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3.8. Virtual Network Interface Cards

Virtual network interface cards (vNICs) are virtual network interfaces that are based on the physical NICs of a host. Each host can have multiple NICs, and each NIC can be a base for multiple vNICs.

When you attach a vNIC to a virtual machine, the Red Hat Virtualization Manager creates several associations between the virtual machine to which the vNIC is being attached, the vNIC itself, and the physical host NIC on which the vNIC is based. Specifically, when a vNIC is attached to a virtual machine, a new vNIC and MAC address are created on the physical host NIC on which the vNIC is based. Then, the first time the virtual machine starts after that vNIC is attached, libvirt assigns the vNIC a PCI address. The MAC address and PCI address are then used to obtain the name of the vNIC (for example, eth0) in the virtual machine.

The process for assigning MAC addresses and associating those MAC addresses with PCI addresses is slightly different when creating virtual machines based on templates or snapshots:

  • If PCI addresses have already been created for a template or snapshot, the vNICs on virtual machines created based on that template or snapshot are ordered in accordance with those PCI addresses. MAC addresses are then allocated to the vNICs in that order.
  • If PCI addresses have not already been created for a template, the vNICs on virtual machines created based on that template are ordered alphabetically. MAC addresses are then allocated to the vNICs in that order.
  • If PCI addresses have not already been created for a snapshot, the Red Hat Virtualization Manager allocates new MAC addresses to the vNICs on virtual machines based on that snapshot.

Once created, vNICs are added to a network bridge device. The network bridge devices are how virtual machines are connected to virtual machine logical networks.

Running the ip addr show command on a virtualization host shows all of the vNICs that are associated with virtual machines on that host. Also visible are any network bridges that have been created to back logical networks, and any NICs used by the host. 

[root@rhev-host-01 ~]# ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 00:21:86:a2:85:cd brd ff:ff:ff:ff:ff:ff
    inet6 fe80::221:86ff:fea2:85cd/64 scope link
       valid_lft forever preferred_lft forever
3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN qlen 1000
    link/ether 00:21:6b:cc:14:6c brd ff:ff:ff:ff:ff:ff
5: ;vdsmdummy;: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN
    link/ether 4a:d5:52:c2:7f:4b brd ff:ff:ff:ff:ff:ff
6: bond0: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
7: bond4: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
8: bond1: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
9: bond2: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
10: bond3: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
11: ovirtmgmt: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN
    link/ether 00:21:86:a2:85:cd brd ff:ff:ff:ff:ff:ff
    inet 10.64.32.134/23 brd 10.64.33.255 scope global ovirtmgmt
    inet6 fe80::221:86ff:fea2:85cd/64 scope link
       valid_lft forever preferred_lft forever

The console output from the command shows several devices: one loop back device (lo), one Ethernet device (eth0), one wireless device (wlan0), one VDSM dummy device (;vdsmdummy;), five bond devices (bond0, bond4, bond1, bond2, bond3), and one network bridge (ovirtmgmt).

vNICs are all members of a network bridge device and logical network. Bridge membership can be displayed using the brctl show command: 

[root@rhev-host-01 ~]# brctl show
bridge name	bridge id		STP enabled	interfaces
ovirtmgmt		8000.e41f13b7fdd4	no		vnet002
							vnet001
							vnet000
							eth0

The console output from the brctl show command shows that the virtio vNICs are members of the ovirtmgmt bridge. All of the virtual machines that the vNICs are associated with are connected to the ovirtmgmt logical network. The eth0 NIC is also a member of the ovirtmgmt bridge. The eth0 device is cabled to a switch that provides connectivity beyond the host.