Microsoft Product Support Services (PSS) support boundaries for network booting Microsoft Windows Preinstallation Environment (Windows PE) 2.0

This article discusses the support boundaries that PSS provides for Windows PE 2.0 and describes methods for network booting Windows PE 2.0. This article specifically addresses the Pre-Boot Execution Environment (PXE) network boot technology. It does not encompass other network boot technologies that perform block-level I/O over a network connection, such as the Internet small computer system interface (iSCSI) boot technology.

A network booting client performs the following tasks:

• Obtains an IP address
• Discovers a valid network boot server
• Downloads and executes a network boot application

The code execution in the network boot application leads to a full download and to a boot of Windows PE.

Hardware support for network boot operations

The ability to network boot comes from code that is embedded into the ROM of the network adapter or of the motherboard of the computer. This code contains logic that controls, among other things, network boot device initialization, User Datagram Protocol (UDP) transport support, and Trivial File Transfer Protocol (TFTP) support. For a certain class of PCI network interface cards (NICs), you may boot a device that uses software that emulates the function of an onboard PXE ROM in order to enable the device to boot. This external software usually takes the form of a network boot floppy disk.

Hardware PXE ROM issues

PSS support for hardware-related PXE ROM issues is limited to a commercially reasonable effort. If PSS determines that the issue is related to the hardware PXE ROM, you must obtain support directly from the vendor of the computer or from the hardware manufacturer.

Software PXE ROM issues

Microsoft provides software PXE ROM emulation in the form of a Remote Installation Service (RIS) boot floppy disk. This can be used to boot supported network adapters. PSS provides full support for this utility. Additionally, implementations of a software PXE ROM are available from other software vendors. If PSS determines that the issue is related to a non-Microsoft PXE ROM implementation, you must obtain support directly from the software vendor.

Methods to direct a booting PXE client to the correct network boot file

The following methods may be used to direct a booting PXE client to download and execute the correct network boot file:

• The client may contact the network boot server directly to obtain the correct network boot file.
• The client may receive the correct network boot file information from a Dynamic Host Configuration Protocol (DHCP) server.

IP Helper table updates

The PXE network boot method uses DHCP packets for communication. The DHCP packets serve a dual purpose. They are intended to help the client in obtaining an IP address lease from a DHCP server and to locate a valid network boot server. If the booting client, the DHCP server, and the network boot server are all located on the same network segment, usually no additional configuration is necessary. The DHCP broadcasts from the client reach both the DHCP server and the network boot server.

However, if either the DHCP server or the network boot server are on a different network segment than the client, or if they are on the same network segment but the network is controlled by a switch or a router, you may have to update the routing tables for the networking equipment in order to make sure that DHCP traffic is directed correctly.

Such a process is known as performing IP Helper table updates. When you perform this process, you must configure the networking equipment so that all DHCP broadcasts from the client computer are directed to both a valid DHCP server and to a valid network boot server. After the client computer has obtained an IP address, it contacts the network boot server directly in order to obtain the name and the path of the network boot file to download. Again, this process is handled by using DHCP packets.

DHCP options 60, 66, and 67

In some environments, it may be appropriate to use following DHCP options to direct PXE clients to an appropriate network boot file to download:

• 60 = client Identifier set to the string value of PXEClient
• 66 = boot server host name
• 67 = boot file name

When you use these DHCP options, client computers receive the following information directly from the DHCP server:

• An IP address lease
• Information about the correct boot server
• Information about the correct boot file

In this scenario, client computers do not contact the network boot server by using DHCP. Instead, TFTP is used to specify a file that is downloaded directly. This method may be used as an alternative to the IP Helper update method that is described in this article. However, we do not recommend that you use this method for the following reasons:

• Using DHCP options is not as reliable as updating the IP Helper tables. In testing, Microsoft has observed that some client computers do not correctly parse the DHCP options that are returned from the DHCP server. This problem occurs most frequently when older PXE ROMs are used. The result is that booting client computers will report the following error message:
  TFTP Failed
  This problem occurs when the PXE ROM does not process the boot server host name value and tries to download the network boot application directly from the DHCP server.
• If multiple network boot servers are available to service client requests, and if a network boot server name is explicitly provided as part of the DHCP scope, load-balancing may not occur correctly.
• Client computers may be directed to a network boot server that is unavailable. Because client computers do not have to contact a network boot server directly in order to determine the appropriate network boot file to download, you may discover that client computers are directed by the DHCP server to a boot file that does not exist or to a server that is unavailable.
• Client computers may bypass the answer settings for the network boot server. Many network boot servers provide an on/off mechanism that enables you to control whether certain client requests are answered. According to the PXE standard, client computers must contact the network boot server directly to obtain the path and the file name of the network boot application. Additionally, when you use DHCP options 66 and 67, the client computer may completely bypass communication with the network boot server. When this occurs, the settings for the network boot server are not used.

Network boot referrals

A network boot referral, or a PXE boot referral, occurs when a client computer is directed to download a network boot application from a different server than the one with which it is communicating by using DHCP. A network boot referral or a PXE boot referral occurs as part of the discovery process to determine the network boot server name and the network boot file name. Either a network boot server or a DHCP server may initiate this referral.

First order referral from a PXE server

Consider the following scenario:

• Computer A broadcasts a DHCP packet and receives an IP address lease from a DHCP server. Computer A also receives a network boot response from PXE Server 1.
• Computer A contacts PXE Server 1 directly by using port 4011.
• PXE Server 1 directs Computer A to download the \boot\x86\wdsnbp.com file from the TFTP server.
• Computer A downloads the \boot\x86\wdsnbp.com file from the TFTP server.

