Operation Manual – DHCP
H3C S3100 Series Ethernet Switches Chapter 1 DHCP Overview
1-3
1.2.3 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.
1.3 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):
Figure 1-2 DHCP packet format
The fields are described as follows:
z op: Operation types of DHCP packets, 1 for request packets and 2 for response
packets.
z htype, hlen: Hardware address type and length of the DHCP client.
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