Configuring DHCP Relay Agent 613Mechanism of option 82 supporting on DHCP relay agentThe procedure for a DHCP client to obtain an IP address from a DHCP serverthrough a DHCP relay agent is similar to that for the client to obtain an IP addressfrom a DHCP server directly. The following are the mechanism of option 82supporting on DHCP relay agent.1 A DHCP client broadcasts a request packet when it initiates.2 The DHCP relay agent on the local network receives the request packet, and thenchecks whether the packet contains option 82 and processes the packetaccordingly.3 If the packet contains option 82, the DHCP relay agent processes the packetdepending on the configured policy (that is, discards the packet, replaces theoriginal option 82 in the packet with its own, or leaves the original option 82unchanged in the packet), and forwards the packet (if not discarded) to the DHCPserver.4 If the packet does not contain option 82, the DHCP relay agent adds option 82 tothe packet and forwards the packet to the DHCP server. The forwarded packetcontains the port number of the switch to which the DHCP client is connected, theVLAN to which the DHCP client belongs, and the MAC address of the DHCP relayagent.5 Upon receiving the DHCP request packet forwarded by the DHCP relay agent, theDHCP server stores the information contained in the option field and sends apacket that contains DHCP configuration information and option 82 to the DHCPrelay agent.6 Upon receiving the packet returned from the DHCP server, the DHCP relay agentstrips option 82 from the packet and forwards the packet with the DHCPconfiguration information to the DHCP client.n Request packets sent by a DHCP client fall into two categories: DHCP-DISCOVERpackets and DHCP-REQUEST packets. As DHCP servers coming from differentmanufacturers process DHCP request packets in different ways (that is, someDHCP servers process option 82 in DHCP-DISCOVER packets, whereas the restprocess option 82 in DHCP-REQUEST packets), a DHCP relay agent adds option 82to both types of packets to accommodate to DHCP servers of differentmanufacturers.Configuring DHCPRelay AgentIntroduction to DHCPRelay AgentConfiguration TasksTable 481 DHCP relay agent configuration tasksConfiguration task Description Related sectionEnable DHCP Required “Enabling DHCP”on page 614Configure an interface to operate in DHCP relayagent modeRequired “Configuring anInterface to Operatein DHCP RelayAgent Mode” onpage 614
