1-5
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:
z 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.
Figure 1-5 Sub-option 1 in normal padding format
Sub-option type (0x01)
0715
Length (0x06) Circuit ID type (0x00) Length (0x04)
23 31
VLAN ID Interface number
z Sub-option 2: Padded with 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 following figure gives
its format. The value of the sub-option type is 2, and that of the remote ID type is 0.
Figure 1-6 Sub-option 2 in normal padding format
Protocols and Standards
z RFC 2131: Dynamic Host Configuration Protocol
z RFC 2132: DHCP Options and BOOTP Vendor Extensions
z RFC 1542: Clarifications and Extensions for the Bootstrap Protocol
z RFC 3046: DHCP Relay Agent Information Option
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