1-5
z Option 121: Classless route option. It specifies a list of classless static routes (the destination
addresses in these static routes are classless) that the requesting client should add to its routing
table.
z Option 33: Static route option. It specifies a list of classful static routes (the destination addresses
in 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 Options
Some options, such as Option 43, have no unified definitions in RFC 2132. The formats of some
self-defined options are introduced as follows.
Vendor-specific option (Option 43)
DHCP servers and clients exchange vendor-specific information through messages containing the
vendor-specific option (Option 43). Upon receiving a DHCP message requesting Option 43 (in Option
55), the DHCP server returns a response message containing Option 43 to assign vendor-specific
information 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 DHCP
server 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 other
control information from the PXE server.
1) Format of Option 43
Figure 1-5 Format of Option 43
For the sake of scalability, network configuration parameters are carried in different sub-options of
Option 43 so that the DHCP client can obtain more information through Option 43 as shown in
Figure
1-5. 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 indicates
an ACS parameter sub-option. 0x02 indicates a service provider identifier sub-option. 0x80
indicates a PXE server address sub-option.
z Sub-option length: Length of a sub-option excluding the sub-option type and sub-option length
fields.
z Sub-option value: Value of a sub-option.
2) Format of the sub-option value field of Option 43
z As shown in Figure 1-6, the value field of the ACS parameter sub-option is filled in with variable
ACS URL, username, and password separated with a space (0x20) in between.
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