Overview Of Dhcp-Snooping Option 82 - 3Com 4210 9-Port Configuration Manual

Overview of DHCP-Snooping Option 82

Introduction to Option 82
Option 82 is the relay agent information option in the DHCP message. It records the location information
of the DHCP client.
When a DHCP relay agent (or a device enabled with DHCP snooping) receives a client's request, it
adds the 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. If Option 82 is defined, at least one sub-option must be
defined. Currently the DHCP relay agent supports two sub-options: sub-option 1 (circuit ID sub-option)
and sub-option 2 (remote ID sub-option).
Padding content and frame format of Option 82
There is no specification for what should be padded in Option 82. Manufacturers can pad it as required.
By default, the sub-options of Option 82 for Switch 4210 Family (enabled with DHCP snooping) are
padded as follows:
sub-option 1 (circuit ID sub-option): Padded with the port index (smaller than the physical port
number by 1) and VLAN ID of the port that received the client's request.
sub-option 2 (remote ID sub-option): Padded with the bridge MAC address of the DHCP snooping
device that received the client's request.
By default, when Switch 4210 Family serve as DHCP snooping devices, Option 82 adopts the extended
format. Refer to
padding contents). That is, the circuit ID or remote ID sub-option defines the type and length of a circuit
ID or remote ID.
The remote ID type field and circuit ID type field are determined by the option storage format. They are
both set to "0" in the case of HEX format and to "1" in the case of ASCII format.
Figure 3-2 Extended format of the circuit ID sub-option
Figure 3-3 Extended format of the remote ID sub-option
In practice, some network devices do not support the type and length identifiers of the Circuit ID and
Remote ID sub-options. To interwork with these devices, Switch 4210 Family support Option 82 in the
standard format. Refer to
Figure 3-2 and Figure 3-3
Figure 3-4 and
for the extended format of the sub-options (with the default
Figure 3-5 for the standard format of the sub-options (with the
3-2