H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 306)

31-2
Figure 33-1 Typical DHCP relay agent application
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 Dynamically .
2) After receiving the DHCP-DISCOVER or DHCP-REQUEST broadcast from the client, the network
device providing the DHCP relay agent function unicasts the message to the designated DHCP
server based on the configuration.
3) The DHCP server selects an IP address and other parameters and sends the configuration
information to the DHCP relay agent that relays the information to the client (the sending mode is
decided by the flag filed in the client’s DHCP-DISCOVER packet, refer to
DHCP Packet Format for
details).
DHCP Relay Agent Support for 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. With this option, 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 of Option 82
Option 82 has no unified definition in RFC 3046. Its padding information varies with vendors. Currently,
the device that operates as a DHCP relay agent supports the extended padding format of Option 82
sub-options. By default, the sub-options of Option 82 are padded as follows, as shown in
Figure 33-2
and Figure 33-3 . (The content in brackets is the fixed value of each field.)
z
Sub-option 1: 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.
z
Sub-option 2: Padded with the bridge MAC address of the DHCP relay agent device that received
the client’s request.