the server. The remainder
of the packet is unencrypted. Other information, such as username, authorized services, and accounting, can be
captured by a third party.
TACACS+ encrypts the entire body of the packet but leaves a standard TACACS+ header. Within the header
is a field that indicates whether the body is encrypted or not. For debugging purposes, it is useful to have the
body of the packets unencrypted. However, during normal operation, the body of the packet is fully encrypted
for more secure communications.
Authentication and Authorization RADIUS combines authentication and authorization. The access-accept
packets sent by the RADIUS server to the client contain authorization information. This makes it difficult to
decouple authentication and authorization.
TACACS+ uses the AAA architecture, which separates AAA. This allows separate authentication solutions
that can still use TACACS+ for authorization and accounting. For example, with TACACS+, it is possible to
use Kerberos authentication and TACACS+ authorization and accounting. After a NAS authenticates on a
Kerberos server, it requests authorization information from a TACACS+ server without having to
re-authenticate. The
NAS informs the TACACS+ server that it has successfully authenticated on a Kerberos server, and the server
then provides authorization information.
During a session, if additional authorization checking is needed, the access server checks with a TACACS+
server to determine if the user is granted permission to use a particular command. This provides greater control
over the commands that can be executed on the access server while decoupling from the authentication
mechanism.