HP 830 Series Configuration Manual page 26

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 about 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 use Option 82 to locate the DHCP client and further implement security control
and accounting. The DHCP server can use Option 82 to provide individual configuration policies for the
clients.
Option 82 can contain up to 255 sub-options and must have at least one sub-option. The relay agent
option 82 supports two sub-options: sub-option 1 (Circuit ID) and sub-option 2 (Remote ID).
Option 82 has no standard 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.
If you choose normal format or 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 13 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.
Figure 14 Sub-option 2 in normal padding format
Verbose padding format:
•
Sub-option 1—Contains the user-specified access node identifier (ID of the device that adds
Option 82 in DHCP messages), and the type, number, and VLAN ID of the interface that
received the client's request. The VLAN ID field has a fixed length of 2 bytes. All the other
padding contents of sub-option 1 are length variable. See
17
Figure 15.