Networking with neutron

While nova uses the OpenStack Networking service (neutron) to provide network connectivity for instances, nova itself provides some additional features not possible with neutron alone. These are described below.


Changed in version 2014.2: The feature described below was first introduced in the Juno release.

The SR-IOV specification defines a standardized mechanism to virtualize PCIe devices. This mechanism can virtualize a single PCIe Ethernet controller to appear as multiple PCIe devices. Each device can be directly assigned to an instance, bypassing the hypervisor and virtual switch layer. As a result, users are able to achieve low latency and near-line wire speed.

A full guide on configuring and using SR-IOV is provided in the OpenStack Networking service documentation


Nova only supports PCI addresses where the fields are restricted to the following maximum value:

  • domain - 0xFFFF

  • bus - 0xFF

  • slot - 0x1F

  • function - 0x7

Nova will ignore PCI devices reported by the hypervisor if the address is outside of these ranges.

New in version 25.0.0: For information on creating servers with remotely-managed SR-IOV network interfaces of SmartNIC DPUs, refer to the relevant section in Networking Guide.


  • Only VFs are supported and they must be tagged in the Nova Compute configuration in the passthrough_whitelist option as remote_managed: "true". There is no auto-discovery of this based on vendor and product IDs;

  • Either VF or its respective PF must expose a PCI VPD capability with a unique card serial number according to the PCI/PCIe specifications (see the Libvirt docs to get an example of how VPD data is represented and what to expect). If this is not the case, those devices will not appear in allocation pools;

  • Only the Libvirt driver is capable of supporting this feature at the time of writing;

  • The support for VPD capability handling in Libvirt was added in release 7.9.0 - older versions are not supported by this feature;

  • All compute nodes must be upgraded to the Yoga release in order for scheduling of nodes with VNIC_TYPE_REMOTE_MANAGED ports to succeed;

  • The same limitations apply to operations like live migration as with legacy SR-IOV ports;

  • Clearing a VLAN by programming VLAN 0 must not result in errors in the VF kernel driver at the compute host. Before v8.1.0 Libvirt clears a VLAN by programming VLAN 0 before passing a VF through to the guest which may result in an error depending on your driver and kernel version (see, for example, this bug which discusses a case relevant to one driver). As of Libvirt v8.1.0, EPERM errors encountered while programming VLAN 0 are ignored if VLAN clearning is not explicitly requested in the device XML (i.e. VLAN 0 is not specified explicitly).

NUMA Affinity

New in version 18.0.0: The feature described below was first introduced in the Rocky release.


The functionality described below is currently only supported by the libvirt/KVM driver.

As described in CPU topologies, NUMA is a computer architecture where memory accesses to certain regions of system memory can have higher latencies than other regions, depending on the CPU(s) your process is running on. This effect extends to devices connected to the PCIe bus, a concept known as NUMA I/O. Many Network Interface Cards (NICs) connect using the PCIe interface, meaning they are susceptible to the ill-effects of poor NUMA affinitization. As a result, NUMA locality must be considered when creating an instance where high dataplane performance is a requirement.

Fortunately, nova provides functionality to ensure NUMA affinitization is provided for instances using neutron. How this works depends on the type of port you are trying to use.

For SR-IOV ports, virtual functions, which are PCI devices, are attached to the instance. This means the instance can benefit from the NUMA affinity guarantees provided for PCI devices. This happens automatically and is described in detail in PCI-NUMA affinity policies.

For all other types of ports, some manual configuration is required.

  1. Identify the type of network(s) you wish to provide NUMA affinity for.

    • If a network is an L2-type network (provider:network_type of flat or vlan), affinity of the network to given NUMA node(s) can vary depending on value of the provider:physical_network attribute of the network, commonly referred to as the physnet of the network. This is because most neutron drivers map each physnet to a different bridge, to which multiple NICs are attached, or to a different (logical) NIC.

    • If a network is an L3-type networks (provider:network_type of vxlan, gre or geneve), all traffic will use the device to which the endpoint IP is assigned. This means all L3 networks on a given host will have affinity to the same NUMA node(s). Refer to the neutron documentation for more information.

  2. Determine the NUMA affinity of the NICs attached to the given network(s).

    How this should be achieved varies depending on the switching solution used and whether the network is a L2-type network or an L3-type networks.

    Consider an L2-type network using the Linux Bridge mechanism driver. As noted in the neutron documentation, physnets are mapped to interfaces using the [linux_bridge] physical_interface_mappings configuration option. For example:

    physical_interface_mappings = provider:PROVIDER_INTERFACE

    Once you have the device name, you can query sysfs to retrieve the NUMA affinity for this device. For example:

    $ cat /sys/class/net/PROVIDER_INTERFACE/device/numa_node

    For an L3-type network using the Linux Bridge mechanism driver, the device used will be configured using protocol-specific endpoint IP configuration option. For VXLAN, this is the [vxlan] local_ip option. For example:


    Once you have the IP address in question, you can use ip to identify the device that has been assigned this IP address and from there can query the NUMA affinity using sysfs as above.


