kubespray/roles/network_plugin/flannel/templates/flannel-options.conf.j2

[Service]
{% if ansible_os_family in ["CoreOS", "Container Linux by CoreOS"] %}
Environment="DOCKER_OPT_BIP=--bip={{ flannel_subnet }} --mtu={{ flannel_mtu }}"
{% else %}
Environment="DOCKER_NETWORK_OPTIONS=--bip={{ flannel_subnet }} --mtu={{ flannel_mtu }}"
{% endif %}