3.7. Virtual Network Interface Cards

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

When you attach a virtual network interface card to a virtual machine, the Red Hat Virtualization Manager creates several associations between the virtual machine to which the virtual network interface card is being attached, the virtual network interface card itself, and the physical host network interface card on which the virtual network interface card is based. Specifically, when a virtual network interface card is attached to a virtual machine, a new virtual network interface card and MAC address are created on the physical host network interface card on which the virtual network interface card is based. Then, the first time the virtual machine starts after that virtual network interface card is attached, libvirt assigns the virtual network interface card a PCI address. The MAC address and PCI address are then used to obtain the name of the virtual network interface card (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. When PCI addresses have already been created for a template or snapshot, the virtual network interface cards on virtual machines created based on that template or snapshot are ordered in accordance with those PCI addresses and MAC addresses allocated in that order. If PCI addresses have not already been created for a template, the virtual network interface cards on virtual machines created based on that template are allocated in the order of the naming of the virtual network interface cards. If PCI addresses have not already been created for a snapshot, the Red Hat Virtualization Manager allocates new MAC addresses to the virtual network interface cards on virtual machines based on that snapshot.

Once created, virtual network interface cards 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 virtual network interface cards 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 network interface cards 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).

Virtual network interface cards 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 virtual network interface cards are members of the ovirtmgmt bridge. All of the virtual machines that the virtual network interface cards are associated with are connected to the ovirtmgmt logical network. The eth0 network interface card is also a member of the ovirtmgmt bridge. The eth0 device is cabled to a switch that provides connectivity beyond the host.