The flat_networks configuration option defines interfaces that support the use of untagged networks, commonly referred to as a native or access VLAN. This option requires that a provider label be specified. A provider label is an arbitrary label or name that is mapped to a physical interface or bridge on the host. These mappings will be discussed in further detail later in this chapter.
In the following example, the physnet1 interface has been configured to support a flat network:
flat_networks = physnet1
Multiple interfaces can be defined using a comma-separated list:
flat_networks = physnet1,physnet2
Due to the lack of an identifier to segregate untagged traffic on the same interface, an interface can only support a single flat network.
In this environment, the flat_networks option can remain unconfigured.