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

Mechanism drivers and L2 agents

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:

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

Note

For more details，see the Bug 1567792.

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 OpenStack 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 OpenStack 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.

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:

[ml2]
mechanism_drivers = ovs,l2pop

Note

For more details, see the Bug 1567792.

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.

  • 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.

Mechanism drivers and supported VNIC types

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)

SRIOV

direct, macvtap, direct_physical

yes (sriov_driver vnic_type_prohibit_list, see: Configuration Reference)

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:

Mechanism drivers and L2 agents

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:

Reference implementations and other agents

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.

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 SR-IOV.

  • 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.