.. _config-plugin-ml2:

===========
ML2 plug-in
===========

Architecture
~~~~~~~~~~~~

The Modular Layer 2 (ML2) neutron plug-in is a framework allowing OpenStack
Networking to simultaneously use the variety of layer 2 networking
technologies found in complex real-world data centers. The ML2 framework
distinguishes between the two kinds of drivers that can be configured:

* Type drivers

  Define how an OpenStack network is technically realized. Example: VXLAN

  Each available network type is managed by an ML2 type driver. Type drivers
  maintain any needed type-specific network state. They validate the type
  specific information for provider networks and are responsible for the
  allocation of a free segment in project networks.

* Mechanism drivers

  Define the mechanism to access an OpenStack network of a certain type.
  Example: Open vSwitch mechanism driver.

  The mechanism driver is responsible for taking the information established by
  the type driver and ensuring that it is properly applied given the
  specific networking mechanisms that have been enabled.

  Mechanism drivers can utilize L2 agents (via RPC) and/or interact directly
  with external devices or controllers.

Multiple mechanism and type drivers can be used simultaneously to access
different ports of the same virtual network.

.. todo::
    Picture showing relationships

ML2 driver support matrix
-------------------------


.. list-table:: Mechanism drivers and L2 agents
   :header-rows: 1

   * - type driver / mech driver
     - Flat
     - VLAN
     - VXLAN
     - GRE
     - Geneve
   * - Open vSwitch
     - yes
     - yes
     - yes
     - yes
     - yes
   * - Linux bridge
     - yes
     - yes
     - yes
     - no
     - no
   * - OVN
     - yes
     - yes
     - yes (requires OVN 20.09+)
     - no
     - yes
   * - SRIOV
     - yes
     - yes
     - no
     - no
     - no
   * - MacVTap
     - yes
     - yes
     - no
     - no
     - no
   * - L2 population
     - no
     - no
     - yes
     - yes
     - yes

.. note::

   L2 population is a special mechanism driver that optimizes BUM (Broadcast,
   unknown destination address, multicast) traffic in the overlay networks
   VXLAN, GRE and Geneve. It needs to be used in conjunction with either the
   Linux bridge or the Open vSwitch mechanism driver and cannot be used as
   standalone mechanism driver. For more information, see the
   *Mechanism drivers* section below.

Configuration
~~~~~~~~~~~~~

Network type drivers
--------------------

To enable type drivers in the ML2 plug-in. Edit the
``/etc/neutron/plugins/ml2/ml2_conf.ini`` file:

.. code-block:: ini

    [ml2]
    type_drivers = flat,vlan,vxlan,gre

.. note::

   For more details，see the `Bug 1567792 <https://bugs.launchpad.net/openstack-manuals/+bug/1567792>`__.

For more details, see the
`Networking configuration options <../configuration/ml2-conf.html>`__
of Configuration Reference.

The following type drivers are available

* Flat

* VLAN

* GRE

* VXLAN

Provider network types
^^^^^^^^^^^^^^^^^^^^^^

Provider networks provide connectivity like project networks.
But only administrative (privileged) users can manage those
networks because they interface with the physical network infrastructure.
More information about provider networks see
:doc:`intro-os-networking`.

* Flat

  The administrator needs to configure a list of physical network names that
  can be used for provider networks.
  For more details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#ml2-type-flat>`__.

* VLAN

  The administrator needs to configure a list of physical network names that
  can be used for provider networks.
  For more details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#ml2-type-vlan>`__.

* GRE

  No additional configuration required.

* VXLAN

  The administrator can configure the VXLAN multicast group that should be
  used.

  .. note::

     VXLAN multicast group configuration is not applicable for the Open
     vSwitch agent.

     As of today it is not used in the Linux bridge agent. The Linux bridge
     agent has its own agent specific configuration option. For more details,
     see the `Bug 1523614 <https://bugs.launchpad.net/neutron/+bug/1523614>`__.

