DHCP options are internationally agreed-upon numbers, with an associated value, that represent specific information or settings. When a host receives option number 6 and the associated ip address, it knows that this address is the DNS server it should use. The DHCPv4 protocol defines a large set of standardized options that send useful information to hosts. It also defines options that a host can send to the DHCP server for cases where the server may be able to use that information.
When a client broadcasts a request for a configuration, the client includes its vendor client class. If this client class matches any client classes in the dhcptab database, then the options specified for that class are sent to the client, along with other configuration options.
Adding Vendor and Site Options To create additional vendor or site options, you must define: Vendor class, which is the name used to distinguish clients by vendor. It is not specified when you are defining a site option. Value type, which specifies the type of data contained in this option. Vendor codes can be between 1 and ; site codes can be between and Granularity, which specifies how many objects of this value type make up a single instance of this option.
For example, the static route symbol is a list of routes. Each route consists of two IP addresses, so the value type is defined as IP and the granularity is defined as 2. Length, which specifies the minimal granularity permissible in this option.
For example, a subnet mask can be only one IP address, so the length for the subnet mask option is 1. A value of zero means that a variable number of items, up to 16, is permitted.
DHCP servers and clients use Option 43 to exchange vendor-specific configuration information. PXE server address, which is used to obtain the boot file or other control information from the PXE server. Option 43 format Figure Option 43 format Network configuration parameters are carried in different sub-options of Option 43 as shown in Figure Sub-option type—The field value can be 0x01 ACS parameter sub-option0x02 service provider identifier sub-optionor 0x80 PXE server address sub-option. Sub-option length—Excludes the sub-option type and sub-option length fields.
Because site options are specific to your site, you can create any meaningful option. In the case of vendor options, however, you can add only options that are meaningful to a specific client vendor. Some options are predefined, but others must be created.
In that case, you may need what is a vendor option create a list of vendor options on the server that are appropriate to the specific vendor. The list typicall is provided by the client vendor.