Flat network mode removes most of the complexity of VLAN mode by simply bridging all instance interfaces onto a single network.
There are two variations of flat mode that differ mostly in how IP addresses are given to instances.
IP addresses for VM instances are grabbed from a subnet specified by the network administrator, and injected into the image on launch. All instances of the system are attached to the same Linux networking bridge, configured manually by the network administrator both on the network controller hosting the network and on the computer controllers hosting the instances. To recap:
Note that the configuration injection currently only works on linux-style systems that keep networking configuration in /etc/network/interfaces.
IP addresses for VM instances are grabbed from a subnet specified by the network administrator. Similar to the flat network, a single Linux networking bridge is created and configured manually by the network administrator and used for all instances. A DHCP server is started to pass out IP addresses to VM instances from the specified subnet. To recap:
The network nodes do not act as a default gateway in flat mode. Instances are given public IP addresses.
Compute nodes have iptables/ebtables entries created per project and instance to protect against IP/MAC address spoofing and ARP poisoning.
Todo
add flat network mode configuration examples