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.

ML2 driver support matrix

Configuration

Network type drivers

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

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

For more details, see the Networking configuration
options 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 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.

• 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.

• 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.

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 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.

• 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.

• 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.

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:

[ml2]
mechanism_drivers = ovs,l2pop

For more details, see the Configuration
Reference.

• 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 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.

• 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.

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.

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.

• 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.

• 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.

• 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.

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.

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.

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.

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.

Security

L2 agents support some important security configurations.

• Security Groups

  For more details, see the related section in the Configuration
  Reference.

• 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:

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

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 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.