About Cloudstack RedundantRouter some Issue

Hi,everyone No.1 i'm built cloudstack environment for 4.0, Advance Network mode, i'm create a Network offering with Source NAT and other service(such as Static Nat, LB, PF,FW,DNS,DHCP) , and RedundantRouter is Enabled, and GuestType is Isolated, create Guest network and use this Network Offering,

Help Me, i have some question

hi,everyone i have some question about cloudstack, No.1: how can i use External DHCP Server(not use VirtualRouter VM DHCP Server), i set global settings direct.attach.network.externalIpAllocator.enabled = true,and restart management server,but vm's ip address also provide for VirtualRouter, how