ip arp gratuitous through ip dhcp ping packets

ip arp gratuitous

To enable the gratuitous Address Resolution Protocol (ARP) control on the router, use the ip arp gratuitous command in global configuration mode. To disable the ARP control, use the no form of this command.

ip arp gratuitous { local | none | ignore }

no ip arp gratuitous

Syntax Description

local

Accepts only local (same subnet) gratuitous arps.

none

Rejects gratuitous arp control.

ignore

Stops processing all received gratuitous arps.

Command Default

Gratuitous ARP control is enabled.

Gratuitous ARP control is disabled by default on the Cisco NCS 4200 Series routers.

Command Modes

Global configuration (config)

Command History

Release

Modification

15.0(1)M

This command was introduced in a release earlier than Cisco IOS Release 15.0(1)M.

12.2(33)SRC

This command was integrated into a release earlier than Cisco IOS Release 12.2(33)SRC.

12.2(33)SXI

This command was integrated into a release earlier than Cisco IOS Release 12.2(33)SXI.

Cisco IOS XE Release 2.1

This command was integrated into Cisco IOS XE Release 2.1.

Cisco IOS XE Dublin17.10.x

The ignore keyword is added.

Examples

The following example shows how to enable the gratuitous ARP control to accept only local (same subnet) gratuitous arp control: 


Router> enable
Router# configure terminal
Router(config)# ip arp gratuitous local

Related Commands

Command

Description

show arp

Display the entries in the ARP table.

ip arp incomplete

To rectify the Address Resolution Protocol (ARP) retry parameters, use the ip arp incomplete command in global configuration mode. To disable the correction of the retry parameters, use the no form of this command.

ip arp incomplete {entries number-of-IP-addresses | retry number-of-times}

no ip arp incomplete {entries | retry}

Syntax Description

entries

Limits the number of unresolved addresses.

number-of-IP-addresses

Number of IP addresses to resolve. The range is from 1 to 2147483647.

retry

Limits the number of attempts to resolve an address.

number-of-times

Number of times an ARP Request is sent. The range is from 1 to 2147483647.

Command Modes

Global configuration (config)

Command History

Release

Modification

15.0(1)M

This command was introduced in a release earlier than Cisco IOS Release 15.0(1)M.

Usage Guidelines

An incomplete ARP entry is learned through an ARP request but has not yet been completed with the MAC address of the external host.

Examples

The following example shows how to limit the number of unresolved addresses: 


Router> enable
Router# configure terminal
Router(config)# ip arp incomplete entries 100

Related Commands

Command

Description

show arp

Display the entries in the Address Resolution Protocol (ARP) table.

ip arp inspection filter vlan

To permit ARPs from hosts that are configured for static IP when DAI is enabled and to define an ARP access list and apply it to a VLAN, use the ip arp inspection filter vlan command in global configuration mode. To disable this application, use the no form of this command.

ip arp inspection filter arp-acl-name vlan vlan-range [static]

no ip arp inspection filter arp-acl-name vlan vlan-range [static]

Syntax Description

arp-acl-name

Access control list name.

vlan-range

VLAN number or range; valid values are from 1 to 4094.

static

(Optional) Treats implicit denies in the ARP ACL as explicit denies and drops packets that do not match any previous clauses in the ACL.

Command Default

No defined ARP ACLs are applied to any VLAN.

Command Modes

Global configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Usage Guidelines

For vlan-range , you can specify the VLAN to which the switches and hosts belong. You can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma.

When an ARP access control list is applied to a VLAN for dynamic ARP inspection, the ARP packets containing only the IP-to-Ethernet MAC bindings are compared against the ACLs. All other packet types are bridged in the incoming VLAN without validation.

This command specifies that the incoming ARP packets are compared against the ARP access control list, and the packets are permitted only if the access control list permits them.

If the access control lists deny the packets because of explicit denies, the packets are dropped. If the packets are denied because of an implicit deny, they are then matched against the list of DHCP bindings if the ACL is not applied statically.

If you do not specify the static keyword, it means that there is no explicit deny in the ACL that denies the packet, and DHCP bindings determine whether a packet is permitted or denied if the packet does not match any clauses in the ACL.

Examples

This example shows how to apply the ARP ACL static-hosts to VLAN 1 for DAI: 


Router(config)# ip arp inspection filter static-hosts vlan 1

Related Commands

Command

Description

arp access-list

Configures an ARP ACL for ARP inspection and QoS filtering and enters the ARP ACL configuration submode.

show ip arp inspection

Displays the status of DAI for a specific range of VLANs.

ip arp inspection limit (interface configuration)

To limit the rate of incoming ARP requests and responses on an interface and prevent DAI from consuming all of the system’s resources in the event of a DoS attack, use the ip arp inspection limit command in interface configuration mode. To return to the default settings, use the no form of this command.

ip arp inspection limit rate pps [burst interval seconds | none]

no ip arp inspection limit

Syntax Description

rate pps

Specifies the upper limit on the number of incoming packets processed per second; valid values are from 1 to 2048 pps.

burst interval seconds

(Optional) Specifies the consecutive interval in seconds over which the interface is monitored for the high rate of the ARP packets; valid values are from 1 to 15 seconds.

none

(Optional) Specifies that there is no upper limit on the rate of the incoming ARP packets that can be processed.

Command Default

The default settings are as follows:

  • The rate pps is set to 15 packets per second on the untrusted interfaces, assuming that the network is a switched network with a host connecting to as many as 15 new hosts per second.

  • The rate is unlimited on all the trusted interfaces.

  • The burst interval seconds is set to 1 second.

Command Modes

Interface configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Usage Guidelines

You should configure the trunk ports with higher rates to reflect their aggregation. When the rate of the incoming packets exceeds the user-configured rate, the interface is placed into an error-disabled state. You can use the error-disable timeout feature to remove the port from the error-disabled state. The rate applies to both the trusted and nontrusted interfaces. Configure appropriate rates on trunks to handle the packets across multiple DAI-enabled VLANs, or use the none keyword to make the rate unlimited.

The rate of the incoming ARP packets on the channel ports is equal to the sum of the incoming rate of packets from all the channel members. Configure the rate limit for the channel ports only after examining the rate of the incoming ARP packets on the channel members.

After a switch receives more than the configured rate of packets every second consecutively over a period of burst seconds, the interface is placed into an error-disabled state.

Examples

This example shows how to limit the rate of the incoming ARP requests to 25 packets per second: 


Router# configur terminal
Router(config)# interface fa6/3
Router(config-if)# ip arp inspection limit rate 25

This example shows how to limit the rate of the incoming ARP requests to 20 packets per second and to set the interface monitoring interval to 5 consecutive seconds: 


Router# configure terminal
Router(config)# interface fa6/1
Router(config-if)# ip arp inspection limit rate 20 burst interval 5

Related Commands

Command

Description

show ip arp inspection

Displays the status of DAI for a specific range of VLANs.

ip arp inspection log-buffer

To configure the parameters that are associated with the logging buffer, use the ip arp inspection log-buffer command in global configuration mode. To disable the parameters, use the no form of this command.

ip arp inspection log-buffer {entries number | logs number interval seconds}

no ip arp inspection log-buffer {entries | logs}

Syntax Description

entries number

Specifies the number of entries from the logging buffer; valid values are from 0 to 1024.

logs number

Specifies the number of entries to be logged in an interval; valid values are from 0 to 1024.

interval seconds

Specifies the logging rate; valid values are from 0 to 86400 (1 day).

Command Default

The default settings are as follows:

  • When dynamic ARP inspection is enabled, denied, or dropped, the ARP packets are logged.

  • The entries number is 32.

  • The logs number is 5 per second.

  • The interval seconds is 1 second.

Command Modes

Global configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Usage Guidelines

A 0 value for the logs number indicates that the entries should not be logged out of this buffer.

A 0 value for the interval seconds keyword and argument indicates an immediate log.

You cannot enter a 0 for both the logs number and the interval seconds keywords and arguments.

The first dropped packet of a given flow is logged immediately. The subsequent packets for the same flow are registered but are not logged immediately. Registration for these packets occurs in a log buffer that is shared by all the VLANs. Entries from this buffer are logged on a rate-controlled basis.

Examples

This example shows how to configure the logging buffer to hold up to 45 entries: 


Router# configure terminal
Router(config)# ip arp inspection log-buffer entries 45

This example shows how to configure the logging rate for 10 logs per 3 seconds: 


Router(config)# ip arp inspection log-buffer logs 10 interval 3

Related Commands

Command

Description

arp access-list

Configures an ARP ACL for ARP inspection and QoS filtering and enters the ARP ACL configuration submode.

clear ip arp inspection log

Clears the status of the log buffer.

show ip arp inspection log

Shows the status of the log buffer.

ip arp inspection trust

To set a per-port configurable trust state that determines the set of interfaces where incoming ARP packets are inspected, use the ip arp inspection trust command in interface configuration mode. To make the interfaces untrusted, use the no form of this command.

ip arp inspection trust

no ip arp inspection trust

Syntax Description

This command has no arguments or keywords.

Command Default

This command has no default settings.

Command Modes

Interface configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Examples

This example shows how to configure an interface to be trusted: 


Router# configure terminal
Router(config)# interface fastEthernet 6/3
Router(config-if)# ip arp inspection trust

Related Commands

Command

Description

show ip arp inspection

Displays the status of DAI for a specific range of VLANs.

ip arp inspection validate

To perform specific checks for ARP inspection, use the ip arp inspection validate command in global configuration mode. To disable ARP inspection checks, use the no form of this command.

ip arp inspection validate [src-mac] [dst-mac] [ip]

no ip arp inspection validate [src-mac] [dst-mac] [ip]

Syntax Description

src-mac

(Optional) Checks the source MAC address in the Ethernet header against the sender’s MAC address in the ARP body.

dst-mac

(Optional) Checks the destination MAC address in the Ethernet header against the target MAC address in the ARP body.

ip

(Optional) Checks the ARP body for invalid and unexpected IP addresses.

Command Default

Disabled

Command Modes

Global configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Usage Guidelines

The sender IP addresses are checked in all ARP requests and responses and target IP addresses are checked only in ARP responses. Addresses include 0.0.0.0, 255.255.255.255, and all IP multicast addresses.

The src-mac checks are issued against both ARP requests and responses. The dst-mac checks are issued for ARP responses.


Note

When enabled, packets with different MAC addresses are classified as invalid and are dropped.

When enabling the checks, specify at least one of the keywords (src-mac , dst-mac , and ip ) on the command line. Each command overrides the configuration of the previous command. If a command enables src and dst mac validations, and a second command enables IP validation only, the src and dst mac validations are disabled as a result of the second command.

The no form of this command disables only the specified checks. If no check options are enabled, all the checks are disabled.

Examples

This example shows how to enable the source MAC validation: 


Router(config)# ip arp inspection validate src-mac

Related Commands

Command

Description

arp access-list

Configures an ARP ACL for ARP inspection and QoS filtering and enters the ARP ACL configuration submode.

show ip arp inspection

Displays the status of DAI for a specific range of VLANs.

ip arp inspection vlan

To enable DAI on a per-VLAN basis, use the ip arp inspection vlan command in global configuration mode. To disable DAI, use the no form of this command.

ip arp inspection vlan vlan-range

no ip arp inspection vlan vlan-range

Syntax Description

vlan-range

VLAN number or range; valid values are from 1 to 4094.

Command Default

ARP inspection is disabled on all VLANs.

Command Modes

Global configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Usage Guidelines

For vlan-range , you can specify a single VLAN identified by a VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma.

You must specify on which VLANs to enable DAI. DAI may not function on the configured VLANs if the VLAN has not been created or is a private VLAN.

Examples

This example shows how to enable DAI on VLAN 1: 


Router(config)# ip arp inspection vlan 1

Related Commands

Command

Description

arp access-list

Configures an ARP ACL for ARP inspection and QoS filtering and enters the ARP ACL configuration submode.

show ip arp inspection

Displays the status of DAI for a specific range of VLANs.

ip arp inspection vlan logging

To control the type of packets that are logged, use the ip arp inspection vlan logging command in global configuration mode. To disable this logging control, use the no form of this command.

ip arp inspection vlan vlan-range logging {acl-match {matchlog | none} | dhcp-bindings {permit | all | none}}

no ip arp inspection vlan vlan-range logging {acl-match | dhcp-bindings}

Syntax Description

vlan-range

Number of the VLANs to be mapped to the specified instance. The number is entered as a single value or a range; valid values are from 1 to 4094.

acl-match

Specifies the logging criteria for packets that are dropped or permitted based on ACL matches.

matchlog

Specifies that logging of packets matched against ACLs is controlled by the matchlog keyword in the permit and deny access control entries of the ACL.

none

Specifies that ACL-matched packets are not logged.

dhcp-bindings

Specifies the logging criteria for packets dropped or permitted based on matches against the DHCP bindings.

permit

Specifies logging when permitted by DHCP bindings.

all

Specifies logging when permitted or denied by DHCP bindings.

none

Prevents all logging of packets permitted or denied by DHCP bindings.

Command Default

All denied or dropped packets are logged.

Command Modes

Global configuration

Command History

Release

Modification

12.2(18)SXE

Support for this command was introduced on the Supervisor Engine 720.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Usage Guidelines

By default, the matchlog keyword is not available on the ACEs. When you enter the matchlog keyword, denied packets are not logged. Packets are logged only when they match against an ACE that has the matchlog keyword.

The acl-match and dhcp-bindings keywords merge with each other. When you set an ACL match configuration, the DHCP bindings configuration is not disabled. You can use the no form of this command to reset some of the logging criteria to their defaults. If you do not specify either option, all the logging types are reset to log on when the ARP packets are denied. The two options that are available are as follows:

  • acl-match --Logging on ACL matches is reset to log on deny.

  • dhcp-bindings --Logging on DHCP bindings is reset to log on deny.

Examples

This example shows how to configure an ARP inspection on VLAN 1 to add packets to a log that matches the ACLs: 


Router(config)# ip arp inspection vlan 1 logging acl-match matchlog

Related Commands

Command

Description

arp access-list

Configures an ARP ACL for ARP inspection and QoS filtering and enters the ARP ACL configuration submode.

show ip arp inspection

Displays the status of DAI for a specific range of VLANs.

ip arp nat-garp-retry

To enable the efficient mapping of MAC addresses to IP addresses within a local network using the Address Resolution Protocol (ARP) and Gratuitous ARP (GARP), first use the ip arp nat-garp-retry feature enable command.

Following this, to request GARP messages, use the 'garp-interface' option along with the 'ip nat inside source static' command on the BD-VIF interface during NAT mapping configuration. For more information, see the ip nat inside source static command reference.

ip arp nat-garp-retry feature enable

ip arp nat-garp-retry feature disable

Upon activation, the following parameters can be configured:

  • The 'retries' argument can be added to the ip arp nat-garp-retry command to specify the number of NAT GARP Retry messages. The default is 2 times, with a permissible range of 1 to 5 retries for each entry.

    The command for this option is: ip arp nat-garp-retry entries

  • The 'interval' argument can be added to the ip arp nat-garp-retry command to set the time gap between NAT GARP Retry messages. The default interval is 5 seconds, with an acceptable range of 1 to 30 seconds.

    The command for this option is: ip arp nat-garp-retry interval

  • The 'entries' argument can be added to the ip arp nat-garp-retry command to define the maximum number of GARP command executions. The maximum number of BD-VIF interfaces for GARP initiation is capped at 3000 to optimize control plane load.

    The command for this option is: ip arp nat-garp-retry retries

Syntax Description

nat-garp-retry

Activates the NAT Gratuitous ARP (GARP) retry feature.

entries

(Optional) Defines the limit for GARP command executions. The maximum number of BD-VIF interfaces for GARP initiation is capped at 3000 to optimize control plane load.

interval

(Optional) Sets the time gap between NAT GARP Retry messages. The default is 5 seconds, with a permissible range of 2 to 30 seconds.

retries

(Optional) Determines the number of NAT GARP Retry message attempts. The default is 2 times, with a permissible range of 1 to 5 retries for each entry.

Command Default

By default, the NAT Gratuitous ARP (GARP) retry feature is disabled.

Command Modes

Global configuration (config)

Command History

Release

Modification

Cisco IOS XE 17.13.1a

This command was introduced.

Usage Guidelines

The ip arp nat-garp retry command is a fundamental command that controls the parameters of the number of Gratuitous Address Resolution Protocol (GARP) entries, the intervals between the GARP messages, and the number of retries available. This command is specifically designed for use with BD-VIF (Bridge Domain-Virtual Interface) interfaces.

Activation of the ip arp nat-garp retry command is initiated with the 'ip arp nat-garp-retry feature enable' argument, effectively enabling this feature. After enabling, the command includes three additional optional arguments that provide further control over its function.

The ip arp nat-garp-retry entries argument sets the number of GARP entries. The ip arp nat-garp-retry interval argument determines the interval between GARP messages. Finally, the ip arp nat-garp-retry retries argument sets the number of retries available.

These optional arguments enable the user to configure the GARP retry mechanism in detail within the BD-VIF interfaces, thereby enhancing the efficiency of MAC to IP address mapping within the network.

By default, this feature is disabled. However, the user can enable it prior to configuring, to activate the GARP retry for IP NAT static inside CLI. This would offer more control over the GARP messages and their retry mechanism.

Examples

Here is an example of how to use the 'ip arp nat-garp retry' command and its optional arguments:

Router(config)# ip arp nat-garp-retry feature enable
Router(config)# ip arp nat-garp-retry entries 10
Router(config)# ip arp nat-garp-retry interval 30
Router(config)# ip arp nat-garp-retry retries 5

Related Commands

Command

Description

ip nat inside source static

Triggers GARP requests for static NAT mapping configurations on the BD-VIF interface.

ip arp poll

To configure the IP Address Resolution Protocol (ARP) polling for unnumbered interfaces, use the ip arp poll command in global configuration mode. To remove the IP ARP polling for unnumbered interfaces, use the no form of this command.

ip arp poll {queue queue-size | rate packet-rate}

no ip arp poll {queue | rate}

Syntax Description

queue queue-size

Configures the IP ARP polling queue size, in packets. The range is from 0 to 10000. The default is 1000.

rate packet-rate

Configures the IP ARP polling packet rate, in packets per second. The range is from 0 to 10000. The default is 1000.

Command Default

IP ARP polling for unnumbered interfaces has a default queue size of 1000 and packet rate of 1000 packets per second.

Command Modes

Global configuration (config)  

Command History

Release

Modification

15.1(1)SY

This command was introduced.

Examples

The following example shows how to configure the queue size for IP ARP polling for unnumbered interfaces: 

Device(config)# ip arp poll queue 5000

The following example shows how to configure the packet rate for IP ARP polling for unnumbered interfaces: 

Device(config)# ip arp poll rate 5000

Related Commands

Command

Description

show ip arp poll

Displays the IP ARP host polling status.

ip arp proxy disable

To globally disable proxy Address Resolution Protocol (ARP), use the ip arp proxy disable command in global configuration mode. To reenable proxy ARP, use the no form of this command.

ip arp proxy disable

no ip arp proxy disable

Syntax Description

This command has no arguments or keywords.

Command Default

Proxy ARP is enabled.

Command Modes

Global configuration

Command History

Release

Modification

12.2 S

This command was introduced.

12.3(11)T

This command was integrated into 12.3(11)T.

12.2 (18)SXE

This command was integrated into 12.2(18)SXE.

Usage Guidelines

The ip arp proxy disable command overrides any proxy ARP interface configuration. The default ip arp proxy command returns proxy ARP to the default behavior, which is enabled.

Examples

The following example disables proxy ARP: 


ip arp proxy disable

The following example enables proxy ARP: 


no ip arp proxy disable

Related Commands

Command

Description

ip proxy-arp

Enables proxy ARP on an interface.

ip arp queue

To configure the Address Resolution Protocol (ARP) input packet queue size, use the ip arp queue command in global configuration mode. To restore the default, use the no form of this command.

ip arp queue queue-size

no ip arp queue

Syntax Description

queue-size

Size of the ARP input packet queue. Valid values are from 512 to 2147483647.

Command Default

By default, the queue size is configured as 512.

Command Modes

Global configuration (config)

Command History

Release

Modification

15.0(1)M5

This command was introduced.

Usage Guidelines

You can configure the ARP input packet queue size based on the volume of the incoming traffic. The ARP input queue size can be set by the platform during initialization. The ARP input packet size is configurable at the system level but not at the interface level.

Examples

The following example shows how to configure the ARP input packet queue size as 650: 


Router(config)# ip arp queue 650

ip classless

To enable a router to forward packets, which are destined for a subnet of a network that has no network default route, to the best supernet route possible, use the ip classless command in global configuration mode. To disable the functionality, use the no form of this command.

ip classless

no ip classless

Syntax Description

This command has no arguments or keywords.

Command Default

Enabled

Command Modes

Global configuration

Command History

Release

Modification

10.0

This command was introduced.

11.3

The default behavior changed from disabled to enabled.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2SX

This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.

Usage Guidelines

This command allows the software to forward packets that are destined for unrecognized subnets of directly connected networks. The packets are forwarded to the best supernet route.

When this feature is disabled, the Cisco IOS software discards the packets when a router receives packets for a subnet that numerically falls within its subnetwork addressing scheme, no such subnet number is in the routing table, and there is no network default route.


Note

If the supernet or default route is learned by using Intermediate System-to-Intermediate System (IS-IS) or Open Shortest Path First (OSPF), the no ip classless configuration command is ignored.

Examples

The following example prevents the software from forwarding packets destined for an unrecognized subnet to the best supernet possible: 


no ip classless

ip ddns update hostname

To enable a host to be used for Dynamic Domain Name System (DDNS) updates of address (A) and pointer (PTR) Resource Records (RRs), use the ip ddns update hostname command in interface configuration mode. To disable the dynamic updates, use the no form of this command.

ip ddns update hostname hostname

no ip ddns update hostname hostname

Syntax Description

hostname

Specifies a hostname of the server that will receive updates.

Note

 

It is expected that the hostname will be an fully qualified domain name (FQDN). Using an FQDN hostname enables the specification of a hostname in a different domain that the default domain of the device.

Command Default

No host is configured.

Command Modes

Interface configuration

Command History

Release

Modification

12.3(8)YA

This command was introduced.

12.3(14)T

This command was integrated into Cisco IOS Release 12.3(14)T.

Usage Guidelines

The interface configuration overrides the global configuration.

Examples

The following example shows how to configure the testhost host to update A and PTR RRs: 


interface ethernet1/0
 ip ddns update hostname testhost

Related Commands

Command

Description

ip ddns update method

Specifies a method of DDNS updates of A and PTR RRs and the maximum interval between the updates.

ip ddns update method

To specify a method and method name for updating Dynamic Domain Name System (DDNS) address (A) and pointer (PTR) Resource Records (RRs) and enter DDNS-update-method configuration mode, use the ip ddns update method command in global configuration mode. To disable the dynamic updating, use the no form of this command.

ip ddns update method method-name

no ip ddns update method

Syntax Description

method-name

IETF standardized DDNS update method name.

Command Default

No DDNS update method is configured.

Command Modes

Global configuration

Command History

Release

Modification

12.3(8)YA

This command was introduced.

12.3(14)T

This command was integrated into Cisco IOS Release 12.3(14)T.

Usage Guidelines

The interface configuration overrides the global configuration.

Examples

The following example shows how to assign a DDNS update method name: 


ip ddns update method unit-test

Once you have assigned the method name, you can specify the type of update (DDNS or HTTP) and set a maximum interval. Refer to the ddns and http commands for more information.

Related Commands

Command

Description

ddns

Specifies DDNS as the update method for A and PTR RRs.

http

Specifies HTTP as the update method for A and PTR RRs.

ip default-gateway

To define a default gateway (router) when IP routing is disabled, use the ip default-gateway command in global configuration mode. To disable this function, use the no form of this command.

ip default-gateway ip-address

no ip default-gateway ip-address

Syntax Description

ip-address

IP address of the router.

Command Default

Disabled

Command Modes

Global configuration

Command History

Release

Modification

10.0

This command was introduced.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2SX

This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.

Usage Guidelines

The Cisco IOS software sends any packets that need the assistance of a gateway to the address you specify. If another gateway has a better route to the requested host, the default gateway sends an Internet Control Message Protocol (ICMP) redirect message back. The ICMP redirect message indicates which local router the Cisco IOS software should use.

Examples

The following example defines the router on IP address 192.31.7.18 as the default router: 


ip default-gateway 192.31.7.18

Related Commands

Command

Description

ip redirects

Enables the sending of ICMP redirect messages if the Cisco IOS software is forced to resend a packet through the same interface on which it was received.

show ip redirects

Displays the address of a default gateway (router) and the address of hosts for which an ICMP redirect message has been received.

ip dhcp aaa default username

To specify the default user name for non-virtual routing and forwarding (VRF) address pools that have been configured to obtain subnets through authentication, authorization, and accounting (AAA), use the ip dhcp aaa default username command in global configuration mode. To disable this functionality, use the no form of this command.

ip dhcp aaa default username name

no ip dhcp aaa default username name

Syntax Description

name

Name of the address pool.

Command Default

No default behavior or values.

Command Modes

Global configuration

Command History

Release

Modification

12.2(8)T

This command was introduced.

12.2(15)T

The behavior when the username attribute is sent in the AAA request was changed.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

Usage Guidelines

Address pools that are configured with the vrf and origin aaa commands will set the username attribute in the AAA request to the specified VRF name. If the VPN ID as specified in RFC 2685 is configured for the VRF, the VPN ID will be sent instead.

Address pools that are not configured with the vrf command but are configured with the origin aaa command, will set the username attribute in the AAA request to the specified name in the ip dhcp aaa default username command.

Use the debug aaa attribute command to verify the value of the username attribute in the subnet request to the AAA server.

In Cisco IOS Release 12.2(8)T, if this command is not configured, no AAA subnet request from non-VRF ODAPs will be sent.

In Cisco IOS Release 12.2(15)T, if the DHCP pool is not configured with VRF and the ip dhcp aaa default username command is not configured, the AAA request will still be sent with the username attribute set to the Dynamic Host Configuration Protocol (DHCP) pool name.

This command is not needed if all on-demand address pools (ODAPs) on the VHG/provider edge (PE) are VRF-associated.

Examples

The following example sets the username attribute in the AAA request to abc: 


ip dhcp aaa default username abc

Related Commands

Command

Description

debug aaa attribute

Verifies the value of the AAA attributes.

origin

Configures an address pool as an on-demand address pool.

vrf

Associates the on-demand address pool with a VPN routing and forwarding instance.

ip dhcp auto-broadcast

To configure a Dynamic Host Configuration Protocol (DHCP) server on your network to respond only with unicast messages instead of automatically switching to broadcast responses, use the no ip dhcp auto-broadcast command in global configuration mode. The default behavior is represented by the ip dhcp auto-broadcast command.

ip dhcp auto-broadcast

[ no] ip dhcp auto-broadcast

Command Default

The default command, ip dhcp auto-broadcast allows the DHCP server to send broadcast messages to a client after the server has tried sending two unicast messages. Change this default behavior, so that the DHCP server sends unicast messages to a client, by using the "no" form of the command: no ip auto-broadcast .

Command Modes

Global configuration mode.

Command History

Release

Modification

Cisco IOS XE Release 3.9S

This command was integrated into Cisco IOS XE Release 3.9S

Usage Guidelines

Usually, when the client requests a unicast response from the DHCPv4 server, the server responds with a unicast message. However, sometimes these unicast responses can get lost or the client does not have the support to handle unicast messages. In such cases, after sending two unicast offer response messages, if the client still sends the same request packet, the server understands that the client is unable to receive unicast messages and automatically responds with a broadcast message.

You can use the no ip dhcp auto-broadcast command to change this behavior and ensure that the server continues to send unicast messages to the client.

Examples

The following command specifies that a DHCP server sends unicast messages to the client: 


no ip dhcp auto-broadcast

Related Commands

Command

Description

ip dhcp client broadcast-flag

Configures a DHCP client to set or clear the broadcast flag.

ip dhcp bootp ignore

To enable a Dynamic Host Configuration Protocol (DHCP) server to selectively ignore and not reply to received Bootstrap Protocol (BOOTP) request packets, use the ip dhcp bootp ignore command in global configuration mode. To return to the default behavior, use the no form of this command.

ip dhcp bootp ignore

no ip dhcp bootp ignore

Syntax Description

This command has no arguments or keywords.

Command Default

The default behavior is to service BOOTP requests.

Command Modes

Global configuration

Command History

Release

Modification

12.2(8)T

This command was introduced.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

Usage Guidelines

A DHCP server can forward ignored BOOTP request packets to another DHCP server if the ip helper-address command is configured on the incoming interface. If the ip helper-address command is not configured, the router will drop the received BOOTP request.

Examples

The following example shows that the router will ignore received BOOTP requests: 


hostname Router 
!
ip subnet-zero
!
ip dhcp bootp ignore

Related Commands

Command

Description

ip bootp server

Enables the BOOTP service on routing devices.

ip helper-address

Forwards UDP broadcasts, including BOOTP, received on an interface.

ip dhcp class

To define a Dynamic Host Configuration Protocol (DHCP) class and enter DHCP class configuration mode, use the ip dhcp class command in global configuration mode. To remove the class, use the no form of this command.

ip dhcp class class-name

no ip dhcp class class-name

Syntax Description

class-name

Name of the DHCP class.

Command Default

No default behavior or values.

Command Modes

Global configuration

Command History

Release

Modification

12.2(13)ZH

This command was introduced.

12.3(4)T

This command was integrated into Cisco IOS Release 12.3(4)T.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

12.2(33)SRB

This command was integrated into Cisco IOS Release 12.2(33)SRB.

Usage Guidelines

DHCP class configuration provides a method to group DHCP clients based on some shared characteristics other than the subnet in which the clients reside.

Examples

The following example defines three DHCP classes and their associated relay agent information patterns. Note that CLASS3 is considered a “match to any” class because it has no relay agent information pattern configured: 


ip dhcp class CLASS1 
 relay agent information 
! Relay agent information patterns
  relay-information hex 01030a0b0c02050000000123 
  relay-information hex 01030a0b0c02* 
  relay-information hex 01030a0b0c02050000000000 bitmask 0000000000000000000000FF 
ip dhcp class CLASS2 
 relay agent information
! Relay agent information patterns
  relay-information hex 01040102030402020102 
  relay-information hex 01040101030402020102 
ip dhcp class CLASS3 
 relay agent information

Related Commands

Command

Description

relay agent information

Enters relay agent information option configuration mode.

relay-information hex

Specifies a hexadecimal string for the full relay agent information option.

ip dhcp client

To configure the Dynamic Host Configuration Protocol (DHCP) client to associate any added routes with a specified tracked object number, use the ip dhcp client command in interface configuration mode. To restore the default setting, use the no form of this command.

ip dhcp client route track number

no ip dhcp client route track

Syntax Description

route track number

Associates a tracked object number with the DHCP-installed static route. Valid values for the number argument range from 1 to 500.

Command Default

No routes are associated with a track number.

Command Modes

Interface configuration

Command History

Release

Modification

12.3(2)XE

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2(33)SXH

This command was integrated into Cisco IOS Release 12.2(33)SXH.

Usage Guidelines

The ip dhcp client command must be configured before the ip address dhcp command is configured on an interface. The ip dhcp client command is checked only when an IP address is acquired from DHCP. If the ip dhcp client command is specified after an IP address has been acquired from DHCP, the ip dhcp client command will not take effect until the next time the router acquires an IP address from DHCP.

Examples

The following example configures DHCP on an Ethernet interface and associates tracked object 123 with routes generated from this interface: 


interface ethernet 0/0
 ip dhcp client route track 123
 ip address dhcp

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an Ethernet interface from the DHCP.

ip dhcp client authentication key-chain

To specify the key chain to be used in authenticating a request, use the ip dhcp client authentication key-chain command in interface configuration mode. To disable the key-chain authentication, use the no form of this command.

ip dhcp client authentication key-chain name [forcerenew]

no ip dhcp client authentication key-chain

Syntax Description

name

Name of the key chain.

forcerenew

(Optional) Configures DHCP authentication only for FORCERENEW messages.

Command Default

Authentication is not specified.

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

12.4(22)YB

This command was introduced.

15.0(1)M

This command was integrated into Cisco IOS Release 15.0(1)M.

15.1(4)M

This command was modified. The forcerenew keyword was added.

Usage Guidelines

Configure the ip dhcp client authentication key-chain command to send to the server the authentication messages that are encoded by the secret ID and secret value that were configured using the key chain command. When authentication is enabled, all client-server exchanges must be authenticated; the ip dhcp client authentication mode and key chain commands must be configured.

When the ip dhcp client authentication key-chain command is configured, authentication is enabled for all the DHCP messages including FORCERENEW messages that are received through the interface. To configure DHCP authentication only for the FORCERENEW messages, use forcerenew keyword.

Examples

The following example shows how to specify a key chain named chain1 for authentication exchanges: 


Router(config-if)# ip dhcp client authentication key-chain chain1

Related Commands

Command

Description

ip dhcp client authentication mode

Specifies the type of authentication to be used in DHCP messages on the interface.

ip dhcp-client forcerenew

Enables FORCERENEW-message handling on the DHCP client when authentication is enabled.

key chain

Identifies a group of authentication keys for routing protocols.

ip dhcp client authentication mode

To specify the type of authentication to be used in DHCP messages on the interface, use the ip dhcp client authentication mode command in interface configuration mode. To remove the specification, use the no form of this command.

ip dhcp client authentication mode {md5 | token} [forcerenew]

no ip dhcp client authentication mode

Syntax Description

md5

Specifies MD5-based authentication.

token

Specifies token-based authentication.

forcerenew

(Optional) Configures DHCP authentication only for FORCERENEW messages.

Command Default

No authentication mode is configured.

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

12.4(22)YB

This command was introduced.

15.0(1)M

This command was integrated into Cisco IOS Release 15.0(1)M.

15.1(4)M

This command was modified. The forcerenew keyword was added.

Usage Guidelines

Token-based authentication is useful only for basic protection against inadvertently instantiated DHCP servers. Tokens are transmitted in plain text; they provide weak authentication and do not provide message authentication. MD5-based authentication provides better message and entry authentication because it specifies the generation of a temporary value by the source.

When the ip dhcp client authentication key-chain command is configured, authentication is enabled for all the DHCP messages including FORCERENEW messages that are received through the interface. To configure DHCP authentication only for FORCERENEW messages, use the forcerenew keyword.

Examples

The following example shows how to specify chain1 as the key chain and MD5 as the mode for authentication exchanges: 


Router(config-if)# ip dhcp client authentication key-chain chain1
Router(config-if)# ip dhcp client authentication mode md5

Related Commands

Command

Description

ip dhcp client authentication key-chain

Specifies the key chain to be used in DHCP authentication requests.

ip dhcp-client forcerenew

Enables FORCERENEW-message handling on the DHCP client when authentication is enabled.

key chain

Identifies a group of authentication keys for routing protocols.

ip dhcp client broadcast-flag (interface)

To configure a DHCP client to set or clear the broadcast flag, use the ip dhcp client broadcast-flag command in interface configuration mode. To disable the configuration, use the no form of this command.

ip dhcp client broadcast-flag {clear | set}

no ip dhcp client broadcast-flag

Syntax Description

clear

Clears the broadcast flag.

set

Sets the broadcast flag.

Command Default

The broadcast flag is set.

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

15.1(3)T

This command was introduced.

Usage Guidelines

For a DHCP server to work on a Dynamic Multipoint VPN (DMVPN) network, the DHCP client available on the spoke must unicast the DHCP messages from the server to the client. By default, the DHCP client on the spoke broadcasts the DHCP messages. The broadcast flag is set during broadcast. Hence, the DHCP client on the spoke must have an option to clear the DHCP broadcast flag. You can use the ip dhcp client broadcast-flag command to configure the DHCP client to set or clear the broadcast flag.

Examples

The following example shows how to configure a DHCP client to clear the broadcast flag: 


Router(config)# tunnel 1
Router(config-if)# ip dhcp client broadcast-flag clear

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from the DHCP.

ip dhcp support tunnel unicast

Configures a spoke-to-hub tunnel to unicast the DHCP replies over the DMVPN network.

ip dhcp client class-id

To specify the class identifier, use the ip dhcp client class-id command in interface configuration mode. To remove the class identifier, use the no form of this command.

ip dhcp client class-id {string | hex string}

no ip dhcp client class-id {string | hex string}

Syntax Description

string

A unique ASCII string.

hex string

A unique hexadecimal value.

Command Default

No class identifier is specified.

Command Modes

Interface configuration

Command History

Release

Modification

12.3(2)XF

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

Usage Guidelines

The ip dhcp client class-id command is checked only when an IP address is acquired from a Dynamic Host Configuration Protocol (DHCP) server. If the command is specified after an IP address has been acquired from the DHCP server, the command will not take effect until the next time the router acquires an IP address from the DHCP server. 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 specified.

The class identifier is used by vendors to specify the type of device that is requesting an IP address. For example, docsis 1.0 can be used for a cable modem and Cisco Systems, Inc. IP Phone can be used for a Cisco IP phone.

Examples

The following example configures a class identifier with a hexadecimal string of ABCDEF1235: 


interface Ethernet 1
 ip dhcp client class-id hex ABCDEF1235

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from DHCP.

release dhcp

Performs an immediate release of a DHCP lease for an interface.

renew dhcp

Performs an immediate renewal of a DHCP lease for an interface.

ip dhcp client client-id

To specify a client identifier and override the default client identifier, use the ip dhcp client client-id command in interface configuration mode. To return to the default form, use the no form of this command.

ip dhcp client client-id {interface-name | ascii string | hex string | reuse-mac}

no ip dhcp client client-id {interface-name | ascii string | hex string | reuse-mac}

Syntax Description

interface-name

Interface from which the MAC address is used.

ascii string

Specifies a unique ASCII string. The default value is cisco-mac-name where mac is the MAC address of the interface and 'name' is the short form of the interface name.

hex string

Specifies a unique hexadecimal value.

reuse-mac

Reuses the MAC address configured by the atm ether-mac-address command.

