Overview Of Dhcp-Snooping Option 82 - H3C S3100 8C SI Operation Manual

Operation Manual – DHCP
H3C S3100 Series Ethernet Switches

2.1.4 Overview of DHCP-Snooping Option 82

I. 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).
II. 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 S3100-EI Series
Ethernet Switches (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 S3100-EI Series Ethernet Switches serve as DHCP snooping devices,
Option 82 adopts the extended format. Refer to
extended format of the sub-options (with the default 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 2-2 Extended format of the circuit ID sub-option
Chapter 2 DHCP Snooping Configuration
Figure 2-2
2-3
and Figure 2-3 for the