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Using option classes

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Using option classes

Option classes offer an additional method for grouping DHCP-provided configuration details for clients within a scope. For computers running Windows Server 2003 operating systems, there are two types of option classes that can be used for submanaging options:

  • User classes, for assigning options to clients identified as sharing a common need for similar DHCP options configuration.

  • Vendor classes, for assigning vendor-specific options to clients identified as sharing a commonly defined vendor type.

For more information, see Understanding user and vendor classes.

How user classes work

User classes allow DHCP clients to differentiate themselves by specifying a User Class option. When available for client use, this option includes a user-determined class ID that can help to group clients of similar configuration needs within a scope. For example, you might support users and computers with mobile computing needs by configuring a user class at the DHCP server and setting the related class ID at the client computers.

A user class is useful when you need to keep separate options that cover the special needs of identifying client computers, such as providing a shorter lease time for portable computers that move frequently or use remote access often. In this example, you could configure the DHCP server to distribute different options that are specific to the needs of clients.

For instance, shorter leases could be assigned to mobile clients. For more information, see Change the lease time for remote access clients.

The user class feature gives you greater flexibility in configuring DHCP clients on your network, but is not required for standard DHCP use. If user-defined option classes are not configured, options are provided through the applicable server, scope, or client option settings instead. For more information, see Assigning options.

DHCP servers running Windows Server 2003 , which support recognition of user class IDs, perform the following added steps to lease clients identifying themselves according to this process:

  1. The server determines whether the user class identified by the client in its lease request is a recognized class, previously defined on the server.

    If a predefined user classes exists at the server and is configured, class-based assignment is enabled. For other user classes, you must first add and configure them at the server before they are available for use.

  2. If the user class is recognized, the server determines whether any additional DHCP options are configured for this class in the active lease context (either the scope or a client reservation) where the server is leasing the client.

  3. If the scope or reservation options are configured to provide options for the user-defined class of the client, the server returns those options to the client as part of its DHCP acknowledgment message (DHCPACK), which is sent to confirm the lease.

Additional user class examples

You can define specific user class identifiers to convey information about client software configuration, its physical location in a building, or about its user preferences. For example, an identifier can specify that DHCP clients are members of a user-defined class called "2nd floor, West," which has need for a special set of router, DNS, and WINS server settings.

You can also use the Microsoft predefined user classes for isolating configuration details specific for clients with special needs, such as BOOTP or Routing and Remote Access service. For more information, see Assign an advanced (class-based) option.

Notes

  • In addition to configuring or adding user classes at the DHCP server, at computers running Windows 2000, or Windows XP, you can set a DHCP class identifier with the ipconfig /setclassid command. For more information, see Set DHCP class ID information at a client computer.

  • When you configure user classes at the DHCP server, be certain that the class identifying data you set at both the DHCP server and member client computers is identically matching binary or ASCII data. For more information, see Create a new user or vendor class.

  • The user class option only permits one class ID to be used for identifying clients. Each client computer can only be identified as a member of a single user class at the DHCP server. You can use additional user classes and make new hybrids from your other user classes, as needed.

How vendor classes work

Vendor-defined option classes can be used by DHCP clients that are configured to optionally identify themselves by their vendor type to the DHCP server when obtaining a lease. For a client to identify its membership in a vendor class, the client provides a value in the Vendor class identifier option when it requests or selects a lease from the server.

The vendor class identifier information is a string of character data interpreted by a DHCP server. Microsoft supports vendor class identification for its DHCP clients running Windows 98 or Windows XP.

Most vendor types are derived from standard reserved hardware and operating system type abbreviation codes listed in Request for Comments (RFC) 1700. For more information, see TCP/IP RFCs.

Other vendors might choose to define their own specific vendor class IDs for either conveying vendor-specific information or handling special needs for their DHCP clients.

DHCP servers running Windows Server 2003 , which support recognition of vendor class IDs, perform the following additional steps to lease the clients that identify themselves according to this process:

  1. The server determines whether the vendor class identified by the client in its lease request is a recognized class, previously defined on the server.

    Only Microsoft vendor classes are predefined at the server. For other vendor classes, you must manually add and configure these options at your DHCP servers running Windows Server 2003 before they are available for use.

  2. If the vendor class is a recognized one, the server determines whether any additional DHCP options are configured for this class in the active lease context (either the scope or a client reservation) where the server is leasing the client.

  3. If the scope or reservation options are configured to provide options for the vendor-defined class of the client, the server returns them using the Vendor specific information option as part of its DHCP acknowledgment message (DHCPACK), which is sent to confirm the lease.

Vendor classes permit other system vendors to support custom applications for DHCP in mixed vendor environments. Vendor-specific options, when provided, are used in addition to any of the standard DHCP options assigned or required for DHCP.

For more information about DHCP, see "Dynamic Host Configuration Protocol" at the Microsoft Windows Resource Kits Web site or refer directly to RFC 2132.

Notes

  • For more information about DHCP options, see "DHCP Options" at the Microsoft Windows Resource Kits Web site.

  • In most cases, the default (or unspecified) vendor class, DHCP standard options, can provide a useful default for grouping any DHCP clients that do not specify or recognize vendor class IDs.

  • When you add new vendor classes at the DHCP server, be sure that the vendor class ID data you set at the server matches the actual vendor class ID used by clients for your vendor. For more information, contact the appropriate vendor.

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