H3C Technologies H3C S3600 Series Switches - Manual (Page 638)

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DHCP-ACK packet, it broadcasts an ARP packet with the assigned IP address as the destination
address to detect the assigned IP address, and uses the IP address only if it does not receive any
response within a specified period.
z After the client receives the DHCP-ACK message, it will probe whether the IP address assigned by
the server is in use by broadcasting a gratuitous ARP packet. If the client receives no response
within specified time, the client can use this IP address. Otherwise, the client sends a
DHCP-DECLINE message to the server and requests an IP address again.
z If there are multiple DHCP servers, IP addresses offered by other DHCP servers are assignable to
other clients.
Updating IP Address Lease
After a DHCP server dynamically assigns an IP address to a DHCP client, the IP address keeps valid
only within a specified lease time and will be reclaimed by the DHCP server when the lease expires. If
the DHCP client wants to use the IP address for a longer time, it must update the IP lease.
By default, a DHCP client updates its IP address lease automatically by unicasting a DHCP-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 a new IP lease if the server can assign the same IP
address to the client. Otherwise, the DHCP server responds with a DHCP-NAK packet to notify the
DHCP client that the IP address will be reclaimed when the lease time expires.
If the DHCP client fails to update its IP address lease when half of the lease time elapses, it will update
its IP address lease by broadcasting a DHCP-REQUEST packet to the DHCP servers again when
seven-eighths of the lease time elapses. The DHCP server performs the same operations as those
described above.
DHCP Packet Format
DHCP has eight types of packets. They have the same format, but the values of some fields in the
packets are different. The DHCP packet format is based on that of the BOOTP packets. The following
figure describes the packet format (the number in the brackets indicates the field length, in bytes):