Operation Manual – DHCPH3C S7500 Series Ethernet Switches Chapter 3 DHCP Relay Agent Configuration3-2DHCP relay agents can transparently transmit broadcast packets on DHCP clients orservers to the DHCP servers or clients in other network segments.In 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 theydo without the DHCP relay agent. The following sections only describe the forwardingprocess of the DHCP relay agent. For the interaction process of the packets, seeObtaining IP Addresses Dynamically.1) The DHCP client broadcasts the DHCP-DISCOVER packet.2) After receiving the packets, the network device providing the DHCP relay agentfunction unicasts the packet to the designated DHCP server based on theconfiguration.3) The DHCP server assigns IP addresses and transmits the configurationinformation to the clients through the DHCP relay agent so that the clients can beconfigured dynamically. The transmission mode depends on the flag field in theDHCP-DISCOVER packet. For details, see section DHCP Packet Format.3.1.3 Option 82 SupportingI. Introduction to option 82 supportingOption 82 is a relay agent information option in DHCP packets. When a requestpacket from a DHCP client travels through a DHCP relay agent on its way to theDHCP server, the DHCP relay agent adds option 82 into the request packet. Option82 includes many sub-options, but the DHCP server supports only sub-option 1 andsub-option 2 at present. Sub-option 1 defines agent circuit ID (that is, Circuit ID) andsub-option 2 defines remote agent ID (that is, Remote ID).Option 82 enables a DHCP server to track the address information of DHCP relayagents, through which and other proper software, you can achieve the DHCPassignment limitation and accounting functions.II. Primary terminologiesz Option: A length-variable field in DHCP packets, carrying information such aspart of the lease information and packet type. It includes at least one option andat most 255 options.z Option 82: Also known as relay agent information option. This option is a part ofthe Option field in DHCP packet. According to RFC3046, option 82 lies beforeoption 255 and after the other options. Option 82 includes at least one sub-optionand at most 255 sub-options. Currently, the commonly used sub-options inoption 82 are sub-option 1 and sub-option 2.z Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent circuitID, namely Circuit ID. It holds the port number and VLAN-ID of the switch portconnected to the DHCP client, and is usually configured on the DHCP relay
