3com DUA1750-2BAA01 Manual De Usuario

3Com Switch 8800 Configuration Guide 

Chapter 13  DHCP Configuration

 

13-3 

z 

Selection. The DHCP client only receives the first arriving DHCP_Offer packet if 

there are DHCP_Offer packets from several DHCP servers. Then, it retrieves the 

IP address carried in the packet, and broadcasts a DHCP_Request packet to each 

DHCP server. The packet contains the IP address carried by the DHCP_Offer 

packet. 

z 

Acknowledgement. Upon receiving the DHCP_Request packet, the DHCP server 

that owns the IP address the DHCP_Request packet carries sends a DHCP_ACK 

packet to the DHCP client. And then the DHCP client binds TCP/IP protocol 

components to its network adapter. 

z 

IP addresses offered by other DHCP servers (if any) through DHCP_Offer packets 

but not selected by the DHCP client are still available for other clients. 

2) 

Second round registration 

A second round registration goes through the following steps: 

z 

After going through the first round registration successfully and logging out, when 

the DHCP client logs on to the network again, it directly broadcasts a 

DHCP_Request packet that contains the IP address assigned to it in the first 

round registration instead of a DHCP_Discover packet. . 

z 

Upon receiving the DHCP_Request packet, if the IP address carried in the packet 

is still available, the DHCP server owning the IP address answers with a 

DHCP_ACK packet to enable the DHCP client to use the IP address again. 

z 

If the IP address is not available (for example, it is occupied by other DHCP client), 

the DHCP server answers with a DHCP_NAK packet, which enables the DHCP 

client to go through steps in the first round registration. 

3) 

Prolonging the lease time of IP address 

An IP address assigned dynamically is valid for a specified lease time and will be 

reclaimed by the DHCP server when the time expires. So the DHCP client must update 

the lease to prolong the lease time if it is to use the IP address for a longer time. 

By default, a DHCP client updates its IP address lease automatically by sending a 

DHCP_Request packet to the DHCP server when half of the lease time elapses. The 

DHCP server, in turn, answers with a DHCP_ACK packet to notify the DHCP client of 

the new lease. 

General DHCP configuration refers to those that are applicable to both DHCP server 

and DHCP relay.  

The following sections describe the general DHCP configuration tasks: 

z 

z 

z