OWA forms-based authentication with basic authentication disabled

Hi everyone,

I am running form based authentication. For some concern, i would like to disable basic authentication

get-OwaVirtualDirectory "owa (Default Web Site)" |fl *auth*
ClientAuthCleanupLevel : High
InternalAuthenticationMethods : {Basic, Fba}
BasicAuthentication : True WindowsAuthentication : False
DigestAuthentication : False FormsAuthentication : True
LiveIdAuthentication : False AdfsAuthentication : False
OAuthAuthentication : False

ExternalAuthenticationMethods : {Fba}

Note the "InternalAuthenticationMethods" is Basic and Fba.

set-OwaVirtualDirectory "owa (Default Web Site)" -BasicAuthentication $false
get-OwaVirtualDirectory "owa (Default Web Site)" |fl InternalAuthenticationMethods
InternalAuthenticationMethods : {}

with just setting the "BasicAuthentication" to $false it turns off the form based too. So i tried to enabled the form based again.

set-OwaVirtualDirectory "owa (Default Web Site)" -FormsAuthentication $true
ClientAuthCleanupLevel : High
InternalAuthenticationMethods : {Basic, Fba}
BasicAuthentication : True
WindowsAuthentication : False
DigestAuthentication : False
FormsAuthentication : True
LiveIdAuthentication : False
AdfsAuthentication : False
OAuthAuthentication : False
ExternalAuthenticationMethods : {Fba}

The basic authentication was automatically turn on by itself again. So i was thinking the form based required the basic authentication. The information on the internet is kind of scarce. Does anyone has any experience on this? Also note the "ExternalAuthenticationMethods" is {Fba} without Basic? Thanks everyone!