3Com Switch 4800G 24-Port Configuration Manual page 795

Self-Defined Options
Option 67: Bootfile name option. It specifies the bootfile name to be assigned
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to the client.
Option 150: TFTP server IP address option. It specifies the TFTP server IP address
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to be assigned to the client.
For more information about DHCP options, refer to RFC 2132.
Some options have no unified definitions in RFC 2132. The formats of some
self-defined options are introduced as follows.
Relay agent option (Option 82)
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
receives a client's request, it adds Option 82 to the request message and sends it
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 must be
defined. Now 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.
Currently the device supports two padding formats: normal and verbose.
1 Normal padding format
The padding contents for sub-options in the normal padding format are:
sub-option 1: Padded with the VLAN ID and number of the port that received
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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.
Figure 243 Sub-option 1 in normal padding format
0 7
Sub-option type (0x01)
VLAN ID
sub-option 2: Padded with the MAC address of the interface that received the
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client's request. The following figure gives its format. The value of the
sub-option type is 2, and that of the remote ID type is 0.
Figure 244 Sub-option 2 in normal padding format
0 7
Sub-option type (0x02)
15
Length (0x06) Circuit ID type (0x00)
15
Length (0x08) Remote ID type (0x00)
MAC Address
DHCP Options 795
23
Length (0x04)
Interface number
23
Length (0x06)
31
31