The Modular Layer 2 (ML2) plugin is a framework allowing OpenStack
Networking to simultaneously utilize the variety of layer 2 networking
technologies found in complex real-world data centers. It supports the
Open vSwitch, Linux bridge, and Hyper-V L2 agents, replacing and
deprecating the monolithic plugins previously associated with those
agents, and can also support hardware devices and SDN controllers. The
ML2 framework is intended to greatly simplify adding support for new
L2 networking technologies, requiring much less initial and ongoing
effort than would be required for an additional monolithic core
plugin. It is also intended to foster innovation through its
organization as optional driver modules.

The ML2 plugin supports all the non-vendor-specific neutron API
extensions, and works with the standard neutron DHCP agent. It
utilizes the service plugin interface to implement the L3 router
abstraction, allowing use of either the standard neutron L3 agent or
alternative L3 solutions. Additional service plugins can also be used
with the ML2 core plugin.

Drivers within ML2 implement separately extensible sets of network
types and of mechanisms for accessing networks of those types. Unlike
with the metaplugin, multiple mechanisms can be used simultaneously to
access different ports of the same virtual network. Mechanisms can
utilize L2 agents via RPC and/or interact with external devices or
controllers. By utilizing the multiprovidernet extension, virtual
networks can be composed of multiple segments of the same or different
types. Type and mechanism drivers are loaded as python entrypoints
using the stevedore library.

Each available network type is managed by an ML2 type driver.  Type
drivers maintain any needed type-specific network state, and perform
provider network validation and tenant network allocation. As of the
havana release, drivers for the local, flat, vlan, gre, and vxlan
network types are included.

Each available networking mechanism is managed by an ML2 mechanism
driver. All registered mechanism drivers are called twice when
networks, subnets, and ports are created, updated, or deleted. They
are first called as part of the DB transaction, where they can
maintain any needed driver-specific state. Once the transaction has
been committed, they are called again, at which point they can
interact with external devices and controllers. Mechanism drivers are
also called as part of the port binding process, to determine whether
the associated mechanism can provide connectivity for the network, and
if so, the network segment and VIF driver to be used. The havana
release includes mechanism drivers for the Open vSwitch, Linux bridge,
and Hyper-V L2 agents, and for vendor switches/controllers/etc.
It also includes an L2 Population mechanism driver that
can help optimize tunneled virtual network traffic.

For additional information regarding the ML2 plugin and its collection
of type and mechanism drivers, see the OpenStack manuals and
http://wiki.openstack.org/wiki/Neutron/ML2.