Operation Manual – DHCP
H3C S3100 Series Ethernet Switches
Chapter 1 DHCP Overview
1-2
z
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 from the DHCP client, it chooses an
unassigned IP address from the address pool 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 details.
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:
z
After the client receives the DHCP-ACK message, it will probe whether the IP
address assigned by the server is in use by broadcasting a gratuitous ARP packet. If
the client receives no response within specified time, the client can use this IP
address. Otherwise, the client sends a DHCP-DECLINE message to the server and
requests an IP address again.
z
If there are multiple DHCP servers, IP addresses offered by other DHCP servers are
assignable to other clients.