Obtaining Ip Addresses Dynamically; Updating Ip Address Lease - Huawei Quidway S5600 Series Operation Manual

Operation Manual - DHCP
Quidway S5600 Series Ethernet Switches-Release 1510
Dynamic assignment. The DHCP server assigns IP addresses to DHCP clients
for predetermined period of time. In this case, a DHCP client must apply for an IP
address at the expiration of the period. This policy applies to most clients.

1.2.2 Obtaining IP Addresses Dynamically

A DHCP client undergoes the following four phases to dynamically obtain an IP
address from a DHCP server:
1) Discover: In this phase, the DHCP client tries to find a DHCP server by
broadcasting a DHCP-DISCOVER packet.
2) Offer: In this phase, the DHCP server offers an IP address. Each DHCP server
that receives the DHCP-DISCOVER packet chooses an unassigned IP address
from the address pool based on the IP address assignment policy and then
broadcasts a DHCP-OFFER packet to the DHCP client. The broadcast mode is
determined by the flag in the DHCP-DISCOVER packet from the client. For
detailed information, refer to section 1.3 DHCP Packet Format.
3) Select: In this phase, the DHCP client selects an IP address. If more than one
DHCP server sends DHCP-OFFER packets to the DHCP client, the DHCP client
only accepts the DHCP-OFFER packet that first arrives, and then broadcasts a
DHCP-REQUEST packet containing the assigned IP address carried in the
DHCP-OFFER packet.
4) Acknowledge: Upon receiving the DHCP-REQUEST packet, the DHCP server
returns a DHCP-ACK packet to the DHCP client to confirm the assignment of the
IP address to the client, or returns a DHCP-NAK packet to refuse the assignment
of the IP address to the client. When the client receives the 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.
Note:
The IP addresses offered by other DHCP servers (if any) are not used by the DHCP
client and are still available to other clients.

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
Huawei Technologies Proprietary
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Chapter 1 DHCP Overview