Project network types
^^^^^^^^^^^^^^^^^^^^^

Project networks provide connectivity to instances for a particular
project. Regular (non-privileged) users can manage project networks
within the allocation that an administrator or operator defines for
them. More information about project and provider networks see
:doc:`intro-os-networking`.

Project network configurations are made in the
``/etc/neutron/plugins/ml2/ml2_conf.ini`` configuration file on the neutron
server:

* VLAN

  The administrator needs to configure the range of VLAN IDs that can be
  used for project network allocation.
  For more details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#ml2-type-vlan>`__.

* GRE

  The administrator needs to configure the range of tunnel IDs that can be
  used for project network allocation.
  For more details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#ml2-type-gre>`__.

* VXLAN

  The administrator needs to configure the range of VXLAN IDs that can be
  used for project network allocation.
  For more details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#ml2-type-vxlan>`__.

.. note::
   Flat networks for project allocation are not supported. They only
   can exist as a provider network.

Mechanism drivers
-----------------

To enable mechanism drivers in the ML2 plug-in, edit the
``/etc/neutron/plugins/ml2/ml2_conf.ini`` file on the neutron server:

.. code-block:: ini

   [ml2]
   mechanism_drivers = ovs,l2pop

.. note::

   For more details, see the `Bug 1567792 <https://bugs.launchpad.net/openstack-manuals/+bug/1567792>`__.

For more details, see the
`Configuration Reference <../configuration/ml2-conf.html#ml2>`__.

* Linux bridge

  No additional configurations required for the mechanism driver. Additional
  agent configuration is required. For details, see the related *L2 agent*
  section below.

* Open vSwitch

  No additional configurations required for the mechanism driver. Additional
  agent configuration is required. For details, see the related *L2 agent*
  section below.

* OVN

  The administrator must configure some additional configuration options for
  the mechanism driver. When this driver is used, architecture of the Neutron
  application in the cluster is different from what it is with other drivers
  like e.g. Open vSwitch or Linuxbridge.
  For details, see :ref:`OVN reference architecture<refarch-refarch>`.

* SRIOV

  The SRIOV driver accepts all PCI vendor devices.

* MacVTap

  No additional configurations required for the mechanism driver. Additional
  agent configuration is required. Please see the related section.

* L2 population

  The administrator can configure some optional configuration options. For more
  details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#l2pop>`__.

* Specialized

  * Open source

    External open source mechanism drivers exist as well as the neutron
    integrated reference implementations. Configuration of those drivers is not
    part of this document. For example:

    * OpenDaylight
    * OpenContrail

  * Proprietary (vendor)

    External mechanism drivers from various vendors exist as well as the
    neutron integrated reference implementations.

    Configuration of those drivers is not part of this document.

Supported VNIC types
^^^^^^^^^^^^^^^^^^^^

The ``vnic_type_prohibit_list`` option is used to remove values from the
mechanism driver's ``supported_vnic_types`` list.

.. list-table:: Mechanism drivers and supported VNIC types
   :header-rows: 1

   * - mech driver / supported_vnic_types
     - supported VNIC types
     - prohibiting available
   * - Linux bridge
     - normal
     - no
   * - OVN
     - normal, direct, direct_macvtap, direct_physical
     - no
   * - MacVTap
     - macvtap
     - no
   * - Open vSwitch
     - normal, direct
     - yes (ovs_driver vnic_type_prohibit_list, see: `Configuration Reference <../configuration/ml2-conf.html#ovs_driver>`__)
   * - SRIOV
     - direct, macvtap, direct_physical
     - yes (sriov_driver vnic_type_prohibit_list, see: `Configuration Reference <../configuration/ml2-conf.html#sriov_driver>`__)


Extension Drivers
-----------------

The ML2 plug-in also supports extension drivers that allows other pluggable
drivers to extend the core resources implemented in the ML2 plug-in
(``networks``, ``ports``, etc.). Examples of extension drivers include support
for QoS, port security, etc. For more details see the ``extension_drivers``
configuration option in the `Configuration Reference <../configuration/ml2-conf.html#ml2.extension_drivers>`__.


