This simulation does DHCP for both IPv4 and IPv6.   iosv6 has a ip helper address for iosv7.   It is configured to TFTP using IPv6.  iosv1 is the DHCP server.   I have IPv4 and IPv6 DHCP configurations for each interface

Once the VIRL simulation comes up I have to run this on iosv2 to iosv7 to get DHCP to work.  I did set the mac address on iosv1.  Every time VIRL restarts, it assigns new mac addresses.  This keeps the helper with the same address.   The the real world, the MAC address stay the same on hardware.   Just a VIRL quirk.

conf t
int g0/1
no shut


iosv5 uses subinterfaces with DHCP