Resources, such as DHCP and VMs, must be given a port on a network in order to have access to network traffic.
The ES2 ML2 Mechanism Driver implements on-demand VLAN port membership only if a resource that it hosts requests a port connection to that VLAN. The on-demand switch configuration is triggered not by network events, but by port creation, port update, and port deletion events.
When you create a network in the OpenStack Horizon GUI, no ports are created and the network creation does not trigger any action from the ES2 ML2 Mechanism Driver. However, when you create a VM on a network, Neutron sends a port event to the ML2 Plugin which delegates to ES2 ML2 Mechanism Driver. Because the port event contains a host identifier, the ES2 ML2 Mechanism Driver is able to determine what network interface on that host is connected to what port on which ES2 switch, and tells OFM to how configure them.