    The example provided above is merely that: an example. How one should identify this information can vary massively depending on the driver used, whether bonding is used, the type of network used, etc.

  3. Configure NUMA affinity in nova.conf.

    Once you have identified the NUMA affinity of the devices used for your networks, you need to configure this in nova.conf. As before, how this should be achieved varies depending on the type of network.

    For L2-type networks, NUMA affinity is defined based on the provider:physical_network attribute of the network. There are two configuration options that must be set:

    [neutron] physnets

    This should be set to the list of physnets for which you wish to provide NUMA affinity. Refer to the documentation for more information.

    [neutron_physnet_{physnet}] numa_nodes

    This should be set to the list of NUMA node(s) that networks with the given {physnet} should be affinitized to.

    For L3-type networks, NUMA affinity is defined globally for all tunneled networks on a given host. There is only one configuration option that must be set:

    [neutron_tunnel] numa_nodes

    This should be set to a list of one or NUMA nodes to which instances using tunneled networks will be affinitized.

  4. Configure a NUMA topology for instance flavor(s)

    For network NUMA affinity to have any effect, the instance must have a NUMA topology itself. This can be configured explicitly, using the hw:numa_nodes extra spec, or implicitly through the use of CPU pinning (hw:cpu_policy=dedicated) or PCI devices. For more information, refer to CPU topologies.


Take an example for deployment using L2-type networks first.

physnets = foo,bar

numa_nodes = 0

numa_nodes = 2, 3

This configuration will ensure instances using one or more L2-type networks with provider:physical_network=foo must be scheduled on host cores from NUMA nodes 0, while instances using one or more networks with provider:physical_network=bar must be scheduled on host cores from both NUMA nodes 2 and 3. For the latter case, it will be necessary to split the guest across two or more host NUMA nodes using the hw:numa_nodes extra spec, as discussed here.

Now, take an example for a deployment using L3 networks.

numa_nodes = 0

This is much simpler as all tunneled traffic uses the same logical interface. As with the L2-type networks, this configuration will ensure instances using one or more L3-type networks must be scheduled on host cores from NUMA node 0. It is also possible to define more than one NUMA node, in which case the instance must be split across these nodes.

virtio-net Multiqueue

New in version 12.0.0: (Liberty)

Changed in version 25.0.0: (Yoga)

Support for configuring multiqueue via the hw:vif_multiqueue_enabled flavor extra spec was introduced in the Yoga (25.0.0) release.


The functionality described below is currently only supported by the libvirt/KVM driver.

Virtual NICs using the virtio-net driver support the multiqueue feature. By default, these vNICs will only use a single virtio-net TX/RX queue pair, meaning guests will not transmit or receive packets in parallel. As a result, the scale of the protocol stack in a guest may be restricted as the network performance will not scale as the number of vCPUs increases and per-queue data processing limits in the underlying vSwitch are encountered. The solution to this issue is to enable virtio-net multiqueue, which can allow the guest instances to increase the total network throughput by scaling the number of receive and transmit queue pairs with CPU count.

Multiqueue virtio-net isn’t always necessary, but it can provide a significant performance benefit when:

  • Traffic packets are relatively large.

  • The guest is active on many connections at the same time, with traffic running between guests, guest to host, or guest to an external system.

  • The number of queues is equal to the number of vCPUs. This is because multi-queue support optimizes RX interrupt affinity and TX queue selection in order to make a specific queue private to a specific vCPU.

However, while the virtio-net multiqueue feature will often provide a welcome performance benefit, it has some limitations and therefore should not be unconditionally enabled:

  • Enabling virtio-net multiqueue increases the total network throughput, but in parallel it also increases the CPU consumption.

  • Enabling virtio-net multiqueue in the host QEMU config does not enable the functionality in the guest OS. The guest OS administrator needs to manually turn it on for each guest NIC that requires this feature, using ethtool.

  • In case the number of vNICs in a guest instance is proportional to the number of vCPUs, enabling the multiqueue feature is less important.

Having considered these points, multiqueue can be enabled or explicitly disabled using either the hw:vif_multiqueue_enabled flavor extra spec or equivalent hw_vif_multiqueue_enabled image metadata property. For example, to enable virtio-net multiqueue for a chosen flavor:

$ openstack flavor set --property hw:vif_multiqueue_enabled=true $FLAVOR

Alternatively, to explicitly disable multiqueue for a chosen image:

$ openstack image set --property hw_vif_multiqueue_enabled=false $IMAGE


If both the flavor extra spec and image metadata property are provided, their values must match or an error will be raised.

Once the guest has started, you must enable multiqueue using ethtool. For example:

$ ethtool -L $devname combined $N

where $devname is the name of the network device, and $N is the number of TX/RX queue pairs to configure corresponding to the number of instance vCPUs. Alternatively, you can configure this persistently using udev. For example, to configure four TX/RX queue pairs for network device eth0:

# cat /etc/udev/rules.d/50-ethtool.rules
ACTION=="add", SUBSYSTEM=="net", NAME=="eth0", RUN+="/sbin/ethtool -L eth0 combined 4"

For more information on this feature, refer to the original spec.