Operation Manual – DHCPH3C S3100 Series Ethernet Switches Chapter 2 DHCP Snooping Configuration2-32.1.4 Overview of DHCP-Snooping Option 82I. Introduction to Option 82Option 82 is the relay agent information option in the DHCP message. It records thelocation information of the DHCP client.When a DHCP relay agent (or a device enabled with DHCP snooping) receives aclient’s request, it adds the Option 82 to the request message and sends it to the server.The administrator can locate the DHCP client to further implement security control andaccounting. The Option 82 supporting server can also use such information to defineindividual 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 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 and frame format of Option 82There is no specification for what should be padded in Option 82. Manufacturers canpad it as required. By default, the sub-options of Option 82 for S3100-EI SeriesEthernet Switches (enabled with DHCP snooping) are padded as follows:z sub-option 1 (circuit ID sub-option): Padded with the port index (smaller than thephysical port number by 1) and VLAN ID of the port that received the client’srequest.z sub-option 2 (remote ID sub-option): Padded with the bridge MAC address of theDHCP snooping device that received the client’s request.By default, when S3100-EI Series Ethernet Switches serve as DHCP snooping devices,Option 82 adopts the extended format. Refer to Figure 2-2 and Figure 2-3 for theextended format of the sub-options (with the default padding contents). That is, thecircuit ID or remote ID sub-option defines the type and length of a circuit ID or remoteID.The remote ID type field and circuit ID type field are determined by the option storageformat. They are both set to “0” in the case of HEX format and to “1” in the case of ASCIIformat.Figure 2-2 Extended format of the circuit ID sub-option
