Could not set ForgedTransmits of uplink port groups to Reject?

Hello everyone,

I am a newbie to VMWare. While studying about port groups, I found out that I can not set ForgedTransmits of uplink port group to Reject because the listbox is disabled.

Please tell me the reason why the listbox is disabled and is there any way that I can set the ForgedTransmits to Reject for uplink port groups?

Thanks in advance,