Overview of Networking in oVirt

  1. Overview of Networking in oVirt
    1. Introduction
    2. Host Networking
    3. VM Networking
      1. Supported Networking Types
    4. In which scenario to choose which kind of networking
      1. Interaction with the Guest
      2. Relation to Host Networking


This document provides an introductory general view on networking in oVirt, to enable the reader to dive deeper into details.

Host Networking

On the data link layer (layer 2), oVirt allows the configuration of Linux bonds, to connect to VLANs and to define the MTU for network interfaces. These networks can be shared via Linux bridges to VMs.

For SR-IOV, the number of virtual functions and their mapping to logical networks can be configured.

There are also VLANs managed by FCoE and (application layer) iSCSI bonds, which are exclusively for storage access and neither visible for VMs nor in RHVs network configuration.

On the network layer (layer 3), IP addresses and gateways can be managed.

The DNS resolver used by the host can be configured.

It is also possible to manage network roles and QoS.

VM Networking

Supported Networking Types

In oVirt virtual NICs of VMs can connect to:

  • Linux bridges
  • OVN
    • oVirt internal OVN
    • OpenStack Neutron OVN

The following diagram shows the structure of these three approaches: Graphical comparison of VM network types

The following table compares the four kinds of networks:

  Linux bridge SR-IOV oVirt internal OVN OpenStack Neutron OVN
Isolation from physical host networks Layer 3, Separate IP network possible Layer 2, Separate VLANs possible Isolated Isolated
Live Migration x x x x
QoS x     x
Port Mirroring x      
Configuration of plugged vNIC x x    
MAC address management x x x x
MTU propagation x   x x
VLAN filtering, might require configuration on the physical switch x x Technology Preview Technology Preview
MAC Spoofing Protection x   x x
IP Spoofing Protection x   x x
Predefined Network Filters x      
Custom Layer 3/4 Filtering     x x
NAT       x
DHCP/Router Advertisements     x x
Layer 3 Router     x x
Performance ** *** * *
VM network data encapsulation flat, VLAN flat, VLAN Stable: GENEVE; Technology Preview: flat, VLAN Stable: GENEVE; Technology Preview: flat, VLAN

In which scenario to choose which kind of networking

Linux bridge is the default way to go. It is the most proven option which fits most use cases.

For scenarios that require very low network latency or a high count of Ethernet frames, it might be worth investing in SR-IOV, which requires hardware support and additional configuration steps.

oVirt’s internal OVN networks enable VMs to communicate with each other without additional effort.

If a full-featured SDN is required, OpenStack Neutron can be used to manage OVN.

To utilize the full potential of SDNs, like oVirt internal OVN, OpenStack Neutron OVN, or third party SDN, an additional client like CloudForms is required, because oVirt Engine provides only for a subset of the SDN features an user interface.

All kinds of networks can be combined on a single host and can be connected to the same VM, with a single exception: oVirt’s internal OVN can not be used with Neutron in the same cluster.

Interaction with the Guest

oVirt supports the initial configuration of the guest by providing configuration data via cloud-init. If the qemu-guest-agent runs inside the guest, oVirt is able to report the IP addresses of the guest.

If the guest VM is using a VirtIO NIC, the MTU of the oVirt logical networks are provided to the guest OS. The guest can pick up the MTU from DHCPv4 or IPv6 router advertisements if supported by the logical network.

Relation to Host Networking

Linux bridge networking separates VM and host networking on OSI layer 3. This means that the networking configuration below, like VLAN, bonding, and MTU, is shared between host and VMs.

This implies that hosts should not assign an IP address to VLANs which are connected to VMs to reduce the surface of the host which might be confused by any VM traffic.