3Com Switch 7757 Configuration Manual Download

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590

HAPTER

56: DHCP O

VERVIEW

■

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.

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 sends a
DHCP-OFFER packet (which carries the IP address and other configuration
information) to the DHCP client. The transmission mode depends on the flag field
in the DHCP-DISCOVER packet. For details, see “DHCP Packet Format” on page
590.

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.

The IP addresses offered by other DHCP servers (if any) are not used by the DHCP
client and are still available 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.

If the DHCP client fails to update its IP address lease when half of the lease time
elapses, it will update its IP address lease by broadcasting a DHCP-REQUEST packet
to the DHCP server again when seven-eighths of the lease time elapses. The DHCP
server performs the same operations as those described in the previous section.

DHCP Packet Format

DHCP has eight types of packets. They have the same format, but the values of
some fields in the packets are different. The DHCP packet format is based on that

3Com Switch 7757, Configuration Manual

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