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, for Arista and Cisco switches, and for the Tail-f NCS. 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.