In Chapter 3, Installing Neutron, I briefly discussed the process of how metadata is provided to instances through the Neutron metadata proxy. There are two ways in which an instance can get its metadata over the network: through the router namespace or through the DHCP namespace. Both methods are described in the upcoming sections.
Although routers will be described and configured in the next chapter, it is important to know their function with regard to metadata. Neutron assumes that when a gateway IP is set on a subnet, one of the following two things is true: