Cisco Systems 32369 User Manual

Cisco IOS DHCP Server

Feature Overview

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 Servercannot satisfy 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 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, a domain name, and a lease for the IP address) to the client in a DHSCPOFFER unicast message.

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

Host A

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.

DHCPDISCOVER (broadcast)

DHCPOFFER (unicast)

DHCPREQUEST (broadcast)

DHCPACK (unicast)

Cisco IOS

DHCP server

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Cisco IOS DHCP Server 1

Benefits

Beneﬁts

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 DHCPDISCOVERbroadcast 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 negotiationof the parameters or the client hasbeen 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 assignment at 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.

Supported Platforms

This Cisco IOS DHCP Server feature is supported on the following routers and access servers:

• Catalyst 5000 family switches with an installed 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 12.0[2]T)

Release 12.0(1)T

• Cisco 2500 series

• Cisco 2600 series

• Cisco 3600 series

• Cisco 3800 series

• Cisco MC3810 series

• Cisco 4000 series

• Cisco AS5100 access server

• Cisco AS5200 universal access server

• Cisco AS5300 universal access server

• Cisco 7000 series

• Cisco 7100 series

• Cisco 7200 series

• Cisco MGX 8800 with an installed Route Processor Module

• Cisco 12000 series

• Cisco uBR900 series

• Cisco uBR7200 series

Supported Standards, MIBs, and RFCs

Standards

No new or modiﬁed standards are supported by this feature.

MIBs

No new or modiﬁed MIBs are supported by this feature. For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

RFCs

• RFC 951, Bootstrap Protocol (BOOTP)

• RFC 1542, Clariﬁcations and Extensions for the Bootstrap Protocol

• RFC 2131, Dynamic Host Conﬁguration Protocol

• RFC 2132, DHCP Options and BOOTP Vendor Extensions

Cisco IOS DHCP Server 3

Prerequisites

Before you conﬁgure the Cisco IOS DHCP Server feature, complete the following tasks:

• Identify an external File Transport Protocol (FTP), Trivial File Transfer Protocol (TFTP), or

remote copy protocol (rcp) server that you will use to store the DHCP bindings database.

• Identify the IP addresses that you will enable the DHCP server to assign, and the IP addresses

that you will exclude.

• Identify DHCP options for devices where necessary, including:

— Default boot image name — Default router(s) — Domain Name System (DNS) server(s) — NetBIOS name server

• Decide on a NetBIOS node type (b, p, m, or h).

• Decide on a DNS domain name.

DHCP Conﬁguration Task List

The DHCP server database is organized as a tree. The root of the tree is the address pool for natural networks, branches are subnetwork address pools, and leaves are manual bindings to clients. Subnetworks inherit network parameters and clients inherit subnetwork parameters. Therefore, common parameters, for example the domain name, should be conﬁgured at the highest (network or subnetwork) level of the tree.

Note Inherited parameters can be overridden. For example, if a parameter is deﬁned in both the

natural network and a subnetwork, the deﬁnition of the subnetwork is used.

Address leases are not inherited. If a lease is not speciﬁed for an IP address, by default, the DHCP server assigns a one-day lease for the address.

To conﬁgure the Cisco IOS DHCP Server feature, ﬁrst conﬁgure a database agent or disable conﬂict logging, then conﬁgure IP addresses that the DHCP server should not assign (excluded addresses) and should assign (a pool of available IP addresses) to requesting clients. These conﬁguration tasks are explained in the following sections. Each task in the following list is identiﬁed as required or optional.

• Conﬁguring a DHCP Database Agent or Disabling DHCP Conﬂict Logging (Required)

• Excluding IP Addresses (Required)

• Conﬁguring a DHCP Address Pool (Required)

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• Conﬁguring Manual Bindings (Optional)

• Conﬁguring a DHCP Server Boot File (Optional)

• Conﬁguring the Number of Ping Packets (Optional)

• Conﬁguring the Timeout Value for Ping Packets (Optional)

• Enabling the Cisco IOS DHCP Server Feature (Optional)

Configuring a DHCP Database Agent or Disabling DHCP Conflict Logging

Conﬁguring a DHCP Database Agent or Disabling DHCP Conﬂict Logging

A DHCP database agent is any host, for example, an FTP, TFTP, or RCP server that stores the DHCP bindings database. You can conﬁgure multiple DHCP database agents and you can conﬁgure the interval between database updates and transfers for each agent. To conﬁgure a database agent and database agent parameters, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp database url [timeout seconds | write-delay seconds]

If you choose not to conﬁgure a DHCP database agent, disable the recording of DHCP address conﬂicts on the DHCP server. To disable DHCP address conﬂict logging, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# no ip dhcp conﬂict logging

Conﬁgures the database agent and the interval between database updates and database transfers.

Disables DHCP address conﬂict logging.

Excluding IP Addresses

The DHCP server assumes that all IP addresses in a DHCP address pool subnet are available for assigning to DHCP clients. You must specify the IP address that the DHCP server should not assign to clients. To do so, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp excluded-address low-address

[high-address]

Speciﬁes the IP addresses that the DHCP server should not assign to DHCP clients.

Conﬁguring a DHCP Address Pool

You can conﬁgure a DHCP address pool with a name that is a symbolic string (such as “engineering”) or an integer (such as 0). Conﬁguring a DHCP address pool also places you in DHCP pool conﬁguration mode—identiﬁed by the (conﬁg-dhcp)# prompt—from which you can conﬁgure pool parameters (for example, the IP subnet number and default router list). To conﬁgure a DHCP address pool, complete the required tasks in the following sections.

Conﬁguring the DHCP Address Pool Name and Entering DHCP Pool Conﬁguration Mode

To conﬁgure the DHCP address pool name and enter DHCP pool conﬁguration mode, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp pool name Creates a name for the DHCP server address pool and places

you in DHCP pool conﬁguration mode (identiﬁed by the conﬁg-dhcp# prompt).

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DHCP Configuration Task List

Conﬁguring the DHCP Address Pool Subnet and Mask

To conﬁgure a subnet and mask for the newly created DHCP address pool, which contains the range of available IP addresses that the DHCP server may assign to clients, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# network network-number [mask | /preﬁx-length]

Note You can not conﬁgure manual bindings within the same pool that is conﬁgured with the

Speciﬁes the subnet network number and mask of the DHCP address pool.

The preﬁx length speciﬁes the number of bits that comprise the address preﬁx. The preﬁx is an alternative way of specifying the network mask of the client. The preﬁx length must be preceded by a forward slash (/).

network command. To conﬁgure manual bindings, see the “Conﬁguring Manual Bindings” section.

Conﬁguring the Domain Name for the Client

The domain name of a DHCP client places the client in the general grouping of networks that make up the domain. To conﬁgure a domain name string for the client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# domain-name

domain

Speciﬁes the domain name for the client.

Conﬁguring the Domain Name System IP Servers for the Client

DHCP clients query DNS IP servers when they need to correlate host names to IP addresses. To conﬁgure the DNS IP servers that are available to a DHCP client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# dns-server address [address2 ... address8]

Speciﬁes the IP address of a DNS server that is available to a DHCP client. One IP address is required; however, you can specify up to eight IP addresses in one command line.

Conﬁguring the NetBIOS Windows Internet Naming Service IP Servers for the Client

WindowsInternet Naming Service(WINS) is a name resolutionservice that Microsoft DHCP clients use to correlate host names to IP addresses within a general grouping of networks. Toconﬁgure the NetBIOS WINS servers that are available to a Microsoft DHCP client, use the following command in DHCP pool conﬁguration mode:

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Command Purpose

Router(conﬁg-dhcp)#netbios-name-server address [address2 ... address8]

Speciﬁes the NetBIOS WINS server that is available to a Microsoft DHCP client. One address is required; however, you can specify up to eight addresses in one command line.

Configuring the NetBIOS Node Type for the Client

Conﬁguring the NetBIOS Node Type for the Client

The NetBIOS node type for Microsoft DHCP clients can be one of four settings: broadcast, peer-to-peer,mixed, or hybrid. Toconﬁgure the NetBIOS node type for a Microsoft DHCP,use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# netbios-node-type

type

Speciﬁes the NetBIOS node type for a Microsoft DHCP client.

Conﬁguring the Default Router for the Client

Aftera DHCP client has booted, the client begins sending packetsto its default router.The IP address of the default router should be on the same subnet as the client. To conﬁgure a default router for a DHCP client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# default-router address [address2 ... address8]

Speciﬁes the IP address of the default router for a DHCP client. One IP address is required, although you can specify up to eight addresses in one command line.

Conﬁguring the Address Lease Time

By default, each IP address assigned by a DHCP server comes with a one-day lease, which is the amount of time that the address is valid. To change the lease value for an IP address, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# lease {days [hours][minutes] | inﬁnite}

Conﬁguring Manual Bindings

An address binding is a mapping between the IP address and Media Access Control (MAC) address of a client. The IP address of a client can be assigned manually by an administrator or assigned automatically from a pool by a DHCP server.

Manual bindings are IP addresses that have been manually mapped to the MAC addresses of hosts that are found in the DHCP database. Manual bindings are stored in NVRAM on the DHCP server. Manual bindings are just special address pools. There is no limit on the number of manual bindings but you can only conﬁgure one manual binding per host pool.

Automatic bindings are IP addresses that have been automatically mapped to the MACaddresses of hosts that are found in the DHCP database. Automatic bindings are stored on a remote host called a database agent. The bindings are saved as text records for easy maintenance.

To conﬁgure a manual binding, ﬁrst create a host pool, then specify the IP address and hardware address of the client or client identiﬁer. The hardware address is the MAC address. The client identiﬁer, which is required for Microsoft clients (instead of hardware addresses), is formed by concatenating the media type and the MAC address of the client. Refer to the “Address Resolution Protocol Parameters” section of RFC 1700, Assigned Numbers, for a list of media type codes.

Speciﬁes the duration of the lease. The default is a a one-day lease.

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DHCP Configuration Task List

Toconﬁgure manual bindings, use the following commands beginningin global conﬁguration mode:

Step Command Purpose

1 Router(conﬁg)# ip dhcp pool name Creates a name for the a DHCP server address

2 Router(conﬁg-dhcp)# host address [mask |

3 Router(conﬁg-dhcp)# hardware-address

4 Router(conﬁg-dhcp)# client-name name (Optional) Speciﬁes the name of the client using

/preﬁx-length]

hardware-address type

or Router(conﬁg-dhcp)# client-identiﬁer

unique-identiﬁer

pool and places you in DHCP pool conﬁguration mode—identiﬁed by the (conﬁg-dhcp)# prompt.

Speciﬁes the IP address and subnet mask of the client.

The preﬁx length speciﬁes the number of bits that comprise the address preﬁx. The preﬁx is an alternativeway of specifying the network mask of the client. The preﬁx length must be preceded by a forward slash (/).

Speciﬁes a hardware address for the client.

Speciﬁes the distinct identiﬁcation of the client in dotted-hexadecimal notation, for example, 01b7.0813.8811.66, where 01 represents the Ethernet media type.

any standard ASCII character. The client name should not include the domain name. For example, the name mars should not be speciﬁed as mars.cisco.com.

Configuring a DHCP Server Boot File

The boot ﬁle is used to store the boot image for the client. The boot image is generally the operating system the client uses to load. Tospecifya boot ﬁle for the DHCP client, use the following command in DHCP pool conﬁguration mode:

Command Purpose

Router(conﬁg-dhcp)# bootﬁle ﬁlename Speciﬁes the name of the ﬁle that is used as a boot image.

Conﬁguring the Number of Ping Packets

By default, the DHCP server pings a pool address twice before assigning the address to a requesting client. If the ping is unanswered, the DHCP server assumes (with a high probability) that the address is not in use and assigns the address to the requesting client. To change the number of ping packets the DHCP server should send to the pool address before assigning the address, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp ping packets

number

Speciﬁes the number of ping packets the DHCP server sends to a pool address before assigning the address to a requesting client. The default is two packets.

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Configuring the Timeout Value for Ping Packets

Conﬁguring the Timeout Value for Ping Packets

By default, the DHCP server waits 500 milliseconds before timing out a ping packet. To change the amount of time the server waits, use the following command in global conﬁguration mode:

Command Purpose

Router(conﬁg)# ip dhcp ping timeout

milliseconds

Speciﬁes the amount of time the DHCP server must wait before timing out a ping packet. The default 500 milliseconds.

Enabling the Cisco IOS DHCP Server Feature

By default, the Cisco IOS DHCP Server feature is enabled on your router. If the feature is disabled, use the following command in global conﬁguration mode to reenable the Cisco IOS DHCP Server feature on your router:

Command Purpose

Router(conﬁg)# service dhcp Enables the Cisco IOS DHCP Server feature on your router.

Use the no form of this command to disable the Cisco IOS DHCP Server feature.

Monitoring and Maintaining the DHCP Server

To clear DHCP server variables, use the following commands in privileged EXEC mode, as needed:

Command Purpose

Router# clear ip dhcp binding address | * Deletes an automatic address binding from the DHCP

database. Specifying address clears the automatic binding for a speciﬁc (client) IP address whereas specifying asterisk (*) clears all automatic bindings.

Router# clear ip dhcp conﬂict address | * Clears an address conﬂict from the DHCP database. Specify-

ing address clears the conﬂict for a speciﬁc IP address whereas specifying an asterisk (*) clears conﬂicts for all addresses.

Router# clear ip dhcp server statistics Resets all DHCP server counters to 0.

To enable DHCP server debugging, use the following command in privileged EXEC mode, as needed:

Command Purpose

Router# debug ip dhcp server {events | packets | linkage}

Enables debugging on the DHCP server.

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Configuration Examples

To display DHCP server information, use the following commands in EXEC mode, as needed:

Command Purpose

Router> show ip dhcp binding [address] Displays a list of all bindings created on a speciﬁc DHCP

Router> show ip dhcp conﬂict [address] Displays a list of all address conﬂicts recorded by a speciﬁc

Router# show ip dhcp database [url] Displays recent activity on the DHCP database.

Router> show ip dhcp server statistics Displays count information about server statistics and mes-

Conﬁguration Examples

This section provides the following conﬁguration examples:

• DHCP Database Agent Conﬁguration Example

• DHCP Address Pool Conﬁguration Example

server.

DHCP server.

Note Use this command in privileged EXEC mode.

sages sent and received.

• Manual Bindings Conﬁguration Example

DHCP Database Agent Conﬁguration Example

The following example stores bindings on host 172.16.4.253. The ﬁle transfer protocol is FTP.The server should wait 2 minutes (120 seconds) before writing database changes.

ip dhcp database ftp://user:password@172.16.4.253/router-dhcp write-delay 120

DHCP Address Pool Conﬁguration Example

In the following example, three DHCP address pools are created: one in network 172.16.0.0, one in subnetwork172.16.1.0,and one in subnetwork 172.16.2.0. Attributesfromnetwork172.16.0.0, such as the domain name, DNS server, NetBIOS name server, and NetBIOS node type, are inherited in subnetworks 172.16.1.0 and 172.16.2.0. In each pool, clients are granted 30-day leases and all addresses in each subnetwork, except the excluded addresses, are available to the DHCP server for assigning to clients. Table 1 lists the IP addresses for the devices in three DHCP address pools.

Table 1 DHCP Address Pool Devices Pool 0 (Network 172.16.0.0) Pool 1 (Subnetwork 172.16.1.0) Pool 2 (Subnetwork 172.16.2.0)

Device IP Address Device IP Address Device IP Address

Default routers – Default routers 172.16.1.100

172.16.1.101

DNS server 172.16.1.102 — — — —

172.16.2.102

NetBIOS name server 172.16.1.103 — — — —

172.16.2.103

NetBIOS node type h-node — — — —

Default routers 172.16.2.100

172.16.2.101

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ip dhcp database ftp://user:password@172.16.4.253/router-dhcp write-delay 120 ip dhcp excluded-address 172.16.1.100 172.16.1.103 ip dhcp excluded-address 172.16.2.100 172.16.2.103 ! ip dhcp pool 0

network 172.16.0.0 /16 domain-name cisco.com dns-server 172.16.1.102 172.16.2.102 netbios-name-server 172.16.1.103 172.16.2.103

netbios-node-type h-node ! ip dhcp pool 1

network 172.16.1.0 /24

default-router 172.16.1.100 172.16.1.101

lease 30 ! ip dhcp pool 2

network 172.16.2.0 /24

default-router 172.16.2.100 172.16.2.101

lease 30

Manual Bindings Conﬁguration Example

The following example creates a manual binding for a client named Mars.cisco.com. The MAC address of the client is 02c7.f800.0422 and the IP address of the client is 172.16.2.254.

ip dhcp pool Mars

host 172.16.2.254

hardware-address 02c7.f800.0422 ieee802

client-name Mars

Manual Bindings Configuration Example

Because attributes are inherited, the previous conﬁguration is equivalent to the following:

ip dhcp pool Mars

host 172.16.2.254 mask 255.255.255.0

hardware-address 02c7.f800.0422 ieee802

client-name Mars

default-router 172.16.2.100 172.16.2.101

domain-name cisco.com

dns-server 172.16.1.102 172.16.2.102

netbios-name-server 172.16.1.103 172.16.2.103

netbios-node-type h-node

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Command Reference

This section documents new or modiﬁed commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.

• bootﬁle

• clear ip dhcp binding

• clear ip dhcp conﬂict

• clear ip dhcp server statistics

• client-identiﬁer

• client-name

• default-router

• dns-server

• domain-name

• hardware-address

• host

• ip dhcp conﬂict logging

• ip dhcp database

• ip dhcp excluded-address

• ip dhcp ping packets

• ip dhcp ping timeout

• ip dhcp pool

• ip dhcp relay information check

• ip dhcp relay information option

• ip dhcp relay information policy

• lease

• netbios-name-server

• netbios-node-type

• network (DHCP)

• next-server

• option

• service dhcp

• show ip dhcp binding

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• show ip dhcp conﬂict

• show ip dhcp database

• show ip dhcp server statistics

Manual Bindings Configuration Example

In Cisco IOS Release 12.0(1)T or later, you can search and ﬁlter the output for show and more commands. This functionality is useful when you need to sort through large amounts of output, or if you want to exclude output that you do not need to see.

To use this functionality, enter a show or more command followed by the “pipe” character (|), one of the keywords begin, include, or exclude, and an expression that you want to search or ﬁlter on:

command | {begin | include | exclude} regular-expression Following is an example of the show atm vc command in which you want the command output to

begin with the ﬁrst line where the expression “PeakRate” appears: show atm vc | begin PeakRate For more information on the search and ﬁlter functionality, refer to the Cisco IOS Conﬁguration

Fundamentals Conﬁguration Guide.

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Command Reference

bootﬁle

Syntax Description

Defaults

Command Modes

Tospecify the name of the defaultboot image for a Dynamic Host Configuration Protocol (DHCP) client, use the bootﬁle DHCP pool conﬁguration command. Use the no form of this command to delete the boot image name.

bootﬁle ﬁlename no bootﬁle

ﬁlename Speciﬁes the name of the ﬁle that is used as a boot image.

No default behavior or values.

DHCP pool conﬁguration

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example speciﬁes xllboot as the name of the boot ﬁle:

bootfile xllboot

ip dhcp pool next-server

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clear ip dhcp binding

To delete an automatic address binding from the Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server database, use the clear ip dhcp binding privileged EXEC command.

clear ip dhcp binding address | *

Syntax Description

address The address of the binding you want to clear.

* Clears all automatic bindings.

Command Modes

Privileged EXEC

Usage Guidelines

Typically, the address denotes the client’s IP address. If the asterisk (*) character is used as the address parameter, DHCP clears all automatic bindings.

clear ip dhcp binding

Examples

Related Commands

Use the no ip dhcp pool global conﬁguration command to delete a manual binding.

The following example deletes the address binding 10.12.1.99 from a DHCP server database:

clear ip dhcp binding 10.12.1.99

show ip dhcp binding

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Command Reference

clear ip dhcp conﬂict

To clear an address conﬂict from the Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server database, use the clear ip dhcp conﬂict privileged EXEC command.

clear ip dhcp conﬂict address | *

Syntax Description

address The IP address of the host that contains the conﬂicting address you

* Clears all address conﬂicts.

Command Modes

Privileged EXEC

want to clear.

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The server detects conﬂicts using a ping session. The client detects conﬂicts using gratuitous

Address Resolution Protocol (ARP). If the asterisk (*) character is used as the address parameter, DHCP clears all conﬂicts.

The following example shows an address conﬂict of 10.12.1.99 being deleted from the DHCP server database:

clear ip dhcp conflict 10.12.1.99

show ip dhcp conﬂict

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clear ip dhcp server statistics

To reset all Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server counters, use the

clear ip dhcp server statistics privileged EXEC command.

clear ip dhcp server statistics

Syntax Description

This command has no arguments or keywords.

Command Modes

Privileged EXEC

Usage Guidelines

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The show ip dhcp serverstatistics command displays DHCP counters. All counters are cumulative.

The counters will be initialized, or set to zero, with this command.

clear ip dhcp server statistics

Examples

Related Commands

The following example resets all DHCP counters to zero:

clear ip dhcp server statistics

show ip dhcp server statistics

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Command Reference

client-identiﬁer

To specify a Microsoft Dynamic Host Configuration Protocol (DHCP) client’s unique identiﬁer (in dotted-hexadecimal notation), use the client-identiﬁer DHCP pool conﬁguration command. It is valid for manual bindings only. Use the no form of this command to delete the client identiﬁer.

client-identiﬁer unique-identiﬁer no client-identiﬁer

Syntax Description

unique-identiﬁer The distinct identiﬁcation of the client in dotted-hexadecimal notation,

Defaults

None

Command Modes

DHCP pool conﬁguration

for example, 01b7.0813.8811.66.

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. Microsoft DHCP clients require client identiﬁers instead of hardware addresses. The client identiﬁer

is formed by concatenating the media type and the MAC address. For example, the Microsoft client identiﬁer for Ethernet address b708.1388.f166 is 01b7.0813.88f1.66, where 01 represents the Ethernet media type. For a list of media type codes, refer to the “Address Resolution Protocol Parameters” section of RFC 1700, Assigned Numbers.

The following example speciﬁes the client identiﬁer for Mac address b7.0813.8811.66 in dotted-hexadecimal notation:

client-identifier 01b7.0813.8811.66

hardware-address host ip dhcp pool

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client-name

Syntax Description

Defaults

client-name

To specify the name of a Dynamic Host Configuration Protocol (DHCP) client, use the client-name DHCP pool conﬁguration command. The client name should not include the domain name. Use the no form of this command to remove the client name.

client-name name no client-name

name Speciﬁes the client’s name, using any standard ASCII character. The

client name should not include the domain name. For example, the name mars should not be speciﬁed as mars.cisco.com.

None

Command Modes

Usage Guidelines

Examples

Related Commands

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example speciﬁes a string client1 that will be the name of the client:

client-name client1

host ip dhcp pool

Cisco IOS DHCP Server 19

Command Reference

default-router

To specify the default router list for a Dynamic Host Configuration Protocol (DHCP) client, use the default-router DHCP pool conﬁguration command. Use the no form of this command to remove the default router list.

Syntax Description

address Speciﬁes the IP address of a router. One IP address is required,

Defaults

None

Command Modes

DHCP pool conﬁguration

default-router address [address2 ... address8] no default-router

although you can specify up to eight addresses in one command line.

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The IP address of the router should be on the same subnet as the client subnet. You can specify up

to eight routers in the list. Routers are listed in order of preference (address1 is the most preferred router, address2 is the next most preferred router, and so on).

The following example speciﬁes 10.12.1.99 as the IP address of the default router:

default-router 10.12.1.99

ip dhcp pool

Release 12.0(1)T

dns-server

Syntax Description

Defaults

dns-server

To specify the Domain Name System (DNS) IP servers available to a Dynamic Host Configuration Protocol (DHCP) client, use the dns-server DHCP pool conﬁguration command. Use the no form of this command to remove the DNS server list.

dns-server address [address2 ... address8] no dns-server

address Speciﬁes the IP address of a DNS server. One IP address is required,

although you can specify up to eight addresses in one command line.

If DNS IP servers are not conﬁgured for a DHCP client, the client cannot correlate host names to IP addresses.

Command Modes

Usage Guidelines

Examples

Related Commands

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. Servers are listed in order of preference (address1 is the most preferred server, address2 is the next

most preferred server, and so on).

The following example speciﬁes 10.12.1.99 as the IP address of the domain name server of the client:

dns-server 10.12.1.99

domain-name ip dhcp pool

Cisco IOS DHCP Server 21

Command Reference

domain-name

To specify the domain name for a Dynamic Host Configuration Protocol (DHCP) client, use the domain-nameDHCP pool conﬁguration command. Use the no form of this command to remove the domain name.

Syntax Description

domain Speciﬁes the client’s domain name string.

Defaults

None.

Command Modes

DHCP pool conﬁguration

domain-name domain no domain-name

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example speciﬁes cisco.com as the domain name of the client:

domain-name cisco.com

dns-server ip dhcp pool

Release 12.0(1)T

hardware-address

To specify the hardware address of a Dynamic Host Configuration Protocol (DHCP) client, use the hardware-address DHCP pool conﬁguration command. It is valid for manual bindings only. Use the no form of this command to remove the hardware address.

hardware-address hardware-address type no hardware-address

Syntax Description

hardware-address Speciﬁes the MAC address of the client’s hardware platform. type Indicates the protocol of the hardware platform. Strings and values are

hardware-address

acceptable. The string options are:

• ethernet

• ieee802

The value options are:

• 1 10Mb Ethernet

• 6 IEEE 802

If no type is speciﬁed, the default protocol is Ethernet.

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

Ethernet is the default type if none is speciﬁed.

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example speciﬁes b708.1388.f166 as the MAC address of the client:

hardware-address b708.1388.f166

client-identiﬁer host ip dhcp pool

Cisco IOS DHCP Server 23

Command Reference

host

Syntax Description

Defaults

Tospecify the IP address and network mask for a manual binding to a Dynamic Host Configuration Protocol (DHCP) client, use the host DHCP pool conﬁguration command. Use the no form of this command to remove the client’s IP address.

host address [mask | /preﬁx-length] no host

address Speciﬁes the IP address of the client. mask (Optional) Speciﬁes the network mask of the client.

/preﬁx-length (Optional) Speciﬁes the number of bits that comprise the address

preﬁx. The preﬁx is an alternative way of specifying the network mask of the client. The preﬁx length must be preceded by a forward slash (/).

None

Command Modes

Usage Guidelines

Examples

Related Commands

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. If the mask and preﬁx length are unspeciﬁed, DHCP examines its address pools. If no mask is found

in the pool database, the Class A, B, or C natural mask is used. This command is valid for manual bindings only.

There is no limit on the number of manual bindings but you can only conﬁgure one manual binding per host pool.

The following example speciﬁes 10.12.1.99 as the client’s IP address and 255.255.248.0 as the subnet mask:

host 10.12.1.99 255.255.248.0

client-identiﬁer hardware-address ip dhcp pool network (DHCP)

Release 12.0(1)T

ip dhcp conﬂict logging

To enable conﬂict logging on a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server, use the ip dhcp conﬂict logging global conﬁguration command. Use the no form of this command to disable conﬂict logging.

ip dhcp conﬂict logging no ip dhcp conﬂict logging

Syntax Description

This command has no arguments or keywords.

Defaults

Conﬂict logging is enabled.

Command Modes

Global conﬁguration

ip dhcp conflict logging

Usage Guidelines

Example

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. Cisco recommends using a DHCP server database agent to store automatic bindings. If you decide

not to use a DHCP server database agent to store automatic bindings, use the no ip dhcp conﬂict logging command to disable the recording of address conﬂicts. By default, the Cisco IOS DHCP Server records DHCP address conﬂicts in a log ﬁle.

The following example disables the recording of DHCP address conﬂicts:

no ip dhcp conflict logging

clear ip dhcp conﬂict ip dhcp database show ip dhcp conﬂicts

Cisco IOS DHCP Server 25

Command Reference

ip dhcp database

You can conﬁgure a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server to save automatic bindings on a remote host called a database agent. To conﬁgure a DHCP server database agent and database agent parameters, use the ip dhcp database global conﬁguration command. Use the no form of this command to remove the database agent.

ip dhcp database url [timeout seconds | write-delay seconds] no ip dhcp database url

Syntax Description

url Speciﬁes the remote ﬁle used to store the automatic bindings.

timeout seconds (Optional) Speciﬁes how long, in seconds, the DHCP server should

Following are the acceptable URL ﬁle formats:

• tftp://host/ﬁlename

• ftp://user:password@host/ﬁlename

• rcp://user@host/ﬁlename

wait before aborting a database transfer. Transfers that exceed the timeout period are aborted. By default, DHCP waits 300 seconds before aborting a database transfer. Inﬁnity is deﬁned as 0 seconds.

Defaults

Command Modes

Usage Guidelines

Example

write-delay seconds (Optional) Speciﬁes how soon the DHCP server should send database

updates. By default, DHCP waits 300 seconds (5 minutes) before sending database changes. The minimum delay is 60 seconds.

DHCP waits 300 seconds for both a write delay and a timeout.

Global conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. Theadministrator may conﬁgure multiple database agents. Bindings are transferred by using the File

Transfer Protocol (FTP), Trivial File Transport Protocol (TFTP), or remote copy protocol (RCP).

The following example speciﬁes the DHCP database transfer timeout value at 80 seconds:

ip dhcp database ftp://user:password@172.16.1.1/router-dhcp timeout 80

Release 12.0(1)T

The following example speciﬁes the DHCP database update delay value at 100 seconds:

ip dhcp database tftp://172.16.1.1/router-dhcp write-delay 100

Related Commands

ip dhcp database

show ip dhcp database

Cisco IOS DHCP Server 27

Command Reference

ip dhcp excluded-address

To specify IP addresses that a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server should not assign to DHCP clients, use the ip dhcp excluded-address global conﬁguration command. Use the no form of this command to remove the excluded IP addresses.

ip dhcp excluded-address low-address [high-address] no ip dhcp excluded-address low-address [high-address]

Syntax Description

low-address The excluded IP address, or ﬁrst IP address in an excluded address

high-address (Optional) The last IP address in the excluded address range.

Defaults

All IP pool addresses are assignable.

range.

Command Modes

Usage Guidelines

Example

Related Commands

Global conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The DHCP server assumes that all pool addresses may be assigned to clients. Use this command to

exclude a single IP address or a range of IP addresses.

The following example conﬁgures an excluded IP address range from 172.16.1.100 through

172.16.1.199:

ip dhcp excluded-address 172.16.1.100 172.16.1.199

ip dhcp pool network (DHCP)

Release 12.0(1)T

ip dhcp ping packets

To specify the number of packets a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server sends to a pool address as part of a ping operation, use the ip dhcp ping packets global conﬁguration command. Use the no form of this command to prevent the server from pinging pool addresses.

ip dhcp ping packets count no ip dhcp ping packets

Syntax Description

count Indicatesthe number of ping packets that are sent before assigning the

Defaults

Two packets

ip dhcp ping packets

address to a requesting client. The default value is two packets.

Command Modes

Usage Guidelines

Examples

Related Commands

Global conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The DHCP server pings a pool address before assigning the address to a requesting client. If the ping

is unanswered, the DHCP server assumes (with a high probability) that the address is not in use and assigns the address to the requesting client.

The following example speciﬁes ﬁve ping attempts by the DHCP server before ceasing any further ping attempts:

ip dhcp ping packets 5

clear ip dhcp conﬂicts ip dhcp ping timeout show ip dhcp conﬂicts

Cisco IOS DHCP Server 29

Command Reference

ip dhcp ping timeout

To specify how long a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server waits for a ping reply from an address pool, use the ip dhcp ping timeout global conﬁguration command. Use the no form of this command to restore the default number of milliseconds (500) of the timeout.

ip dhcp ping timeout milliseconds no ip dhcp ping timeout

Syntax Description

milliseconds The amount of time in milliseconds that the DHCP server waits for a

Defaults

500 milliseconds

ping reply before it stops attempting to reach a pool address for client assignment. The maximum timeout is 10000 milliseconds (10 seconds). The default timeout is 500 milliseconds.

Command Modes

Usage Guidelines

Examples

Related Commands

Global conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. This command speciﬁes how long to wait for a ping reply in milliseconds.

The following example speciﬁes that the DHCP server will wait 800 milliseconds for a ping reply before considering the ping a failure:

ip dhcp ping timeout 800

clear ip dhcp conﬂicts ip dhcp ping packets show ip dhcp conﬂicts

Release 12.0(1)T

ip dhcp pool

Syntax Description

Defaults

Command Modes

ip dhcp pool

Toconﬁgure a Dynamic Host ConfigurationProtocol (DHCP) address pool on a Cisco IOS DHCP Server and enter DHCP pool conﬁguration mode, use the ip dhcp pool global conﬁguration command. Use the no form of this command to remove the address pool.

ip dhcp pool name no ip dhcp pool name

name Can either be a symbolic string (such as “engineering”) or an integer

(such as 0).

DHCP address pools are not conﬁgured.

Global conﬁguration

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. During execution, the conﬁguration mode changes to DHCP pool conﬁguration mode, identiﬁed by

the (conﬁg-dhcp)# prompt. In this mode, the administrator can conﬁgure pool parameters, like the IP subnet number and default router list.

The following example conﬁgures pool1 as the DHCP address pool:

ip dhcp pool pool1

host ip dhcp excluded-address network (DHCP)

Cisco IOS DHCP Server 31

Command Reference

ip dhcp relay information check

To conﬁgure a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server to validate the relay agent information option in forwarded BOOTREPLY messages, use the ip dhcp relay information check global conﬁguration command. Use the no form of this command to disable an information check.

ip dhcp relay information check no ip dhcp relay information check

Syntax Description

This command has no arguments or keywords.

Defaults

The DHCP server checks relay information. Invalid messages are dropped.

Command Modes

Global conﬁguration

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. This command is used by cable modem termination systems. By default, DHCP checks relay

information. Invalid messages are dropped.

