Operation Manual – DHCPH3C S7500 Series Ethernet Switches Chapter 1 DHCP Overview1-3Note:The IP addresses offered by other DHCP servers (if any) are not used by the DHCPclient and are still available to other clients.1.2.3 Updating IP Address LeaseAfter a DHCP server dynamically assigns an IP address to a DHCP client, the IPaddress keeps valid only within a specified lease time and will be reclaimed by theDHCP server when the lease expires. If the DHCP client wants to use the IP addressfor a longer time, it must update the IP lease.By default, a DHCP client updates its IP address lease automatically by unicasting aDHCP-REQUEST packet to the DHCP server when half of the lease time elapses.The DHCP server responds with a DHCP-ACK packet to notify the DHCP client of anew IP lease if the server can assign the same IP address to the client. Otherwise, theDHCP server responds with a DHCP-NAK packet to notify the DHCP client that the IPaddress will be reclaimed when the lease time expires.If the DHCP client fails to update its IP address lease when half of the lease timeelapses, it will update its IP address lease by broadcasting a DHCP-REQUESTpacket to the DHCP server again when seven-eighths of the lease time elapses. TheDHCP server performs the same operations as those described in the previoussection.1.3 DHCP Packet FormatDHCP has eight types of packets. They have the same format, but the values of somefields in the packets are different. The DHCP packet format is based on that of theBOOTP packets. The following table describes the packet format (the number in thebrackets indicates the field length, in bytes):
