DHCP Overview
DHCP communications consist of several messages sent between the DHCP client and
DHCP server by broadcast messages. This exchange of messages consists of the following
events:
Step 1 The client broadcasts a DHCPDISCOVER message on its local subnet
to locate available DHCP servers that can provide it an IP address.
Step 2 Any DHCP servers that receive the DHCPDISCOVER message can
respond with a DHCPOFFER message that includes an available IP
address and other configuration parameters.
Step 3 Based on the DHCPOFFER messages received, the client chooses one of
the offers. It then broadcasts a DHCPREQUEST message requesting
the offered parameters from the chosen DHCP server and implicitly
declining all of the other offers received.
Step 4 The DHCP server selected in the DHCPREQUEST message responds
with a DHCPACK message containing the configuration parameters for
the requesting client.
NOTE Because the DHCP client does not know the IP address of the DHCP server, the
initial DHCP requests are broadcast to every host on the network segment. Instead of
deploying a DHCP server on every network segment, you can configure your IOS router
to forward the DHCP requests to a single DHCP server by using the ip helper-address
command.
NOTE If the selected DHCP server cannot satisfy the DHCPREQUEST (for instance, the
requested address has already been assigned to another system), it sends a DHCPNAK
message to the DHCP client.