Bypassing LDAP for MongoDb Users
Solution:
the two different machanisms need to be added in the configuration. Like:

authenticationMechanisms=PLAIN,MONGODB-CR


After that, it's necessary to create the users which should be authenticated via LDAP as followed:

db.getSiblingDB("$external").createUser({ user : "syranno", roles: [ { role: "readWrite", db: "testdb" } ] } )


By adding the user to $external, MongoDb knows to authenticate this user against LDAP.