DHCP Server does not provide both Server- and Scope-defined DHCP Vendor Class Options

Applies to: Microsoft Windows Server 2003 R2 Datacenter Edition (32-Bit x86)Microsoft Windows Server 2003 R2 Datacenter x64 EditionMicrosoft Windows Server 2003 R2 Datacenter x64 Edition with Service Pack 2 More

Symptoms


In a situation where DHCP Vendor Class Options are required on DHCP Server, there might be the desire to not configure all options at either the server or the scope level. In the case where there are some DHCP vendor class options that apply globally, e.g. valid for all clients, and some options relevant just for clients acting in a specific environment or area, differentiating between the server and scope level would be the easiest way the fulfill this need.

Although the DHCP Server snap-in shows the vendor-specific DHCP options configured at the server level as additional options in the scope, DHCP Server only provides the configured scope level vendor-specific DHCP options to the clients. 


Cause


This behavior is by design.

Resolution


To provide the DHCP clients with all required vendor-specific DHCP options, only configure these options either at the server or scope level, not both.

More Information


The way the DHCP Server provides vendor-specific options to the client works as follows:
  • Option lookup happens per option requested in the Parameter Request List (PRL)
  • Vendor-specific options are requested by a single option (option 43) in the client's PRL
  • Similar to lookup for all other options in the PRL, option 43 is sought from the highest level, e.g. reservation, user class specific to the lowest level server default
  • The option lookup for option 43 specifically concatenates all options found at a particular level into a single byte buffer
  • At this point the option is considered complete and the lookup returns with the concatenated options
DHCP Server does not combine the vendor-specific options configured on the server level with the configured vendor-specific options on the scope level.
This is also true for the available Microsoft DHCP vendor classes. This behavior is by design.