Operation Manual – DHCPH3C S7500 Series Ethernet Switches Chapter 4 DHCP Snooping Configuration4-4z If the message contains option 82, the switch replaces the original option 82 in themessage with its own option 82, and then broadcasts the request message.z If the request message does not contain option 82, the DHCP-Snooping-enabledswitch inserts option 82 into the message, and then broadcast this message.3) By now, the request message contains the number of the switch port connected tothe DHCP client, the VLAN to which the port belongs to, and the MAC address ofthe DHCP-Snooping-enabled switch.4) After receiving the DHCP request message broadcast by theDHCP-Snooping-enabled device, the DHCP server records the informationcarried by the options in the message, and then sends the message containingDHCP configuration information and option 82 information to theDHCP-Snooping-enabled device.5) After receiving the returned message from the DHCP server, theDHCP-Snooping-enabled switch checks the option 82 field in the message.z If the option 82 field is inserted by the switch, the switch removes the option 82field from the message, and then forwards the message containing the DHCPconfiguration information to the DHCP client.z If the option 82 field is not inserted by the switch, the switch obtains the VLANinformation contained in this field and broadcasts the returned message in thisVLAN.Note:There are two types of request messages from a DHCP client: DHCP_DISCOVER andDHCP_REQUEST. The DHCP servers of different vendors process the requestmessages differently. Some devices process the option 82 information in theDHCP_DISCOVER message, whereas other devices process the option 82information in the DHCP_REQUEST message, so a DHCP-Snooping-enabled switchinserts option 82 into both messages.4.2.2 Enabling DHCP-Snooping Option 82I. Configuration prerequisitesBefore configuring DHCP-Snooping option 82, you need to:z Configure network parameters of the DHCP-Snooping-enabled switch.z Enable DHCP-Snooping.z Configure network parameters of the DHCP server, address pool, and addresslease time, and other address assignment policies.
