DHCP Client

The Cisco Dynamic Host Configuration Protocol (DHCP) Client feature allows a Cisco device to act as a host requesting configuration parameters, such as an IP address, from a DHCP server.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Restrictions for the DHCP Client

The DHCP client can be configured on Ethernet interfaces.

Information About the DHCP Client

DHCP Client Operation

The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network. A DHCP client is an Internet host using DHCP to obtain configuration parameters such as an IP address. The figure below 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 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.

Figure 1. DHCP Request for an IP Address from a DHCP Server

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. 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.

DHCP Client Overview

The configurable dynamic host configuration protocol client functionality allows a DHCP client to use a user-specified client identifier, class identifier, or suggested lease time when requesting an address from a DHCP server.

Configuration parameters and other control information are carried in tagged data items that are stored in the options field of the DHCP message. The DHCP client provides flexibility by allowing the following options to be configured for a DHCP client:

  • Option 12—This option specifies the name of the client. The name may or may not be qualified with the local domain.

  • Option 51—This option is used in a client request (DHCPDISCOVER or DHCPREQUEST) to allow the client to request a lease time for the IP address.

  • Option 55—This option allows the DHCP client to request certain options from the DHCP server. The ip dhcp client request command allows the system administrator to turn off some of the requested options, thus removing them from the request list.

  • Option 60—This option allows the user to configure the vendor class identifier string to use in the DHCP interaction.

  • Option 61—This option is used by DHCP clients to specify their unique identifier. DHCP servers use this value to index their database of address bindings. This value is expected to be unique for all clients in an administrative domain.

  • Option 120—This option is used to specify a 32-bit (binary) IPv4 address to be used by the Session Initiation Protocol (SIP) client to locate a SIP server.

  • Option 121—This option is used to configure classless static routes by specifying classless network destinations; that is, each routing table entry includes a subnet mask. Upto ten classless static routes are supported using option 121 on the DHCP client.

    Note

    If a request includes both static routes and classless static routes, the client uses only the classless static routes. If the DHCP server returns both a classless static route option and a router option, the DHCP client ignores the router option.

  • Option 125—This option is used by DHCP clients and servers to exchange vendor-specific information.

How to Configure the DHCP Client

Configuring the DHCP Client

Cisco devices running Cisco software include the Dynamic Host Configuration Protocol (DHCP) server and relay agent software, which are enabled by default. Your device can act as both the DHCP client and the DHCP server. Use the ip address dhcp command to obtain IP address information for the configured interface.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    interface type number

    4.    ip address dhcp

    5.    end

    6.    debug dhcp detail

    7.    debug ip dhcp server packets


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 interface type number


    Example:
    Device(config)# interface GigabitEthernet 0/0/1
     

    Configures an interface type and enters interface configuration mode.

     
    Step 4 ip address dhcp


    Example:
    Device(config-if)# ip address dhcp
     

    Acquires an IP address on an interface from DHCP.

     
    Step 5 end


    Example:
    Device(config-if)# end
     

    Returns to privileged EXEC mode.

     
    Step 6debug dhcp detail


    Example:
    Device# debug dhcp detail
     

    Displays the DHCP packets that were sent and received.

     
    Step 7debug ip dhcp server packets


    Example:
    Device# debug ip dhcp server packets
     

    Displays the server side of the DHCP interaction.

     

    Configuration Examples for the DHCP Client

    Example: Configuring the DHCP Client

    The figure below shows a simple network diagram of a Dynamic Host Configuration Protocol (DHCP) client on an Ethernet LAN.

    Figure 2. Topology Showing a DHCP Client with a Gigabit Ethernet Interface

    On the DHCP server, the configuration is as follows: 

    ip dhcp pool 1
 network 10.1.1.0 255.255.255.0
 lease 1 6

    On the DHCP client, the configuration is as follows on interface E2: 

    interface GigabitEthernet 0/0/0
 ip address dhcp

    This configuration allows the DHCP client to acquire an IP address from the DHCP server through Gigabit Ethernet interface 0/0/0.

    Additional References

    Related Documents

    Related Topic

    Document Title

    Cisco IOS commands

    Cisco IOS Master Command List, All Releases

    DHCP commands

    Cisco IOS IP Addressing Services Command Reference

    DHCP conceptual information

    “DHCP Overview” module in the IP Addressing: DHCP Configuration Guide

    RFCs

    RFCs

    Title

    RFC 2131

    Dynamic Host Configuration Protocol

    RFC 2132

    DHCP Options and BOOTP Vendor Extensions

    Technical Assistance

    Description

    Link

    The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

    http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

    Feature Information for the DHCP Client

    The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

    Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to . An account on Cisco.com is not required.
    Table 1 Feature Information for the DHCP Client

    Feature Name

    Releases

    Feature Information

    DHCP Client

    12.1(3)T

    12.1(14)EA1

    12.2(2)T

    12.2(27)SBA

    Cisco IOS XE Release 2.3

    The DHCP client is defined as an Internet host using DHCP to obtain configuration parameters such as an IP address.