3Com 3C13636 Configuration Manual page 450

3Com Router 3000 Ethernet Family
Configuration Guide
DHCP Client
DHCP Client
Figure 9-1 Network diagram for a DHCP server application
In order to obtain a valid dynamic IP address, a DHCP client should exchange different
information with the server in different stages, three usual situations are:
1) First network access of DHCP client
In this case, the DHCP client should undergo four stages in order to set up a connection
with a DHCP server.
Discover stage where the DHCP client is searching for a DHCP server. In this
stage, the client broadcast a DHCP_Discover packet on the network and only
DHCP servers respond to it.
Offer stage where DHCP servers offer IP addresses to the client. Upon receipt of
the DHCP_Discover packet from the client, each DHCP server sends a
DHCP_Offer packet carrying an unassigned IP address selected from its IP
address pool to the client with other settings. To guarantee that the offered IP
address is unique, each of them does an ARP probe before that.
Selecting stage where the DHCP client picks one IP address out of all the offers. If
the client receives offers from multiple DHCP servers, it only accepts the one
reaching first. Then, it broadcasts a DHCP_Request packet containing the IP
address, which was assigned and wrapped in the DHCP_Offer packet by DHCP
server, to all the DHCP servers.
Final check stage where the DHCP client checks the offered IP address. After
receiving a DHCP_ACK packet, the client broadcasts an ARP packet destined to
the offered IP address. If no response is received after a specified period of time,
the client uses the IP address.
Except for the selected DHCP server, all other DHCP servers can allocate their
offered IP addresses to other requesting clients.
2) Non-first network access of DHCP client
If it is not the first time for the DHCP client to access the network, it should undergo the
following procedure in order to set up a connection with a DHCP server.
When a DHCP client that has a successful access record accesses the network
again, it only needs to broadcast a DHCP_Request packet containing the IP
address assigned to it the last time instead of sending a DHCP_Discover packet.
DHCP Client
DHCP Server
LAN
DHCP Client
3Com Corporation
9-3
Chapter 9 DHCP Configuration