External user authentication means that authentication services are provided by an authentication server outside of the SMC.
You can use the following kinds of external authentication services:
Authentication services that support the RADIUS or TACACS+ protocol, such as RSA Authentication Manager or the NPS (Network Policy
Server) of a Windows (Active Directory) server.
LDAP authentication for simple password authentication against the LDAP database on the external directory server where user accounts are
stored.
User authentication is only supported for IPv4 traffic.
Note: RADIUS authentication servers that are used to authenticate administrators support IPv6 addresses.