HP 6125G Configuration Manual page 38

Figure 20 PXE server address sub-option value field
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 or DHCP snooping device 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 can contain up to 255 sub-options and must have one sub-option at least. Option 82 supports
two sub-options: sub-option 1 (Circuit ID) and sub-option 2 (Remote ID). DHCP snooping device supports
three sub-options: sub-option 1 (Circuit ID), sub-option 2 (Remote ID), and sub-option 9.
Option 82 has no unified definition. Its padding formats vary with vendors.
There are two methods for configuring Option 82:
User-defined method—Manually specify the content of Option 82.
•
Non-user-defined method—Pad Option 82 in the default normal format, verbose format, private
•
format, or standard format.
NOTE:
Only the DHCP snooping device supports sub-option 9, padded in either private or standard format.
If you choose normal format and verbose format, you can specify the code type for the sub-options as
ASCII or HEX.
Normal padding format
•
Sub-option 1—Contains the VLAN ID and interface number of the interface that received the
client's request. The value of the sub-option type is 1, and that of the circuit ID type is 0.
Figure 21 Sub-option 1 in normal padding format
Sub-option 2—Contains the MAC address of the DHCP relay agent interface or the MAC
address of the DHCP snooping device that received the client's request. The value of the
sub-option type is 2, and that of the remote ID type is 0.
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