These are the three different types of authentication supported by OSPF.
Null Authentication—This is also called Type 0 and it means no authentication information is included in the packet header. It is the default.
Plain Text Authentication—This is also called Type 1 and it uses simple clear-text passwords.
MD5 Authentication—This is also called Type 2 and it uses MD5 cryptographic passwords.
Authentication does not need to be set. However, if it is set, all peer routers on the same segment must have the same password and authentication method. The examples in this document demonstrate configurations for both plain text and MD5 authentication.