3Com Switch 8800 Advanced Software V5 Configuration Guide

740 CHAPTER 51: DHCP RELAY AGENT CONFIGURATION
Configuring the DHCP
Relay Agent to Support
Option 82
Introduction to Option 82
Option 82 is the relay agent option in the Options 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 so that the
administrator can locate the DHCP client to further implement security control and
accounting.
Option 82 involves at most 255 sub-options. At least one sub-option must be
defined. Now Switch 8800s support two sub-options: sub-option 1 and
sub-option 2.
Option 82 has no unified definition. Its padding formats vary with vendors.
Currently, two padding formats are supported: normal and verbose.
The padding contents for sub-options in the normal padding format are:
sub-option 1: Padded with the VLAN ID and interface number related to the
interface that received the client’s request.
sub-option 2: Padded with the MAC address of the interface that received the
client’s request.
The padding contents for sub-options in the verbose padding format are:
sub-option 1: Padded with specified access node identifier, type of the interface
that received the client’s request, interface number, PVC identifier (used when the
interface type is ATM), and VLAN ID.
sub-option 2: Padded with the MAC address of the interface that received the
client’s request.
Handling strategies for Option 82 on the relay agent
If the DHCP relay agent supports Option 82, it will handle a client’s request
according to the contents defined in Option 82, if any. The handling strategies are
described in the table below.
If a reply returned by the DHCP server contains Option 82, the DHCP relay agent
will remove Option 82 before forwarding the reply to the client.
If a client’s request
message has...
Handling
strategy
Padding
format
The DHCP relay agent will...
Option 82 Drop - Drop the message.
Keep - Forward the message without changing
Option 82.
Replace Normal Forward the message after replacing the
original Option 82 with the Option 82
padded in normal format.
Verbose Forward the message after replacing the
original Option 82 with the Option 82
padded in verbose format.