Creating Multicast Transmissions

Multicasting enables you to deploy an image to numerous client computers at the same time without overburdening the network. By using multicast, you transmit image data only once, drastically reducing the amount of network bandwidth that is used to deploy images from Windows Deployment Services.

Consider implementing multicasting if your organization:

• Has network routers that support multicasting.
• Is a large company that requires many concurrent client installations. If your organization deploys images to only a small number of computers at the same time, multicasting might not be the right choice.
• Wants to use network bandwidth efficiently. With this feature, images are sent over the network only once, and you can specify limitations (for example, to only use 10 percent of your bandwidth). If your organization does not have bandwidth overload issues, multicasting might not be worth the effort.
• Has enough disk space on client computers for the image to be downloaded. When multicasting, Windows Deployment Services downloads the image to the client computer instead of installing it from the server.
• Meets the requirements listed in the following section.

Multicast Prerequisites

To use multicast in your organization, it must meet all the following requirements:

• Routers that support multicasting. In particular, your network infrastructure needs to support the Internet Group Management Protocol (IGMP) to properly forward multicast traffic. Without the IGMP, multicast packets are treated as broadcast packets, which can lead to network flooding.
• At least one install image that you want to transmit on the server.
• The Boot.wim file located in the \Sources folder on Windows 7 or Windows Server 2008 R2 media.
• IGMP snooping should be enabled on all devices. This will cause your network hardware to forward multicast packets only to those devices that are requesting data. If IGMP snooping is turned off, multicast packets are treated as broadcast packets and will be sent to every device in the subnet.

Transmission Types

There are two types of multicast transmissions:

• Auto-Cast This option indicates that as soon as an applicable client requests an install image, a multicast transmission of the selected image begins. Then, as other clients request the same image, they are also joined to the transmission that is already started.
• Scheduled-Cast This option sets the start criteria for the transmission based on the number of clients that are requesting an image, a specific day and time, or both. If you do not select either of the check boxes in Scheduled-Cast, the transmission will not start until you manually start it. Note that in addition to these criteria, you can start a transmission manually at any time by right-clicking it and then clicking Start.