Dynamic Host Control Protocol (DHCP) enables you to automatically assign reusable IP addresses to DHCP clients. The Cisco IOS DHCP Server feature is a full DHCP server implementation that assigns and manages IP addresses from specified address pools within the router to DHCP clients. If the Cisco IOS DHCP Server cannot satisfy a DHCP request from its owndatabase, it can forward the request to one or more secondary DHCP servers defined by the network administrator. Figure 1 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 Cisco IOS DHCP Server. A DHCP server offers configuration parameters (such as an IP address, a MAC address, adomain name, and a lease for the IP address) to the client in a DHCPOFFER unicast message.
Figure 1 DHCP Request for an IP Address from a DHCP Server
DHCPDISCOVER (broadcast) Host A DHCPOFFER (unicast) DHCPREQUEST (broadcast)
Cisco IOS 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 thefirst 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 serverconfirms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.
Cisco IOS DHCP Server 1
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 clientcan 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 configurationparameters 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.
The Cisco IOS DHCP Server feature offers the following benefits:
Reduced Internet access costs Using automatic IP address assignmentat each remote site substantially reduces Internet access costs. Static IP addresses are considerably more expensive to purchase than are automatically allocated IP addresses.
Reduced client configuration tasks and costs Because DHCP is easy to configure, it minimizes operational overhead and costs associated with device configuration tasks and eases deployment by nontechnical users.Centralized management Because the DHCP server maintains configurations for several subnets, an administrator only needs to update a single, central server when configuration parameters change.
This Cisco IOS DHCP Server feature is supported on the following routers and access servers:
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2 Release 12.0(1)T
Catalyst 5000 family switches with aninstalled Route Switch Module Catalyst 6000 family switches with an installed MultiLayer Switch Feature Card Catalyst 8500 series Cisco 800 series Cisco 1000 series Cisco 1400 series Cisco 1600 series Cisco 1700 series (support for the Cisco 1700 series was added in Cisco IOS Release 12.0T) Cisco 2500 series Cisco 2600 series Cisco 3600 series Cisco 3800 series
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