3Com MSR 50 Series Configuration Manual page 570

570 C 32: DHCP O
HAPTER VERVIEW
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
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 server supporting Option 82 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. Now 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.
Currently the device supports two padding formats: normal and verbose.
1 Normal padding format
The padding contents for sub-options in the normal padding format are:
sub-option 1: Padded with the VLAN ID and interface number of the interface
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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 167 Sub-option 1 in normal padding format
0
Sub-option type (0x01)
VLAN ID
sub-option 2: Padded with the MAC address of the interface that received the
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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 168 Sub-option 2 in normal padding format
0
Sub-option type (0x02)
2 Verbose padding format:
The padding contents for sub-options in the verbose padding format are:
sub-option 1: Padded with the user-specified access node identifier (ID of the
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device that adds Option 82 in DHCP messages), and type, number, PVC
identifier (used when the interface type is ATM), and VLAN ID of the interface
that received the client's request. Its format is shown in the following figure.
7 15
Length (0x06)
7 15
Length (0x08) Remote ID type (0x00)
MAC Address
23
Circuit ID type (0x00)
Interface number
23
31
Length (0x04)
31
Length (0x06)