H3C S9500 Series Operation Manual page 172

Operation Manual – Network Protocol
H3C S9500 Series Routing Switches
2) Second round registration
A second round registration goes through the following steps:
After going through the first round registration successfully and logging out, when
the DHCP client logs on to the network again, it directly broadcasts a
DHCP_Request packet that contains the IP address assigned to it in the first
round registration instead of a DHCP_Discover packet. .
Upon receiving the DHCP_Request packet, if the IP address carried in the packet
is still available, the DHCP server owning the IP address answers with a
DHCP_ACK packet to enable the DHCP client to use the IP address again.
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 DHCP
client to go through steps in the first round registration.
3) Prolonging the lease time of IP address
An IP address assigned dynamically is valid for a specified lease time and will be
reclaimed by the DHCP server when the time expires. So the DHCP client must update
the 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 a
DHCP_Request packet to the DHCP server when half of the lease time elapses. The
DHCP server, in turn, answers with a DHCP_ACK packet to notify the DHCP client of
the new lease.
4.2 Configuring General DHCP
General DHCP configuration refers to those that are applicable to both DHCP server
and DHCP relay.
The following sections describe the general DHCP configuration tasks:
Enabling/Disabling DHCP Service
Configuring Processing Method of DHCP Packets
Enabling/Disabling Fake DHCP Server Detection
4.2.1 Enabling/Disabling DHCP Service
For both DHCP server and DHCP relay, you must enable the DHCP service first before
performing other DHCP configurations. The other related DHCP configurations take
effect only after the DHCP service is enabled.
Perform the following configuration in system view.
4-4
Chapter 4 DHCP Configuration