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Per https://wiki.openstack.org/wiki/Neutron/ML2

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 currently works with the existing openvswitch, linuxbridge, and hyperv L2 agents, and is intended to replace and deprecate the monolithic plugins associated with those L2 agents. The ml2 framework is also intended to greatly simplify adding support for new L2 networking technologies, requiring much less initial and ongoing effort than would be required to add a new monolithic core plugin. A modular agent may be developed as a follow-on effort

Neutron OVS plugin is, actually, L2 agent. After Grizzly Quantum was renamed Neutron. View also
https://openstack.redhat.com/Neutron_with_OVS_and_VLANs

Per https://wiki.openstack.org/wiki/Neutron/ML2After Grizzly release Quantum was renamed Neutron. View:
http://docs.openstack.org/training-guides/content/operator-network-node.html
https://openstack.redhat.com/Neutron_with_OVS_and_VLANs

The Modular Layer 2 (ml2) plugin is a framework allowing OpenStack Networking to simultaneously utilize L2 agent - agent which talks with the variety of layer 2 networking technologies found in complex real-world data centers. It currently works with the existing openvswitch, linuxbridge, and hyperv L2 agents, and is intended to replace and deprecate the monolithic plugins associated with those L2 agents. The ml2 framework is also intended to greatly simplify adding support for new L2 networking technologies, requiring much less initial and ongoing effort than would be required to add a new monolithic core plugin. A modular agent may be developed as a follow-on effort
plugin like OVS (Open vSwitch) or LB (Linux bridge) etc.

Neutron OVS plugin is, actually, L2 agent. After Grizzly Quantum was renamed Neutron. View also
https://openstack.redhat.com/Neutron_with_OVS_and_VLANs

After Grizzly release Quantum was renamed Neutron. View:
http://docs.openstack.org/training-guides/content/operator-network-node.html
https://openstack.redhat.com/Neutron_with_OVS_and_VLANs

L2 agent - agent The original OpenStack Compute network implementation assumed a very basic model of performing all isolation through Linux VLANs and IP tables. OpenStack Networking introduces the concept of a plug-in, which talks with the layer 2 plugin like OVS (Open vSwitch) is a pluggable back-end implementation of the OpenStack Networking API. A plug-in can use a variety of technologies to implement the logical API requests. Some OpenStack Networking plug-ins might use basic Linux VLANs and IP tables, while others might use more advanced technologies, such as L2-in-L3 tunneling or LB (Linux bridge) etc.
OpenFlow, to provide similar benefits.

OpenStack Networking also includes additional agents that might be required depending on your deployment:

plugin agent (quantum-*-agent):Runs on each hypervisor to perform local vswitch configuration. Agent to be run depends on which plug-in you are using, as some plug-ins do not require an agent.
dhcp agent (quantum-dhcp-agent):Provides DHCP services to tenant networks. This agent is the same across all plug-ins.
l3 agent (quantum-l3-agent):Provides L3/NAT forwarding to provide external network access for VMs on tenant networks. This agent is the same across all plug-ins.

After Grizzly release Quantum was renamed Neutron. View:
https://openstack.redhat.com/Neutron_with_OVS_and_VLANs

L2 agent - agent which talks with the layer 2 plugin like OVS (Open vSwitch) or LB (Linux bridge) etc.

http://docs.openstack.org/training-guides/content/operator-network-node.html
https://openstack.redhat.com/Neutron_with_OVS_and_VLANs

The original OpenStack Compute network implementation assumed a very basic model of performing all isolation through Linux VLANs and IP tables. OpenStack Networking introduces the concept of a plug-in, which is a pluggable back-end implementation of the OpenStack Networking API. A plug-in can use a variety of technologies to implement the logical API requests. Some OpenStack Networking plug-ins might use basic Linux VLANs and IP tables, while others might use more advanced technologies, such as L2-in-L3 tunneling or OpenFlow, to provide similar benefits.

OpenStack Networking also includes additional agents that might be required depending on your deployment:

plugin agent (quantum-*-agent):Runs on each hypervisor to perform local vswitch configuration. Agent to be run depends on which plug-in you are using, as some plug-ins do not require an agent.
dhcp agent (quantum-dhcp-agent):Provides DHCP services to tenant networks. This agent is the same across all plug-ins.
l3 agent (quantum-l3-agent):Provides L3/NAT forwarding to provide external network access for VMs on tenant networks. This agent is the same across all plug-ins.