Obtaining Ip Addresses Dynamically; Updating Ip Address Lease - Huawei Quidway S3900 Series Operation Manual

Operation Manual - DHCP
Quidway S3900 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:
Discover: In this phase, the DHCP client tries to find a DHCP server by broadcasting a
DHCP-DISCOVER packet.
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.
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.
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.
Huawei Technologies Proprietary
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Chapter 1 DHCP Overview