4-2Figure 4-1 Typical network diagram for DHCP snooping applicationDHCP snooping listens the following two types of packets to retrieve the IP addresses the DHCP clientsobtain from DHCP servers and the MAC addresses of the DHCP clients:z DHCP-REQUEST packetz DHCP-ACK packetIntroduction to DHCP-Snooping Option 82Introduction to Option 82For details about Option 82, refer to Option 82 Support on DHCP Relay Agent.Padding content and frame format of Option 82There is no specification for what should be padded in Option 82. Manufacturers can pad it as required.By default, the sub-options of Option 82 for S5600 Series Ethernet Switches (enabled with DHCPsnooping) are padded as follows:z sub-option 1 (circuit ID sub-option): Padded with the port index (smaller than the physical portnumber by 1) and VLAN ID of the port that received the client’s request.z sub-option 2 (remote ID sub-option): Padded with the bridge MAC address of the DHCP snoopingdevice that received the client’s request.By default, when S5600 Series Ethernet Switches serve as DHCP snooping devices, Option 82 adoptsthe extended format. Refer to Figure 4-2 and Figure 4-3 for the extended format of the sub-options (withthe default padding contents). That is, the circuit ID or remote ID sub-option defines the type and lengthof a circuit ID or remote ID.The remote ID type field and circuit ID type field are determined by the option storage format. They areboth set to 0 in the case of HEX format and to 1 in the case of ASCII format.Figure 4-2 Extended format of the circuit ID sub-option
