Practical Cloud Knowledge: Install, Configure, and Manage - Deploying and Managing a Cloud Infrastructure - page 198

■ There must be a connected host network interface controller. To be in bridged mode,
a host must have a NIC and a physical network to attach it to. Otherwise, what would
be the point of bridging?
■ The bridge driver enables the virtual NIC to communicate with the host's NIC and
allows for the bridging to occur. For VMware products, this is called the VMware
Bridge protocol, while for Microsoft's virtualization solutions, it is called the Virtual
PC Network Filer driver. Both should be enabled through the host computer's network
settings for the associated network adapter.
■ The host's Ethernet adapter has to be set to promiscuous mode to enable all traffic to
enter instead of just discarding those that are not meant for its own MAC address. This
allows traffic meant for the bridged NIC to be received and routed to the bridge.
Virtual machines with virtual network interface controllers do not directly interact
with the outside physical network for obvious reasons. All data and signals go through
virtualization intermediaries (VIs) , which are special programs that handle communica-
tion between the virtual machines and the host. The VI can be either a part of the hyper-
visor or the host itself.
The virtualization intermediaries are used for safely sharing I/O, meaning that multiple
operating systems may share the same PCI device through the virtualization intermediary.
The virtualization intermediary can perform this by multiplexing the I/O flows from each
operating system and performing the PCI I/O transactions in behalf of the participating
OSs. The VI also performs as a communication medium for all the OSs running on the
same hypervisor. This is assuming that the PCI device being used is of the modern PCIe
(PCI Express) variety, which supports multiple-Mac addresses to allow for a unique MAC
address for each OS, and has state-of-the art IP stack accelerators and functions like trans-
mit and receive queue pairs.
Virtual Disks
Along with all things virtual, a virtual disk is simply a software implementation—in this
case, a file that presents itself to the user and the guest operating system as a hard disk
drive. To the user and the virtual machine, the virtual disk behaves and looks like a real
disk drive. The virtual disk resides in a single physical hard drive or, in the case of large
cloud computing implementations, is striped across multiple physical drives and possibly
across multiple availability zones, complete with backups.
To a virtual machine, a virtual disk is as important as a physical disk to a physical
computer. Without it, the machine cannot boot nor do anything because there is no place
to install an operating system. Take note that a virtual machine is often called a guest
operating system, so without a drive to install that OS, there is no virtual machine, just a
bunch of settings ready for prime time.
A virtual disk can be created from scratch as a single file on the physical hard drive as
part of the process for creating a virtual machine, or it can be part of a virtual machine tem-
plate or even separately created inside a storage repository. There are different file types used
for virtual disks, and it varies depending on the virtualization software provider. Microsoft
uses VHD, Oracle uses VDI, and VMware uses VMDK.