In Chapter 3, Installing Neutron, we briefly covered the process instances accessing metadata over the network, through either a proxy in the router namespace or the DHCP namespace. The latter is described in the following section.
Instances can access metadata at http://169.254.169.254, followed by a URI that corresponds to the version of metadata, usually /latest. When an instance is connected to a network that does not utilize a Neutron router as the gateway, the instance must learn how to reach the metadata service. This can be accomplished in a few different ways, including:
Setting a route manually on the instance
Allowing DHCP to provide a route
When enable_isolated_metadata is set to true in the DHCP configuration file at /etc/neutron/dhcp_agent.ini, each DHCP namespace provides a proxy to the metadata service running on the controller node. The proxy service listens directly on port 80, as shown in the following figure...