H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 200)

26-5
Figure 26-4 DHCP option format
Option type Option length
0 7 15
Value (variable)
Introduction to DHCP options
The common DHCP options are as follows:
Option 6: DNS server option. It specifies the DNS server IP address to be assigned to the client.
Option 51: IP address lease option.
Option 53: DHCP message type option. It identifies the type of the DHCP message.
Option 55: Parameter request list option. It is used by a DHCP client to request specified
configuration parameters. The option contains values that correspond to the parameters requested
by the client.
Option 66: TFTP server name option. It specifies a TFTP server to be assigned to the client.
Option 67: Bootfile name option. It specifies the bootfile name to be assigned to the client.
Option 150: TFTP server IP address option. It specifies the TFTP server IP address to be assigned
to the client.
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.
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.
Introduction to Option 82
Some options, such as Option 82, have no unified definitions in RFC 2132.
Option 82 is the relay agent option in the option field of the DHCP message. It records the location
information of the DHCP client. When a DHCP relay agent or DHCP snooping device receives a client’s
request, it adds Option 82 to the request message before forwarding the message to the server.
The administrator can locate the DHCP client to further implement security control and accounting. The
Option 82 supporting server can also use such information to define individual assignment policies of IP
address and other parameters for the clients.
Option 82 involves at most 255 sub-options. At least one sub-option is defined. Currently the DHCP
relay agent supports two sub-options: sub-option 1 (Circuit ID) and sub-option 2 (Remote ID).
Option 82 has no unified definition. Its padding formats vary with vendors.
By default, the normal padding format is used on the device. You can specify the code type for the
sub-options as ASCII or HEX. The padding contents for sub-options in the normal padding format are as
follows:
Sub-option 1: Padded with the VLAN ID and interface number of the interface that received the
client’s request. The following figure gives its format. The value of the sub-option type is 1, and that
of the circuit ID type is 0.