Chapter 3 Authentication for FortiOS 5.0 : Authentication servers : LDAP servers : Components and topology : Binding
  
Binding
Binding is the step where the LDAP server authenticates the user. If the user is successfully authenticated, binding allows the user access to the LDAP server based on that user’s permissions.
The FortiGate unit can be configured to use one of three types of binding:
anonymous - bind using anonymous user search
regular - bind using username/password and then search
simple - bind using a simple password authentication without a search
You can use simple authentication if the user records all fall under one domain name (dn). If the users are under more than one dn, use the anonymous or regular type, which can search the entire LDAP database for the required username.
If your LDAP server requires authentication to perform searches, use the regular type and provide values for username and password.