3Com Switch 4210 9-Port, Configuration Manual

Page: 284 / 567

282 C
HAPTER
24: DHCP O
VERVIEW
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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.
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 “DHCP
Packet Format” on page 283 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.
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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.
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If there are multiple DHCP servers, IP addresses offered by other DHCP servers
are assignable to other clients.
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 DHCP server responds with a DHCP-ACK packet to notify the DHCP client of a
new IP lease if the server can assign the same IP address to the client. Otherwise,
the DHCP server responds with a DHCP-NAK packet to notify the DHCP client that
the IP address will be reclaimed when the lease time expires.