DHCP Configuration 285Mechanism of option 82 supporting on DHCP relayThe procedure for a DHCP client to obtain an IP address from a DHCP server througha DHCP relay is exactly the same as that for the client to obtain an IP address from aDHCP server directly. The following are the mechanism of option 82 supporting onDHCP relay.1 A DHCP client broadcasts a request packet when it initiates.2 If a DHCP server exists in the local network, it assigns an IP address to the DHCP clientdirectly. Otherwise, the DHCP relay on this network receives and processes therequest packet. The DHCP relay checks whether the packet contains option 82 andprocesses the packet accordingly.3 If the packet contains option 82, the DHCP relay processes the packet depending onthe configured policy (that is, discards the packet, replaces the original option 82 inthe packet with its own, or leaves the original option 82 unchanged in the packet),and forwards the packet (if not discarded) to the DHCP server.4 If the packet does not contain option 82, the DHCP relay adds option 82 to thepacket and forwards the packet to the DHCP server. The forwarded packet containsthe MAC address of the switch port to which the DHCP client is connected, the VLANto which the DHCP client belongs, and the MAC address of the DHCP relay.5 Upon receiving the DHCP request packet forwarded by the DHCP relay, the DHCPserver stores the information contained in the option field and sends a packet thatcontains DHCP configuration information and option 82 to the DHCP relay.6 Upon receiving the packet returned from the DHCP server, the DHCP relay stripsoption 82 from the packet and forwards the packet with the DHCP configurationinformation to the DHCP client.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, some DHCPservers process option 82 in DHCP-DISCOVER packets, whereas the rest processoption 82 in DHCP-REQUEST packets), a DHCP relay adds option 82 to both types ofpackets to accommodate to DHCP servers of different manufacturers.DHCP ClientConfigurationDHCP client configuration is described in the following section.Configuring a VLAN Interface to Obtain an IP Address Using DHCPPerform the following configuration in VLAN Interface View.By default, the Switch attempts to obtain an IP address by DHCP on VLAN 1.If you are attempting to stop the Switch from transmitting packets, you need todisable all features which may generate packets. By default these are:■ DHCP■ Resilient ARP■ Spanning TreeTable 279 Configuring a VLAN Interface to Obtain an IP Address Using DHCPOperation CommandConfigure VLAN interface to obtain IP address usingDHCPip address dhcp-allocRemove the configuration undo ip address dhcp-alloc
