Configuring DHCP
DHCP Server Overview
IPC-66
Cisco IOS IP Configuration Guide
Figure 14 shows the basic steps that occur when a DHCP client requests an IP address from a DHCP
Server. The client, Host A, sends a DHCPDISCOVER broadcast message to locate a CiscoIOS DHCP
Server. A DHCP Server offers configuration parameters (such as an IP address, a MAC address, a
domain name, and a lease for the IP address) to the client in a DHCPOFFER unicast message.
Figure14 DHCP Request for an IP Address from a DHCP Server
Note A DHCP client may receive offers from multiple DHCP Servers and can accept any one of the offers;
however, the client usually accepts the first offer it receives. Additionally, the offer from the DHCP
Server is not a guarantee that the IP address will be allocated to the client; however, the server usually
reserves the address until the client has had a chance to formally request the address.
The client returns a formal request for the offered IP address to the DHCP Server in a DHCPREQUEST
broadcast message. The DHCP Server confirms that the IP address has been allocated to the client by
returning a DHCPACK unicast message to the client.
Note The formal request for the offered IP address (the DHCPREQUEST message) that is sent by the client
is broadcast so that all other DHCP Servers that received the DHCPDISCOVER broadcast message
from the client can reclaim the IP addresses that they offered to the client.
If the configuration parameters sent to the client in the DHCPOFFER unicast message by the DHCP
Server are invalid (a misconfiguration error exists), the client returns a DHCPDECLINE broadcast
message to the DHCP Server.
The DHCP Server will send to the client a DHCPNAK denial broadcast message, which means the
offered configuration parameters have not been assigned, if an error has occurred during the
negotiation of the parameters or the client has been slow in responding to the DHCPOFFER message
(the DHCP Server assigned the parameters to another client) of the DHCP Server.
DHCP defines a process by which the DHCP Server knows the IP subnet in which the DHCP client
resides, and it can assign an IP address from a pool of valid IP addresses in that subnet.
The DHCP Server identifies which DHCP address pool to use to service a client request as follows:
•If the client is not directly connected (the giaddr field of the DHCPDISCOVER broadcast message
is non-zero), the DHCP Server matches the DHCPDISCOVER with a DHCP pool that has the subnet
that contains the IP address in the giaddr field.
•If the client is directly connected (the giaddr field is zero), the DHCP Server matches the
DHCPDISCOVER with DHCP pool(s) that contain the subnet(s) configured on the receiving
interface. If the interface has secondary IP addresses, the subnets associated with the secondary IP
addresses are examined for possible allocation only after the subnet associated with the primary IP
address (on the interface) is exhausted.
The Cisco IOS DHCP Server feature offers the following benefits:
•Reduced Internet access costs
Host A
DHCPACK (unicast)
DHCPREQUEST (broadcast)
DHCPOFFER (unicast)
DHCPDISCOVER (broadcast) Cisco IOS
DHCP server
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