Operation Manual – DHCPH3C S5600 Series Ethernet Switches Chapter 1 DHCP Overview1-3only accepts the DHCP-OFFER packet that first arrives, and then broadcasts aDHCP-REQUEST packet containing the assigned IP address carried in theDHCP-OFFER packet.4) Acknowledge: In this phase, the DHCP servers acknowledge the IP address.Upon receiving the DHCP-REQUEST packet, only the selected DHCP serverreturns a DHCP-ACK packet to the DHCP client to confirm the assignment of theIP address to the client, or returns a DHCP-NAK packet to refuse the assignmentof the IP address to the client. When the client receives the DHCP-ACK packet, itbroadcasts an ARP packet with the assigned IP address as the destinationaddress to detect the assigned IP address, and uses the IP address only if it doesnot receive any response within a specified period.Note:z After the client receives the DHCP-ACK message, it will probe whether the IPaddress assigned by the server is in use by broadcasting a gratuitous ARP packet. Ifthe client receives no response within specified time, the client can use this IPaddress. Otherwise, the client sends a DHCP-DECLINE message to the server andrequests an IP address again.z If there are multiple DHCP servers, IP addresses offered by other DHCP servers areassignable 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 address fora 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. TheDHCP server responds with a DHCP-ACK packet to notify the DHCP client of a new IPlease if the server can assign the same IP address to the client. Otherwise, the DHCPserver responds with a DHCP-NAK packet to notify the DHCP client that the IP addresswill 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-REQUEST packetto the DHCP servers again when seven-eighths of the lease time elapses. The DHCPserver performs the same operations as those described above.
