Scenario 1: Compute host config

	Note
	There are three distinct type of virtual networking devices: TAP devices, VLAN devices, and Linux bridges. For an ethernet frame to travel from eth0 of virtual machine vm01, to the physical network, it must pass through four devices inside of the host: TAP vnet0, Linux bridge brqXXX, VLAN eth1.101), and, finally, the physical network interface card eth1.

A TAP device, such as vnet0 is how hypervisors such as KVM and Xen implement a virtual network interface card (typically called a VIF or vNIC). An ethernet frame sent to a TAP device is received by the guest operating system.

A VLAN device is associated with a VLAN tag attaches to an existing interface device and adds or removes VLAN tags. In the preceding example, VLAN device eth1.101 is associated with VLAN ID 101 and is attached to interface eth1. Packets received from the outside by eth1 with VLAN tag 101 will be passed to device eth1.101, which will then strip the tag. In the other direction, any ethernet frame sent directly to eth1.101 will have VLAN tag 101 added and will be forward to eth1 for sending out to the network.

A Linux bridge behaves like a hub: you can connect multiple (physical or virtual) network interfaces devices to a Linux bridge. Any ethernet frames that come in from one interface attached to the bridge is transmitted to all of the other devices.