Getting ready

One of the prerequisites to create an instance is to add a virtual machine image to the Glance image service. In our example, we will add a CirrOS image and use this image to create an instance.

How to do it…

This recipe showed you that as a part of instance creation, the Horizon GUI allows users to choose the Network to which the instance needs to be associated.

How it works…

As part of the instance creation process, the user chooses the Network to which the instance will be associated. The instance creation and scheduling is the responsibility of Nova and it sends a create Port request to Neutron in order to associate the instance to the selected Network.

In response to the create Port request, Neutron will ensure that the virtual Network on the hypervisor server is configured so as to provide connectivity to the virtual machine. For the very first instance created on the Network, the Neutron server will also start a DHCP process on the Network node. This happens when DHCP is enabled on the corresponding Network. Once a virtual machine boots up, it will send a DHCP request. In response to this request, the DHCP server for that Network will respond with an IP address.

There's more…

If there is exactly one Network for a given tenant, then the dashboard automatically selects this Network when an instance is created. Additionally, note that the tenants can associate more than one Network to an instance. This will create multiple interfaces in the virtual machine.