This chapter describes how to configure Dynamic Host Configuration Protocol (DHCP). For a complete description of the DHCP commands listed in this chapter, refer to the “DHCP Commands” chapter of the Cisco IOS IP and IP Routing Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or searchonline. As explained in RFC 2131, Dynamic Host Configuration Protocol, DHCP provides configuration parameters to Internet hosts. DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a DHCP server to a host and a mechanism for allocating network addresses to hosts. DHCP is built on a client/server model, where designated DHCP server hosts allocatenetwork addresses and deliver configuration parameters to dynamically configured hosts. DHCP supports three mechanisms for IP address allocation:
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Automatic allocation—DHCP assigns a permanent IP address to a client. Dynamic allocation—DHCP assigns an IP address to a client for a limited period of time (or until the client explicitly relinquishes the address). Manual allocation—The networkadministrator assigns an IP address to a client and DHCP is used simply to convey the assigned address to the client.
The format of DHCP messages is based on the format of Bootstrap Protocol (BOOTP) messages, which ensures support for BOOTP relay agent functionality and interoperability between BOOTP clients and DHCP servers. BOOTP relay agents eliminate the need for deploying a DHCP server oneach physical network segment. BOOTP is explained in RFC 951, Bootstrap Protocol (BOOTP), and RFC 1542, Clarifications and Extensions for the Bootstrap Protocol.
DHCP Server Overview
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 cannotsatisfy a DHCP request from its own database, it can forward the request to one or more secondary DHCP servers defined by the network administrator. 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 Cisco IOS DHCP server. A DHCP server offers configuration parameters (such as anIP address, a MAC address, a domain name, and a lease for the IP address) to the client in a DHCPOFFER unicast message.
Cisco IOS IP and IP Routing Configuration Guide
Configuring DHCP DHCP Server Overview
DHCP Request for an IP Address from a DHCP Server
DHCPDISCOVER (broadcast) Host A DHCPOFFER (unicast) DHCPREQUEST (broadcast)
Cisco IOS DHCP serverNote
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.
The formal request for the offered IP address (the DHCPREQUEST message) that is sent by the client is broadcast so that all other DHCPservers 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...
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