Option 82 Supporting - H3C S7500 Series Operation Manual

Operation Manual – DHCP
H3C S7500 Series Ethernet Switches
DHCP relay agents can transparently transmit broadcast packets on DHCP clients or
servers to the DHCP servers or clients in other network segments.
In the process of dynamic IP address assignment through the DHCP relay agent, the
DHCP client and DHCP server interoperate with each other in a similar way as they
do without the DHCP relay agent. The following sections only describe the forwarding
process of the DHCP relay agent. For the interaction process of the packets, see
Obtaining IP Addresses
1) The DHCP client broadcasts the DHCP-DISCOVER packet.
2) After receiving the packets, the network device providing the DHCP relay agent
function unicasts the packet to the designated DHCP server based on the
configuration.
3) The DHCP server assigns IP addresses and transmits the configuration
information to the clients through the DHCP relay agent so that the clients can be
configured dynamically. The transmission mode depends on the flag field in the
DHCP-DISCOVER packet. For details, see section

3.1.3 Option 82 Supporting

I. Introduction to option 82 supporting
Option 82 is a relay agent information option in DHCP packets. When a request
packet from a DHCP client travels through a DHCP relay agent on its way to the
DHCP server, the DHCP relay agent adds option 82 into the request packet. Option
82 includes many sub-options, but the DHCP server supports only sub-option 1 and
sub-option 2 at present. Sub-option 1 defines agent circuit ID (that is, Circuit ID) and
sub-option 2 defines remote agent ID (that is, Remote ID).
Option 82 enables a DHCP server to track the address information of DHCP relay
agents, through which and other proper software, you can achieve the DHCP
assignment limitation and accounting functions.
II. Primary terminologies
Option: A length-variable field in DHCP packets, carrying information such as
part of the lease information and packet type. It includes at least one option and
at most 255 options.
Option 82: Also known as relay agent information option. This option is a part of
the Option field in DHCP packet. According to RFC3046, option 82 lies before
option 255 and after the other options. Option 82 includes at least one sub-option
and at most 255 sub-options. Currently, the commonly used sub-options in
option 82 are sub-option 1 and sub-option 2.
Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent circuit
ID, namely Circuit ID. It holds the port number and VLAN-ID of the switch port
connected to the DHCP client, and is usually configured on the DHCP relay
Dynamically.
3-2
Chapter 3 DHCP Relay Agent Configuration
DHCP Packet Format.