In this scenario, PSS fully supports referral of x86-based client computers and the referral of x64-based client computers as long as all the following requirements are met:

• The name of the network boot application that the client computer is directed to download from the TFTP server must be wdsnbp.com.
  
  Warning Pointing to pxeboot.com is an unsupported scenario.
• The network boot server (PXE Server 1) that performs the referral must be running Microsoft Windows Deployment Services.

First order referral by using DHCP options

Consider the following scenario:

• Computer A broadcasts a DHCP packet and receives an IP address lease from a DHCP server.
• The lease also contains values for DHCP options 66 and 67. These refer the Computer A to download the \boot\x86\wdsnbp.com file from Server 1.
• Computer A downloads the \boot\x86\wdsnbp.com file from Server 1.

In this scenario, PSS fully supports referral of x86-based client computers and the referral of x64-based client computers as long as all the following requirements are met:

• The name of the network boot application that the TFTP server directs the client computer to download must be wdsnbp.com.

Second order referral by using both DHCP options and PXE server

Consider the following scenario:

• Computer A broadcasts a DHCP packet and receives an IP address lease from a DHCP server.
• The lease also contains values for DHCP options 66 and 67. These options refer Computer A to download the \boot\x86\wdsnbp.com file from the PXE server.
• Computer A downloads the \boot\x86\wdsnbp.com file from the PXE server.
• Computer A executes the \boot\x86\wdsnbp.com file, and Wdsnbp.com contacts the PXE server on port 4011.
• The PXE server refers Computer A to download the \boot\x86\wdsnbp.com file from the TFTP server.
• Computer A downloads the \boot\x86\wdsnbp.com file from the TFTP server.

In this scenario, PSS fully supports referral of x86-based client computers and the referral of x64-based client computers as long as all the following requirements are met:

• The name of the network boot application that the PXE Server directs the client computer to download must be wdsnbp.com.
• The network boot server (PXE Server) that performs the referral must be running Windows Deployment Services.

Network boot of Windows PE 2.0 by using a Microsoft PXE server

At the time of this writing, the following network boot servers, or PXE servers, are available from Microsoft:

• Windows Deployment Services
  We recommend that you use Windows Deployment Services for network booting Windows PE 2.0. This solution is fully supported by PSS.
• Remote Installation Services (RIS)
  PSS does not support the network boot of Windows PE 2.0 from this platform. Upgrade your existing RIS servers to Windows Deployment Services in order to acquire this functionality.
• Automated Deployment Services (ADS)
  PSS does not support the network boot of Windows PE 2.0 from this platform.
• Windows Embedded Remote Boot
  PSS does not support the network boot of Windows PE 2.0 from this platform.

Network boot of Windows PE 2.0 by using a third-party PXE server

PSS support for network boot issues that result from the use of a third-party PXE server is limited to a commercially reasonable effort. If PSS determines that the issue is related to the third-party PXE server implementation, you must obtain support directly from the third-party software vendor. This support policy also covers scenarios in which the server implementation does not handle DHCP traffic and provides only TFTP functionality.

Network boot of Windows PE 2.0 by using a third-party boot application

PSS fully supports the network boot process of Windows PE 2.0 when a Microsoft boot loader is responsible for all the following actions:

• Downloading the boot image
• Creating a RAMDISK
• Booting Windows PE

If you use third-party software to perform any of the actions in this list, the PSS support policy is defined as follows:

• You use a third-party network boot program to call into a Microsoft network boot application
  PSS support for network boot issues that result from the use of a third-party network boot program to boot Windows PE 2.0 is limited to a commercially reasonable effort. If PSS determines that the issue is related to the third-party implementation, you must obtain support directly from the third-party software vendor.
• You use a third-party network boot program to call directly into the Microsoft Windows PE loader
  PSS does not support this method for booting Windows PE 2.0.
• You use a third-party download manager in order to download the Windows PE 2.0 image
  PSS does not support this method for booting Windows PE 2.0.
• You use a third-party loader to create the RAMDISK image and you try to boot Windows PE
  PSS does not support this method of booting Windows PE 2.0.
• You use a third-party boot loader or download manager, or both, as a piece of intermediate software in order to intercept or redirect calls, or both, from a Microsoft network boot program or boot loader
  PSS does not support this method of booting Windows PE 2.0.

Image Formats

Windows PE 2.0 can boot by using two boot image formats. Only one of the supported boot image formats may be used when network booting.

• RAMDISK boot
  By using this boot method, a virtual disk volume is created in RAM to hold a boot image for Windows PE. The Windows PE image is downloaded as part of the network boot process and is put into the virtual disk memory space. Windows PE is then loaded and run directly from that media. This method of booting Windows PE 2.0 is fully supported by PSS as long as the following requirements are met:
  • The Windows PE image is provided in Windows Imaging (WIM) format.
  • The computer that is trying to network boot Windows PE has at least 512mb of RAM.
    Note Additional RAM may be required on the client computer depending on application requirements or the size of the WIM image that contains the Windows PE boot image, or both.
• Flat file boot
  PSS does not support this method of network booting Windows PE 2.0. By using this method, Windows PE files are not stored in a boot image. Instead, Windows PE files are stored in a flat file directory structure. Windows PE is booted directly from the flat file directory structure. If the flat file directory structure exists in a network location, Windows PE tries to load and then execute directly over the network. When this occurs, the files in the flat file directory structure are not copied to the client computer.

For more information about how to configure Windows PE 2.0 for download from a third-party PXE server, refer to the documentation that is included with the Windows OEM Preinstallation Kit (Windows OPK), or the documentation that is provided with Windows Automated Installation Kit (Windows AIK) for Windows Vista. For more information about the PXE client DHCP conversation, click the following article number to view the article in the Microsoft Knowledge Base: