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Table Of Contents
Configuring the Cisco IOS DHCP Client
Restrictions for Configuring the DHCP Client
Information About the DHCP Client
How to Configure the DHCP Client
Forcing a Release or Renewal of a DHCP Lease for a DHCP Client
DHCP Release and Renew CLI Operation
Configuration Examples for the DHCP Client
Configurable DHCP Client: Example
Cisco IOS DHCP Client: Example
ATM Primary Interface (Multipoint) Using aal5snap Encapsulation and Inverse ARP: Example
ATM Point-to-Point Subinterface Using aa15snap Encapsulation: Example
ATM Point-to-Point Subinterface Using aa15nlpid Encapsulation: Example
ATM Point-to-Point Subinterface Using aa15mux PPP Encapsulation: Example
Feature Information for the Cisco IOS DHCP Client
Configuring the Cisco IOS DHCP Client
Cisco IOS Dynamic Host Configuration Protocol (DHCP) client software provides the flexibility to include various configuration options for the DHCP client. A DHCP client is defined as an Internet host using DHCP to obtain configuration parameters such as an IP address. This module describes the concepts and tasks needed to configure the Cisco IOS DHCP client.
Module History
This module was first published on May 2, 2005, and last updated on February 27, 2006.
Finding Feature Information in This Module
Your Cisco IOS software release may not support all features. To find information about feature support and configuration, use the "Feature Information for the Cisco IOS DHCP Client" section.
Contents
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Restrictions for Configuring the DHCP Client
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Restrictions for Configuring the DHCP Client
The DHCP client can be configured on Ethernet interfaces and on PPPoA and certain ATM interfaces. The DHCP client works with ATM point-to-point interfaces and will accept any encapsulation type. For ATM multipoint interfaces, the DHCP client is only supported using the aal5snap encapsulation type combined with Inverse ARP. Inverse ARP, which builds an ATM map entry, is necessary to send unicast packets to the server (or relay agent) on the other end of the connection. Inverse ARP is only supported for the aal5snap encapsulation type.
For multipoint interfaces, an IP address can be acquired using other encapsulation types because broadcast packets are used. However, unicast packets to the other end will fail because there is no ATM map entry and thus DHCP renewals and releases also fail.
Information About the DHCP Client
To configure the DHCP client, you must understand the following concepts:
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DHCP Client Operation
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. 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 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. 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.
DHCP Client Overview
The configurable DHCP 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:
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DHCP Client on WAN Interfaces
The DHCP client on WAN interfaces allows a DHCP client to acquire an IP address over PPP over ATM (PPPoA) and certain ATM interfaces. By using DHCP rather than the IP Control Protocol (IPCP), a DHCP client can acquire other useful information such as DNS addresses, the DNS default domain name, and the default route.
The configuration of PPPoA and Classical IP and ARP over ATM already allows for a broadcast capability over the interface (using the broadcast keyword on the ATM interface). Most changes in this feature are directed at removing already existing restrictions on what types of interfaces are allowed to send out DHCP packets (previously, dialer interfaces have not been allowed). This feature also ensures that DHCP RELEASE messages are sent out the interface before a connection is allowed to be broken.
How to Configure the DHCP Client
This section contains the following tasks:
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Configuring the DHCP Client
Perform this task to configure the DHCP client.
DHCP Client Default Behavior
Cisco routers running Cisco IOS software include DHCP server and relay agent software, which are enabled by default. Your router can act as both the DHCP client and DHCP server. Use the ip address dhcp interface command to obtain IP address information for the configured interface.
Prerequisites
You must configure the ip dhcp client commands before entering the ip address dhcp command on an interface to ensure that the DHCPDISCOVER messages that are generated contain the correct option values. The ip dhcp client commands are checked only when an IP address is acquired from DHCP. If any of the ip dhcp client commands are entered after an IP address has been acquired from DHCP, it will not take effect until the next time the router acquires an IP address from DHCP. This means that the new configuration will only take effect after either the ip address dhcp command or the release dhcp and renew dhcp EXEC commands have been configured.
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DETAILED STEPS
Troubleshooting Tips
To verify the configuration, you can use the debug dhcp detail EXEC command to display the DHCP packets that were sent and received. To display the server side of the DHCP interaction, use the debug ip dhcp server packets command.
The following are troubleshooting tips for DHCP clients on WAN interfaces:
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Forcing a Release or Renewal of a DHCP Lease for a DHCP Client
Perform this task to force a release or renewal of a DHCP lease for a DHCP client.
Forcing a release or renewal of a DHCP lease for a DHCP client provides the ability to perform two independent operations from the command-line interface (CLI) in EXEC mode:
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This functionality provides the following benefits:
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DHCP Release and Renew CLI Operation
Release a DHCP Lease
The release dhcp command starts the process to immediately release a DHCP lease for the specified interface. After the lease is released, the interface address is deconfigured. The release dhcp command does not deconfigure the ip address dhcp command specified in the configuration file for the interface. During a write memory or show running configuration file action, or if the router is rebooted, the ip address dhcp command executes to acquire a DHCP address for the interface.
The original IP address for the interface must be assigned by the DHCP server. If the interface is not assigned an IP address by the DHCP server, the release dhcp command fails and displays the following error message:
Interface does not have a DHCP originated address
Renew a DHCP Lease
The renew dhcp command advances the DHCP lease timer to the next stage, at which point one of the following occurs:
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If there is no response to the RENEW request, the interface remains in the RENEW state. In this case, the lease timer will advance to the REBIND state and subsequently send a REBIND request.
If a NAK response is sent in response to the RENEW request, the interface is deconfigured.
The original IP address for the interface must be assigned by the DHCP server. If the interface is not assigned an IP address by the DHCP server, the renew dhcp command fails and displays the following error message:
Interface does not have a DHCP originated address
Prerequisites
The DHCP client must be assigned an IP address by the DHCP server.
Restrictions
If the DHCP client is not assigned an IP address by the DHCP server, the DHCP release and renew CLI commands will fail.
SUMMARY STEPS
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DETAILED STEPS
Configuration Examples for the DHCP Client
This section provides the following configuration examples:
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Configurable DHCP Client: Example
The following example shows how to configure the DHCP client with various options on Ethernet interface 1:
interface Ethernet 1ip dhcp client client-id ascii my-test1ip dhcp client class-id ascii my-class-idip dhcp client lease 0 1 0ip dhcp client hostname sanfranno ip dhcp client request tftp-server-addressip address dhcpCisco IOS DHCP Client: Example
Figure 2 shows a simple network diagram of a DHCP client on an Ethernet LAN.
Figure 2 Topology Showing DHCP Client with Ethernet Interface
On the DHCP server, the configuration is as follows:
ip dhcp pool 1network 10.1.1.0 255.255.255.0lease 1 6On the DHCP client, the configuration is as follows on interface E2:
interface Ethernet2ip address dhcpThis configuration allows the DHCP client to acquire an IP address from the DHCP server through an Ethernet interface.
ATM Primary Interface (Multipoint) Using aal5snap Encapsulation and Inverse ARP: Example
In the following example, the protocol ip 255.255.255.255 broadcast configuration is needed because there must be an ATM map entry to recognize the broadcast flag on the permanent virtual circuit (PVC). You can use any ATM map entry. The protocol ip inarp configuration is needed so the ATM Inverse ARP can operate on the interface such that the system on the other side can be pinged once an address is assigned by DHCP.
interface atm0ip address dhcppvc 1/100encapsulation aal5snapbroadcastprotocol ip 255.255.255.255 broadcastprotocol ip inarpATM Point-to-Point Subinterface Using aa15snap Encapsulation: Example
The following example shows an ATM point-to-point subinterface configuration using aa15snap encapsulation:
interface atm0.1 point-to-pointip address dhcppvc 1/100encapsulation aal5snapbroadcastATM Point-to-Point Subinterface Using aa15nlpid Encapsulation: Example
The following example shows an ATM point-to-point subinterface configuration using aa15nlpid encapsulation:
interface atm0.1 point-to-pointip address dhcppvc 1/100encapsulation aal5nlpidbroadcastATM Point-to-Point Subinterface Using aa15mux PPP Encapsulation: Example
The following example shows an ATM point-to-point subinterface configuration using aa15mux PPP encapsulation:
interface atm0.1 point-to-pointpvc 1/100encapsulation aal5mux ppp virtual-template1broadcast!interface virtual-template1ip address dhcpRelease DHCP Lease: Example
In the following example, a DHCP release is performed on an interface that was originally assigned an IP address by the DHCP server.
Router# release dhcp ethernet 3/1In the following example, an attempt is made to release the DHCP lease on an interface that was not originally assigned an IP address by the DHCP server.
Router# release dhcp ethernet 3/1Interface does not have a DHCP originated address
In the following example, the release dhcp command is executed without specifying the type and number arguments.
Router# release dhcpIncomplete command.
Renew DHCP Lease: Example
In the following example, the DHCP lease is renewed on an interface that was originally assigned an IP address by the DHCP server.
Router# renew dhcp ethernet 3/1In the following example, an attempt is made to renew the DHCP lease on an interface that was not originally assigned an IP address by the DHCP server.
Router# renew dhcp ethernet 3/1Interface does not have a DHCP originated address
In the following example, the renew dhcp command is executed without specifying the type and number arguments.
Router# renew dhcpIncomplete command.
Additional References
The following sections provide references related to the DHCP client.
Related Documents
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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MIBs
RFCs
RFC 2131
Dynamic Host Configuration Protocol
RFC 2132
DHCP Options and BOOTP Vendor Extensions
Technical Assistance
Feature Information for the Cisco IOS DHCP Client
Table 1 lists the features in this module and provides links to specific configuration information. Only features that were introduced or modified in Cisco IOS Release 12.2(1) or a later release appear in the table.
Not all commands may be available in your Cisco IOS software release. For details on when support for a specific command was introduced, see the command reference documentation.
For information on a feature in this technology that is not documented here, see the "DHCP Features Roadmap".
Cisco IOS software images are specific to a Cisco IOS software release, a feature set, and a platform. Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.
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Table 1 Feature Information for the Cisco IOS DHCP Client
Configurable DHCP Client
12.3(8)T
12.2(28)SB
The Configurable DHCP Client feature provides the flexibility to include various configuration options for the DHCP client. A DHCP client is defined as an Internet host using DHCP to obtain configuration parameters such as an IP address.
The following sections provide information about this feature:
The following commands were introduced by this feature: ip dhcp client class-id, ip dhcp client client-id, ip dhcp client hostname, ip dhcp client lease, ip dhcp client request
DHCP Release and Renew CLI in EXEC Mode
12.3(4)T
12.2(28)SB
This feature provides the ability to perform two independent operations from the CLI: (1) immediately release a DHCP lease for a DHCP client, and (2) force a DHCP renewal of a lease for a DHCP client.
The following section provides information about this feature:
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The following commands were introduced by this feature: release dhcp and renew dhcp.
DHCP Client on WAN Interfaces
12.2(8)T
12.2(28)SB
The DHCP Client on WAN Interfaces feature extends the DHCP to allow a DHCP client to acquire an IP address over PPP over ATM (PPPoA) and certain ATM interfaces.
The following section provides information about this feature:
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No commands were introduced or modified by this feature.
Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.
Copyright © 2006 Cisco Systems, Inc. All rights reserved.
This module first published May 2, 2005. Last updated February 27, 2006.
