Operation Manual – DHCPH3C S5600 Series Ethernet Switches Chapter 3 DHCP Relay Agent Configuration3-2Figure 3-1 Typical DHCP relay agent applicationIn the process of dynamic IP address assignment through the DHCP relay agent, theDHCP client and DHCP server interoperate with each other in a similar way as they dowithout the DHCP relay agent. The following sections only describe the forwardingprocess of the DHCP relay agent. For the interaction process of the packets, seesection Obtaining IP Addresses Dynamically.1) After receiving the DHCP-DISCOVER or DHCP-REQUEST broadcast from theclient, the network device providing the DHCP relay agent function unicasts themessage to the designated DHCP server based on the configuration.2) The DHCP server selects an IP address and other parameters and sends theconfiguration information to the DHCP relay agent that relays the information tothe client (the sending mode is decided by the flag filed in the client’sDHCP-DISCOVER packet, refer to section DHCP Packet Format for details).3.1.3 Option 82 Support on DHCP Relay AgentI. Introduction to Option 82Option 82 is the relay agent information option in the DHCP message. It records thelocation information of the DHCP client. With this option, the administrator can locatethe DHCP client to further implement security control and accounting. The Option 82supporting server can also use such information to define individual assignmentpolicies of IP address and other parameters for the clients.Option 82 involves at most 255 sub-options. If Option 82 is defined, at least onesub-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 of Option 82Option 82 has no unified definition in RFC 3046. Its padding information varies withvendors. Currently, S5600 Series Ethernet Switches that operate as DHCP relayagents support the extended padding format of Option 82 sub-options. By default, the
