1-5z Option 121: Classless route option. It specifies a list of classless static routes (the destinationaddresses in these static routes are classless) that the requesting client should add to its routingtable.z Option 33: Static route option. It specifies a list of classful static routes (the destination addressesin these static routes are classful) that a client should add to its routing table. If Option 121 exists,Option 33 is ignored.For more information about DHCP options, refer to RFC 2132.Self-Defined OptionsSome options, such as Option 43, have no unified definitions in RFC 2132. The formats of someself-defined options are introduced as follows.Vendor-specific option (Option 43)DHCP servers and clients exchange vendor-specific information through messages containing thevendor-specific option (Option 43). Upon receiving a DHCP message requesting Option 43 (in Option55), the DHCP server returns a response message containing Option 43 to assign vendor-specificinformation to the DHCP client.The DHCP client can obtain the following information through Option 43:z Auto-Configuration Server (ACS) parameters, including the ACS URL, username, and password.z Service provider identifier acquired by the customer premises equipment (CPE) from the DHCPserver and sent to the ACS for selecting vender-specific configurations and parameters.z Preboot Execution Environment (PXE) server address for further obtaining the bootfile or othercontrol information from the PXE server.1) Format of Option 43Figure 1-5 Format of Option 43For the sake of scalability, network configuration parameters are carried in different sub-options ofOption 43 so that the DHCP client can obtain more information through Option 43 as shown in Figure1-5. The sub-option fields are described as follows:z Sub-option type: Type of a sub-option. The field value can be 0x01, 0x02, or 0x80. 0x01 indicatesan ACS parameter sub-option. 0x02 indicates a service provider identifier sub-option. 0x80indicates a PXE server address sub-option.z Sub-option length: Length of a sub-option excluding the sub-option type and sub-option lengthfields.z Sub-option value: Value of a sub-option.2) Format of the sub-option value field of Option 43z As shown in Figure 1-6, the value field of the ACS parameter sub-option is filled in with variableACS URL, username, and password separated with a space (0x20) in between.