The following example conﬁgures the DHCP server to check that the relay agent information option in forwarded BOOTREPLY messages is valid:

ip dhcp relay information check

ip dhcp relay information option ip dhcp relay information policy

Release 12.0(1)T

ip dhcp relay information option

To conﬁgure a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server to insert the DHCP relay agent information option in forwarded BOOTREQUEST messages, use the ip dhcp relay information option global conﬁguration command. Use the no form of this command to disable inserting relay information to forwarded BOOTREQUEST messages.

ip dhcp relay information option no ip dhcp relay information option

Syntax Description

This command has no arguments or keywords.

Defaults

The DHCP server does not insert relay information.

Command Modes

Global conﬁguration

ip dhcp relay information option

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. This command is used by cable modem termination systems. By default, DHCP does not insert relay

information.

The following example conﬁgures a DHCP server to insert the DHCP relay agent information option in forwarded BOOTREQUEST messages:

ip dhcp relay information option

ip dhcp relay information check ip dhcp relay information policy

Cisco IOS DHCP Server 33

Command Reference

ip dhcp relay information policy

To conﬁgure a Dynamic Host Configuration Protocol (DHCP) relay agent’s information reforwarding policy (what a DHCP relay agent should do if a message already contains relay information), use the ip dhcp relay information policy global conﬁguration command. Use the no form of this command to restore the default relay information policy.

ip dhcp relay information policy {drop | keep | replace} no ip dhcp relay information policy

Syntax Description

drop Directs the DHCP relay agent to discard messages with existing relay

information if the relay information option is already present.

keep Indicates that existing information is left unchanged on the DHCP

relay agent.

replace Indicates that existing information is overwritten on the DHCP relay

agent.

Defaults

Command Modes

Usage Guidelines

Examples

The DHCP server replaces existing relay information.

Global conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. This command is used by cable modem termination systems. When a DHCP relay agent receives a

message from a another DHCP relay agent, relay information might already be present in the message. By default, the relay information from the previous relay agent is replaced.

The following examples conﬁgure a DHCP relay agent to drop messages with existing relay information, keep existing information, and replace existing information:

ip dhcp relay information policy drop

ip dhcp relay information policy keep

ip dhcp relay information policy replace

Related Commands

Release 12.0(1)T

ip dhcp relay information check ip dhcp relay information option

lease

Syntax Description

lease

To conﬁgure the duration of the lease for an IP address that is assigned from a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server to a DHCP client, use the lease DHCP pool conﬁguration command. Use the no form of this command to restore the default value.

lease {days [hours][minutes] | inﬁnite} no lease

days Speciﬁes the duration of the lease in numbers of days. hours (Optional) Speciﬁes the number of hours in the lease. A days value

must be supplied before you can conﬁgure an hours value.

minutes (Optional) Speciﬁes the number of minutes in the lease. A days value

and an hours value must be supplied before you can conﬁgure a minutes value.

inﬁnite Speciﬁes the duration of the lease is unlimited.

Defaults

Command Modes

Usage Guidelines

Examples

One day

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example shows a one-day lease:

lease 1

The following example shows a one-hour lease:

lease 0 1

The following example shows a one-minute lease:

lease 0 0 1

The following example shows an inﬁnite (unlimited) lease:

lease infinite

Related Commands

ip dhcp pool

Cisco IOS DHCP Server 35

Command Reference

netbios-name-server

To conﬁgure NetBIOS Windows Internet Naming Service (WINS) name servers that are available to Microsoft Dynamic Host ConfigurationProtocol (DHCP) clients, use the netbios-name-server DHCP pool conﬁguration command. Use the no form of this command to remove the NetBIOS name server list.

netbios-name-server address [address2...address8] no netbios-name-server

Syntax Description

address Speciﬁes the IP address of the NetBIOS WINS name server.

Defaults

None

Command Modes

DHCP pool conﬁguration

Usage Guidelines

Examples

Related Commands

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. One IP address is required, although you can specify up to eight addresses in one command line.

Servers are listed in order of preference (address1 is the most preferred server, address2 is the next most preferred server, and so on).

The following example speciﬁes the IP address of a NetBIOS name server available to the client:

netbios-name-server 10.12.1.90

dns-server domain-name ip dhcp pool netbios-node-type

Release 12.0(1)T

netbios-node-type

Toconﬁgure the NetBIOS node type for Microsoft Dynamic Host Configuration Protocol (DHCP) clients, use the netbios-node-type DHCP pool conﬁguration command. Use the no form of this command to remove the NetBIOS node type.

netbios-node-type type no netbios-node-type

Syntax Description

type Speciﬁes the NetBIOS node type. Valid types are:

Defaults

None

netbios-node-type

• b-node Broadcast

• p-node Peer-to-peer

• m-node Mixed

• h-node Hybrid (recommended)

Command Modes

Usage Guidelines

Examples

Related Commands

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The recommended type is h-node (hybrid).

The following example speciﬁes the client’s NetBIOS type as hybrid:

netbios node-type h-node

ip dhcp pool netbios-name-server

Cisco IOS DHCP Server 37

Command Reference

network (DHCP)

To conﬁgure the subnet number and mask for a Dynamic Host Configuration Protocol (DHCP) address pool on a Cisco IOS DHCP Server, use the network DHCP pool conﬁguration command. Use the no form of this command to remove the subnet number and mask.

network network-number [mask | /preﬁx-length] no network

Syntax Description

network-number The IP address of the DHCP address pool. mask (Optional) The bit combination that renders which portion of the

/preﬁx-length (Optional) Speciﬁes the number of bits that comprise the address

address of the DHCP address pool refers to the network or subnet and which part refers to the host.

preﬁx. The preﬁx is an alternative way of specifying the network mask of the client. The preﬁx length must be preceded by a forward slash (/).

Defaults

Command Modes

Usage Guidelines

Examples

None

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. This command is valid for DHCP subnetwork address pools only. If the mask or preﬁx length is not

speciﬁed, the class A, B, or C natural mask is used. The DHCP server assumes that all host addresses are available. The system administrator can exclude subsets of the address space by using the ip dhcp excluded-address command.

You can not conﬁgure manual bindings within the same pool that is conﬁgured with the network command.

The following example conﬁgures 172.16.0.0/16 as the DHCP pool’ssubnetworknumber and mask:

network 172.16.0.0 /16

Related Commands

Release 12.0(1)T

host ip dhcp excluded-address ip dhcp pool

next-server

Syntax Description

Defaults

next-server

To conﬁgure the next server in a Dynamic Host Configuration Protocol (DHCP) client’s boot process, use the next-server DHCP pool conﬁguration command. Use the no form of this command to remove the boot server list.

next-server address [address2...address8] no next-server address

address Speciﬁes the IP address of the next server in the boot process, which is

typically a Trivial File Transfer Protocol (TFTP) server. One IP address is required, although you can specify up to eight addresses in one command line.

If the next-server command is not used to conﬁgure a boot server list, the DHCP server uses inbound interface helper addresses as boot servers.

Command Modes

Usage Guidelines

Examples

Related Commands

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. You can specify up to eight servers in the list. Servers are listed in order of preference (address1 is

the most preferred server, address2 is the next most preferred server, and so on).

The following example speciﬁes 10.12.1.99 as the IP address of the next server in the boot process:

next-server 10.12.1.99

bootﬁle ip dhcp pool ip helper-address option

Cisco IOS DHCP Server 39

Command Reference

option

Syntax Description

To conﬁgure Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server options, use the option DHCP pool conﬁguration command. Use the no form of this command to remove the options.

option code [instance number] {ascii string | hex string | ip address} no option code [instance number]

code Speciﬁes the DHCP option code.

instance number (Optional) Speciﬁes a number from 0 to 255. ascii string Speciﬁes an NVT ASCII character string. ASCII character strings that

contain white space must be deliminated by quotation marks.

hex string Speciﬁes dotted-hexadecimal data. Each byte in hexidecimal character

strings is two hexidecimal digits—each byte can be separated by a period, colon, or white space.

Defaults

Command Modes

Usage Guidelines

Examples

ip address Speciﬁes an IP address.

The default instance number is 0.

DHCP pool conﬁguration

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. DHCP provides a framework for passing conﬁguration information to hosts on a TCP/IP network.

Conﬁguration parameters and other control information are carried in tagged data items that are stored in the options ﬁeld of the DHCP message. The data items themselves are also called options. The current set of DHCP options are documented in RFC 2131, Dynamic Host Conﬁguration Protocol.

The following example conﬁgures DHCP option 19, which speciﬁes whether the client should conﬁgure its IP layer for packet forwarding. A value of 0 means disable IP forwarding; a value of 1 means enable IP forwarding. IP forwarding is enabled in the following example:

option 19 hex 01

Release 12.0(1)T

The following example conﬁgures DHCP option 72, which speciﬁes the World Wide Web servers for DHCP clients. World Wide Web servers 172.16.3.252 and 172.16.3.253 are conﬁgured in the following example:

option 72 ip 172.16.3.252 172.16.3.253

Related Commands

option

ip dhcp pool

Cisco IOS DHCP Server 41

Command Reference

service dhcp

Syntax Description

Defaults

Command Modes

To enable the Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server feature on your router, use the service dhcp global conﬁguration command. Use the no form of this command to disable the Cisco IOS DHCP Server feature.

service dhcp no service dhcp

This command has no keywords or arguments.

The feature is enabled.

Global conﬁguration

Usage Guidelines

Examples

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The following example enables DHCP services on the DHCP server:

service dhcp

Release 12.0(1)T

show ip dhcp binding

To display address bindings on the Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server, use the show ip dhcp binding EXEC command.

show ip dhcp binding [address]

Syntax Description

address (Optional) Speciﬁes the IP address of the DHCP client for which

Defaults

None

Command Modes

EXEC

show ip dhcp binding

bindings will be displayed.

Usage Guidelines

Examples

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. If the address is not speciﬁed, all address bindings are shown. Otherwise, only the binding for the

speciﬁed client is displayed.

The following examples show the DHCP binding address parameters, including an IP address, an associated MAC address, a lease expiration date, and the type of address assignment that have occurred. Table 2 lists descriptions of the ﬁelds in each example.

Router> show ip dhcp binding 172.16.1.11

IP address Hardware address Lease expiration Type

172.16.1.11 00a0.9802.32de Feb 01 1998 12:00 AM Automatic

Router> show ip dhcp binding 172.16.3.254

IP address Hardware address Lease expiration Type

172.16.2.254 02c7.f800.0422 Infinite Manual

Table 2 show ip dhcp Field Descriptions Field Description

IP address The IP address of the host as recorded on the DHCP server. Hardware address The MAC address or client identiﬁer of the host as recorded

on the DHCP server. Lease expiration The lease expiration date of the IP address of the host. Type The manner in which the IP address was assigned to the host.

Cisco IOS DHCP Server 43

Command Reference

Related Commands

clear ip dhcp binding

Release 12.0(1)T

show ip dhcp conﬂict

To display address conﬂicts found by a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server when addresses are offered to the client, use the show ip dhcp conﬂict EXEC command.

show ip dhcp conﬂict [address]

Syntax Description

address (Optional) Speciﬁes the IP address of the conﬂict found.

Defaults

None

Command Modes

EXEC

show ip dhcp conflict

Usage Guidelines

Examples

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T. The server detects conﬂicts using ping. The client detects conﬂicts using gratuitous Address

Resolution Protocol (ARP). If an address conﬂict is detected, the address is removed from the pool and the address will not be assigned until an administrator resolves the conﬂict.

The following example displays the detection method and detection time for all IP addresses the DHCPserver has offered that have conﬂicts with other devices. Table 3 lists descriptions of the ﬁelds in the example.

Router> show ip dhcp conflict

IP address Detection Method Detection time

172.16.1.32 Ping Feb 16 1998 12:28 PM

172.16.1.64 Gratuitous ARP Feb 23 1998 08:12 AM

Table 3 show ip dhcp conﬂict Field Descriptions Field Description

IP Address The IP address of the host as recorded on the DHCP server. Detection Method The manner in which the IP address of the hosts were found

on the DHCP server. Can be a ping or a gratuitous ARP. Detection time The time when the conﬂict was found.

Related Commands

clear ip dhcp conﬂict ip dhcp ping packets ip dhcp ping timeout

Cisco IOS DHCP Server 45

Command Reference

show ip dhcp database

To display Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server database agent information, use the show ip dhcp database Privileged EXEC command.

show ip dhcp database [url]

Syntax Description

url (Optional) Speciﬁes the remote ﬁle used to store automatic DHCP

Defaults

If a URL is not speciﬁed, all database agent records are shown. Otherwise, only information about the speciﬁed agent is displayed.

bindings. Following are the acceptable URL ﬁle formats:

• tftp://host/ﬁlename

• ftp://user:password@host/ﬁlename

• rcp://user@host/ﬁlename

Command Modes

Usage Guidelines

Examples

Privileged EXEC

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

ThefollowingexampleshowsallDHCPserverdatabase agent information. Table 4 listsdescriptions for each ﬁeld in the example.

Router# show ip dhcp database

URL : ftp://user:password@172.16.4.253/router-dhcp Read : Dec 01 1997 12:01 AM Written : Never Status : Last read succeeded. Bindings have been loaded in RAM. Delay : 300 seconds Timeout : 300 seconds Failures : 0 Successes : 1

Table 4 show ip dhcp database Field Descriptions Field Description

URL Speciﬁes the remote ﬁle used to store automatic DHCP

bindings. Following are the acceptable URL ﬁle formats:

• tftp://host/ﬁlename

• ftp://user:password@host/ﬁlename

• rcp://user@host/ﬁlename

Release 12.0(1)T

Related Commands

show ip dhcp database

Table 4 show ip dhcp database Field Descriptions (continued) Field Description

Read The last time bindings were read from the ﬁle server. Written The last time bindings were written to the ﬁle server. Status Indication of whether the last read or write of host bindings

was successful. Delay The amount of time to wait before updating the database. Timeout The amount of time before the ﬁle transfer is aborted. Failures The number of failed ﬁle transfers. Successes The number of successful ﬁle transfers.

ip dhcp database

Cisco IOS DHCP Server 47

Command Reference

show ip dhcp server statistics

To display Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server statistics, use the

show ip dhcp server statistics EXEC command.

show ip dhcp server statistics

Syntax Description

This command has no arguments or keywords.

Defaults

None

Command Modes

EXEC

Usage Guidelines

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

Examples

The following example displays DHCP server statistics. Table 5 lists descriptions for each ﬁeld in the example.

Router> show ip dhcp server statistics

Memory usage 40392 Address pools 3 Database agents 1 Automatic bindings 190 Manual bindings 1 Expired bindings 3 Malformed messages 0

Message Received BOOTREQUEST 12 DHCPDISCOVER 200 DHCPREQUEST 178 DHCPDECLINE 0 DHCPRELEASE 0 DHCPINFORM 0

Message Sent BOOTREPLY 12 DHCPOFFER 190 DHCPACK 172 DHCPNAK 6

Release 12.0(1)T

show ip dhcp server statistics

Table 5 show ip dhcp server statistics Field Descriptions Field Description

Memory usage The number of bytes of RAM allocated by the DHCP server. Address pools The number of conﬁgured address pools in the DHCP

database. Database agents The number of database agents conﬁgured in the DHCP

database. Automatic bindings The number of IP addresses that have been automatically

mapped to the MAC addresses of hosts that are found in the

DHCP database. Manual bindings The number of IP addresses that have been manually mapped

to the MAC addresses of hosts that are found in the DHCP

database. Expired bindings The number of expired leases. Malformed messages The number of truncated or corrupted messages that were

received by the DHCP server. Message The DHCP message type that was received by the DHCP

server. Received The number of DHCP messages that were received by the

DHCP server. Sent The number of DHCP messages that were sent by the DHCP

server.

Related Commands

clear ip dhcp server statistics

Cisco IOS DHCP Server 49

Debug Commands

This section describes the following new debug command:

• debug ip dhcp server

Release 12.0(1)T

debug ip dhcp server

To enable Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server debugging, use the debug ip dhcp server privileged EXEC command. Use the no form of this command to disable DHCP server debugging.

debug ip dhcp server {events | packets | linkage} no debug ip dhcp server {events | packets | linkage}

Syntax Description

events Reports server events, like address assignments and database updates. packets Decodes DHCP receptions and transmissions. linkage Displays database linkage information (such as parent-child

Defaults

DHCP server debugging is not enabled.

debug ip dhcp server

relationships in a radix tree).

Usage Guidelines

Examples

This command ﬁrst appeared in Cisco IOS Release 12.0(1)T.

The ﬁrst example below shows a combination of DHCP server events and decoded receptions and transmissions. The second example below shows database linkage information.

Router# debug ip dhcp server events Router# debug ip dhcp server packets DHCPD:DHCPDISCOVER received from client 0b07.1134.a029 through relay 10.1.0.253. DHCPD:assigned IP address 10.1.0.3 to client 0b07.1134.a029. DHCPD:Sending DHCPOFFER to client 0b07.1134.a029 (10.1.0.3). DHCPD:unicasting BOOTREPLY for client 0b07.1134.a029 to relay 10.1.0.253. DHCPD:DHCPREQUEST received from client 0b07.1134.a029. DHCPD:Sending DHCPACK to client 0b07.1134.a029 (10.1.0.3). DHCPD:unicasting BOOTREPLY for client 0b07.1134.a029 to relay 10.1.0.253. DHCPD:checking for expired leases.

Router# debug ip dhcp server linkage DHCPD:child pool:10.1.0.0 / 255.255.0.0 (subnet10.1) DHCPD:parent pool:10.0.0.0 / 255.0.0.0 (net10) DHCPD:child pool:10.0.0.0 / 255.0.0.0 (net10) DHCPD:pool (net10) has no parent. DHCPD:child pool:10.1.0.0 / 255.255.0.0 (subnet10.1) DHCPD:parent pool:10.0.0.0 / 255.0.0.0 (net10) DHCPD:child pool:10.0.0.0 / 255.0.0.0 (net10) DHCPD:pool (net10) has no parent.

Related Commands

show ip dhcp bindings show ip dhcp database

Cisco IOS DHCP Server 51

Glossary

address binding—A mapping between the client’s IP and hardware (MAC) addresses. The client’s IP address may be conﬁgured by the administrator (manual address allocation) or assigned from a poolby the DHCP server (automaticaddress allocation). The binding also contains a lease expiration date. The default for the lease expiration date is one day.

address conﬂict—A duplication of use of the same IP address by two hosts. During address assignment, DHCP checks for conﬂicts using ping and gratuitous ARP. If a conﬂict is detected, the address is removed from the pool. The address will not be assigned until the administrator resolves the conﬂict.

address pool—The range of IP addresses assigned by the DHCP server. Address pools are indexed by subnet number.

client—Any host requesting conﬁguration parameters. database—A collection of address pools and bindings. databaseagent—Anyhost storing the DHCP bindings database, for example,a TrivialFileTransfer

Protocol (TFTP) server. DNS—Domain Name System. A system used in the Internet for translating names of network nodes

into addresses. automatic address allocation—An address assignment method where a network administrator

obtains an IP address for a client for a ﬁnite period of time or until the client explicitly relinquishes the address. Automatic allocation is particularly useful for assigning an address to a client that will be connected to the network only temporarily or for sharing a limited pool of IP addresses among a group of clients that do not need permanent IP addresses. Automatic allocation may also be a good choice for assigning an IP address to a new client being permanently connected to a network where IP addresses are sufﬁciently scarce that it is important to reclaim them when old clients are retired.

Dynamic Host Conﬁguration Protocol (DHCP)—A protocol that provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.

manual address allocation—An address assignment method that allocates an administratively assignedIP address to a host. Manual allocationallowsDHCP to be usedto eliminate the error-prone process of manually conﬁguring hosts with IP addresses.

server—Any host providing conﬁguration parameters.

Release 12.0(1)T

Cisco Systems 32369 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Cisco IOS DHCP Server

Feature Overview

Benefits

Supported Platforms

Supported Standards, MIBs, and RFCs

Standards

MIBs

RFCs

Prerequisites

Configuring a DHCP Database Agent or Disabling DHCP Conflict Logging

Excluding IP Addresses

Configuring the NetBIOS Node Type for the Client

Configuring a DHCP Server Boot File

Configuring the Timeout Value for Ping Packets

Monitoring and Maintaining the DHCP Server

Configuration Examples

Manual Bindings Configuration Example

Command Reference

Syntax Description

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

clear ip dhcp binding

Syntax Description

Command Modes

Usage Guidelines

Examples

Related Commands

Syntax Description

Command Modes

Usage Guidelines

Examples

Related Commands

clear ip dhcp server statistics

Syntax Description

Command Modes

Usage Guidelines

Examples

Related Commands

Syntax Description

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

client-name

Syntax Description

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

default-router

Syntax Description

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

dns-server

Syntax Description

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

domain-name

Syntax Description

Defaults

Command Modes

Usage Guidelines

Examples

Related Commands

hardware-address