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2-2
Figure 2-1 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 section
Obtaining IP Addresses Dynamically.
1) 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.
2) 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 section
DHCP Packet
Format
for details).
Option 82 Support on DHCP Relay Agent
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,
S4500 Series Ethernet Switches that operate as DHCP relay agents support 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 2-2 and Figure 2-3. (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.