Obtaining Ip Addresses Dynamically; Updating Ip Address Lease - H3C S3100-52P Operation Manual

Operation Manual – DLDP
H3C S3100-52P Ethernet Switch
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 again 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. After the DHCP server
receives the DHCP-DISCOVER packet, it chooses an unassigned IP address
according to the priority order of IP address assignment and then sends the IP
address and other configuration information together in a DHCP-OFFER packet to
the DHCP client. The sending mode is decided by the flag filed in the
DHCP-DISCOVER packet, refer to section 1.3 "DHCP Packet Format" for detail.
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: In this phase, the DHCP servers acknowledge the IP address.
Upon receiving the DHCP-REQUEST packet, only the selected 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 but not used by the DHCP client 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.
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Chapter 1 DHCP Overview