R3721-F3210-F3171-HP High-End Firewalls Network Management Configuration Guide-6PW101

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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 involves at most 255 sub-options. At least one sub-option must be defined. 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.
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 or verbose format.
If you choose the second method, you can specify the code type for the sub-options as ASCII or HEX.
1. Normal padding format
{ Sub-option 1—Padded with 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 86 Sub-option 1 in normal padding format
{ 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 value of the
sub-option type is 2, and that of the remote ID type is 0.
Figure 87 Sub-option 2 in normal padding format
2. Verbose padding format
{ Sub-option 1—Padded with 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 Figure 88.
Figure 88 Sub-option 1 in verbose pa
dding format