Agents
------

L2 agent
^^^^^^^^

An L2 agent serves layer 2 (Ethernet) network connectivity to OpenStack
resources. It typically runs on each Network Node and on each Compute Node.

* Open vSwitch agent

  The Open vSwitch agent configures the Open vSwitch to realize L2 networks for
  OpenStack resources.

  Configuration for the Open vSwitch agent is typically done in the
  ``openvswitch_agent.ini`` configuration file. Make sure that on agent start
  you pass this configuration file as argument.

  For a detailed list of configuration options, see the related section in the
  `Configuration Reference <../configuration/openvswitch-agent.html>`__.

* Linux bridge agent

  The Linux bridge agent configures Linux bridges to realize L2 networks for
  OpenStack resources.

  Configuration for the Linux bridge agent is typically done in the
  ``linuxbridge_agent.ini`` configuration file. Make sure that on agent start
  you pass this configuration file as argument.

  For a detailed list of configuration options, see the related section in the
  `Configuration Reference <../configuration/linuxbridge-agent.html>`__.

* SRIOV Nic Switch agent

  The sriov nic switch agent configures PCI virtual functions to realize L2
  networks for OpenStack instances. Network attachments for other resources
  like routers, DHCP, and so on are not supported.

  Configuration for the SRIOV nic switch agent is typically done in the
  ``sriov_agent.ini`` configuration file. Make sure that on agent start
  you pass this configuration file as argument.

  For a detailed list of configuration options, see the related section in the
  `Configuration Reference <../configuration/sriov-agent.html>`__.

* MacVTap agent

  The MacVTap agent uses kernel MacVTap devices for realizing L2
  networks for OpenStack instances. Network attachments for other resources
  like routers, DHCP, and so on are not supported.

  Configuration for the MacVTap agent is typically done in the
  ``macvtap_agent.ini`` configuration file. Make sure that on agent start
  you pass this configuration file as argument.

  For a detailed list of configuration options, see the related section in the
  `Configuration Reference <../configuration/macvtap-agent.html>`__.

L3 agent
^^^^^^^^

The L3 agent offers advanced layer 3 services, like virtual Routers and
Floating IPs. It requires an L2 agent running in parallel.

Configuration for the L3 agent is typically done in the
``l3_agent.ini`` configuration file. Make sure that on agent start
you pass this configuration file as argument.

For a detailed list of configuration options, see the related section in the
`Configuration Reference <../configuration/l3-agent.html>`__.

DHCP agent
^^^^^^^^^^

The DHCP agent is responsible for DHCP (Dynamic Host Configuration
Protocol) and RADVD (Router Advertisement Daemon) services.
It requires a running L2 agent on the same node.

Configuration for the DHCP agent is typically done in the
``dhcp_agent.ini`` configuration file. Make sure that on agent start
you pass this configuration file as argument.

For a detailed list of configuration options, see the related section in the
`Configuration Reference <../configuration/dhcp-agent.html>`__.

Metadata agent
^^^^^^^^^^^^^^

The Metadata agent allows instances to access cloud-init meta data and user
data via the network. It requires a running L2 agent on the same node.

Configuration for the Metadata agent is typically done in the
``metadata_agent.ini`` configuration file. Make sure that on agent start
you pass this configuration file as argument.

For a detailed list of configuration options, see the related section in the
`Configuration Reference <../configuration/metadata-agent.html>`__.

L3 metering agent
^^^^^^^^^^^^^^^^^

The L3 metering agent enables layer3 traffic metering. It requires a running L3
agent on the same node.

Configuration for the L3 metering agent is typically done in the
``metering_agent.ini`` configuration file. Make sure that on agent start
you pass this configuration file as argument.

For a detailed list of configuration options, see the related section in the
`Configuration Reference <../configuration/metering-agent.html>`__.

