3Com Switch 8800 Configuration Guide Chapter 13 DHCP Configuration13-3z Selection. The DHCP client only receives the first arriving DHCP_Offer packet ifthere are DHCP_Offer packets from several DHCP servers. Then, it retrieves theIP address carried in the packet, and broadcasts a DHCP_Request packet to eachDHCP server. The packet contains the IP address carried by the DHCP_Offerpacket.z Acknowledgement. Upon receiving the DHCP_Request packet, the DHCP serverthat owns the IP address the DHCP_Request packet carries sends a DHCP_ACKpacket to the DHCP client. And then the DHCP client binds TCP/IP protocolcomponents to its network adapter.z IP addresses offered by other DHCP servers (if any) through DHCP_Offer packetsbut not selected by the DHCP client are still available for other clients.2) Second round registrationA second round registration goes through the following steps:z After going through the first round registration successfully and logging out, whenthe DHCP client logs on to the network again, it directly broadcasts aDHCP_Request packet that contains the IP address assigned to it in the firstround registration instead of a DHCP_Discover packet. .z Upon receiving the DHCP_Request packet, if the IP address carried in the packetis still available, the DHCP server owning the IP address answers with aDHCP_ACK packet to enable the DHCP client to use the IP address again.z If the IP address is not available (for example, it is occupied by other DHCP client),the DHCP server answers with a DHCP_NAK packet, which enables the DHCPclient to go through steps in the first round registration.3) Prolonging the lease time of IP addressAn IP address assigned dynamically is valid for a specified lease time and will bereclaimed by the DHCP server when the time expires. So the DHCP client must updatethe lease to prolong the lease time if it is to use the IP address for a longer time.By default, a DHCP client updates its IP address lease automatically by sending aDHCP_Request packet to the DHCP server when half of the lease time elapses. TheDHCP server, in turn, answers with a DHCP_ACK packet to notify the DHCP client ofthe new lease.13.2 Configuring General DHCPGeneral DHCP configuration refers to those that are applicable to both DHCP serverand DHCP relay.The following sections describe the general DHCP configuration tasks:z Enabling/Disabling DHCP Servicez Configuring Processing Method of DHCP Packetsz Enabling/Disabling Fake DHCP Server Detection
