Operation Manual – DHCPH3C S9500 Series Routing Switches Chapter 3 DHCP Relay Agent Configuration3-9Caution:Do not change or delete the IP address of the interface enabled with the DHCP Relayagent; otherwise users will be unable to obtain IP addresses.3.2 DHCP Option 82 Configuration3.2.1 Introduction to Option 82 Support on DHCP Relay AgentOption 82 is the relay agent information option in the DHCP packets. With Option 82support enabled, a DHCP relay agent adds Option 82 into a received DHCP request.Option 82 can be composed of many sub-options, but Option 82 mentioned in thischapter only supports sub-option 1, sub-option 2 and sub-option 5. Sub-option 1defines the agent circuit ID (that is, Circuit ID), and sub-option 2 defines the agentremote ID (that is, Remote ID). sub-option 5 is used for link selection, which includesthe IP address that the DHCP relay agent adds.With Option 82, the information about the addresses of the DHCP clients and theDHCP relay agents can be recorded on the DHCP server. Option 82 together with othersoftware can implement the DHCP address allocation restrictions and the accountingfunction.I. Conceptsz OptionA DHCP packet has a field called options, which contains part of the lease informationand the packet type. The options field is length-variable and consists of one option atleast and 255 options at most.z Option 82Option 82 is also called relay agent information option and is part of the options field ina DHCP packet. Option 82 is defined in RFC 3046 before Option 255 and behind otheroptions. You can define a minimum of one sub-option and a maximum of 255sub-options for Option 82. At present, the commonly used sub-options in Option 82include sub-option 1, sub-option 2 and sub-option 5.z sub-option 1Sub-option 1 belongs to Option 82 and defines the Circuit ID. The option identifies theVLAN ID and Layer 2 port number of the receiving port on the relay agent. Generally,sub-option 1 and sub-option 2 are used together to identify a DHCP client.z Sub-option 2