Security
--------

L2 agents support some important security configurations.

* Security Groups

  For more details, see the related section in the
  `Configuration Reference <../configuration/ml2-conf.html#securitygroup>`__.

* Arp Spoofing Prevention

  Configured in the *L2 agent* configuration.


Reference implementations
~~~~~~~~~~~~~~~~~~~~~~~~~

Overview
--------

In this section, the combination of a mechanism driver and an L2 agent is
called 'reference implementation'. The following table lists these
implementations:

.. list-table:: Mechanism drivers and L2 agents
   :header-rows: 1

   * - Mechanism Driver
     - L2 agent
   * - Open vSwitch
     - Open vSwitch agent
   * - Linux bridge
     - Linux bridge agent
   * - OVN
     - No (there is ovn-controller running on nodes)
   * - SRIOV
     - SRIOV nic switch agent
   * - MacVTap
     - MacVTap agent
   * - L2 population
     - Open vSwitch agent, Linux bridge agent

The following tables shows which reference implementations support which
non-L2 neutron agents:

.. list-table:: Reference implementations and other agents
   :header-rows: 1

   * - Reference Implementation
     - L3 agent
     - DHCP agent
     - Metadata agent
     - L3 Metering agent
   * - Open vSwitch & Open vSwitch agent
     - yes
     - yes
     - yes
     - yes
   * - Linux bridge & Linux bridge agent
     - yes
     - yes
     - yes
     - yes
   * - OVN
     - no (own L3 implementation)
     - no (DHCP provided by OVN, fully distributed)
     - yes (running on compute nodes, fully distributed)
     - no
   * - SRIOV & SRIOV nic switch agent
     - no
     - no
     - no
     - no
   * - MacVTap & MacVTap agent
     - no
     - no
     - no
     - no

.. note::
   L2 population is not listed here, as it is not a standalone mechanism.
   If other agents are supported depends on the conjunctive mechanism driver
   that is used for binding a port.

   More information about L2 population see the
   `OpenStack Manuals <https://networkop.co.uk/blog/2016/05/06/neutron-l2pop/>`_.


Buying guide
------------

This guide characterizes the L2 reference implementations that currently exist.

* Open vSwitch mechanism and Open vSwitch agent

  Can be used for instance network attachments as well as for attachments of
  other network resources like routers, DHCP, and so on.

* Linux bridge mechanism and Linux bridge agent

  Can be used for instance network attachments as well as for attachments of
  other network resources like routers, DHCP, and so on.

* OVN mechanism driver

  Can be used for instance network attachments as well as for attachments of
  other network resources like routers, metadata ports, and so on.

* SRIOV mechanism driver and SRIOV NIC switch agent

  Can only be used for instance network attachments (device_owner = compute).

  Is deployed besides an other mechanism driver and L2 agent such as OVS or
  Linux bridge. It offers instances direct access to the network adapter
  through a PCI Virtual Function (VF). This gives an instance direct access to
  hardware capabilities and high performance networking.

  The cloud consumer can decide via the neutron APIs VNIC_TYPE attribute, if
  an instance gets a normal OVS port or an SRIOV port.

  Due to direct connection, some features are not available when using SRIOV.
  For example, DVR, security groups, migration.

  For more information see the :ref:`config-sriov`.

* MacVTap mechanism driver and MacVTap agent

  Can only be used for instance network attachments (device_owner = compute)
  and not for attachment of other resources like routers, DHCP, and so on.

  It is positioned as alternative to Open vSwitch or Linux bridge support on
  the compute node for internal deployments.

  MacVTap offers a direct connection with very little overhead between
  instances and down to the adapter. You can use MacVTap agent on the
  compute node when you require a network connection that is performance
  critical. It does not require specific hardware (like with SRIOV).

  Due to the direct connection, some features are not available when using
  it on the compute node. For example, DVR, security groups and arp-spoofing
  protection.
