[libvirt] when define NAT network, assign what kind of forward dev is better

hi all: in my case, my physical machine has one ethernet, i want to use both bridge and nat network. i create a bridge outbr0(addif eth0), when creating NAT network, there are 3 options for forward-dev 1.<forward mode="nat" /> 2.<forward mode="nat" dev="outbr0"/> 3.<forward mode="nat" dev="eth0"/> i do not know which one is the best? thanks