Note

 

The reuse-mac keyword is to be used only on ATM subinterfaces along with the atm ether-mac-address command.

Command Default

The client identifier is an ASCII value in the form cisco-mac -name where mac is the MAC address of the interface and name is the short form of the interface name.

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

12.3(2)XF

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

15.1(4)M4

This command was modified and integrated into Cisco IOS Release 15.1(4)M4. The reuse-mac keyword was added.

Usage Guidelines

The ip dhcp client client-id command is specified only when an IP address is acquired from a DHCP server. If the command is specified after an IP address has been acquired from the DHCP server, the command will not take effect until the next time the device acquires an IP address from the DHCP server. 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 specified.

When the no form of this command is specified, the configuration is removed and the system returns to the default form. To configure the system, a client identifier must be included.

Examples

The following example shows how to configure a client identifier named test-client-id: 


Device> enable
Device# configure terminal
Device(config)# interface Ethernet 1
Device(config-if)# ip dhcp client client-id ascii test-client-id

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from the DHCP server.

release dhcp

Performs an immediate release of a DHCP lease for an interface.

renew dhcp

Performs an immediate renewal of a DHCP lease for an interface.

ip dhcp client default-router distance

To configure the default Dynamic Host Configuration Protocol (DHCP) administrative distance, use the ip dhcp client default-router distance command in interface configuration mode. To disable the configuration, use the no form of this command.

ip dhcp client default-router distance metric-value

no ip dhcp client default-router distance

Syntax Description

metric-value

Default route metric value. Range: 1 to 255. Default: 254.

Command Default

The default administrative distance is 254.

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

12.4(15)T

This command was introduced.

Usage Guidelines

While you are adding the default route the administrative distance is calculated as follows:

  • Interface configuration is given the highest preference if the metric value is not set to the default value.

  • If a metric value is not configured on an interface, then the existing global configuration command will get preference.

  • If the administrative distance is not configured in both interface configuration mode and global configuration mode, then the global configuration default distance of 254 is used.

Examples

The following example shows how to configure the DHCP default route metric to 2: 


Router # configure terminal
Router(config)# interface FastEthernet 0/2
Router(config-if)# ip dhcp client default-router distance 2

Related Commands

Command

Description

debug dhcp client

Displays debugging information about the DHCP client activities and monitors the status of DHCP packets.

ip dhcp-client default-router distance

Configures a default DHCP administrative distance for clients in global configuration mode.

show ip route dhcp

Displays the routes added to the routing table by the DHCP server and relay agent.

ip dhcp client hostname

To specify or modify the hostname sent in a Dynamic Host Configuration Protocol (DHCP) message, use the ip dhcp client hostname command in interface configuration mode. To remove the hostname, use the no form of this command.

ip dhcp client hostname host-name

no ip dhcp client hostname host-name

Syntax Description

host-name

Name of the host.

Command Default

The hostname is the globally configured hostname of the router.

Command Modes

Interface configuration(config-if)

Command History

Release

Modification

12.3(2)XF

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

Usage Guidelines

The ip dhcp client hostname command is checked only when an IP address is acquired from a DHCP server. If the command is specified 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 the DHCP server. 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 specified.

This command is applicable only for DHCP requests generated by Cisco IOS software. This command is ignored when Cisco IOS software relays requests (for example, from Distributed Route Processor PPP clients).

Examples

The following example shows how to specify the hostname of the DHCP client as hostA: 


interface Ethernet 1
 ip dhcp client hostname hostA

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from DHCP.

release dhcp

Performs an immediate release of a DHCP lease for an interface.

renew dhcp

Performs an immediate renewal of a DHCP lease for an interface.

ip dhcp client lease

To configure the duration of the lease for an IP address that is requested from a Dynamic Host Configuration Protocol (DHCP) client to a DHCP server, use the ip dhcp client lease command in interface configuration mode. To restore to the default value, use the no form of this command.

ip dhcp client lease days [hours] [minutes]

no ip dhcp client lease

Syntax Description

days

Specifies the duration of the lease in days.

hours

(Optional) Specifies the number of hours in the lease. A days value must be supplied before an hours value can be configured.

minutes

(Optional) Specifies the number of minutes in the lease. A days value and an hours value must be supplied before a minutes value can be configured.

Command Default

A default lease time is not included in the DHCP DISCOVER messages sent by the client. The client accepts the lease time that the DHCP server sends.

Command Modes

Interface configuration

Command History

Release

Modification

12.3(2)XF

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

Usage Guidelines

The ip dhcp client lease command is checked only when an IP address is acquired from a DHCP server. If the command is specified 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 the DHCP server. 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 specified.

Examples

The following example shows a one-day lease: 


ip dhcp client lease 1

The following example shows a one-hour lease: 


ip dhcp client lease 0 1

The following example shows a one-minute lease: 


ip dhcp client lease 0 0 1

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from DHCP.

lease

Configures the duration of the lease for an IP address that is assigned from a DHCP server to a DHCP client

release dhcp

Performs an immediate release of a DHCP lease for an interface.

renew dhcp

Performs an immediate renewal of a DHCP lease for an interface.

ip dhcp client mobile renew

To configure the number of renewal attempts and the interval between attempts for renewing an IP address acquired by a Dynamic Host Configuration Protocol (DHCP) client, use the ip dhcp client mobile renew command in interface configuration mode. To disable the functionality, use the no form of this command.

ip dhcp client mobile renew count number interval ms

no ip dhcp client mobile renew count number interval ms

Syntax Description

count number

Number of attempts to renew a current IP address before starting the DHCP discovery process. The range is from 0 to 10 attempts. The default is 2 attempts.

interval ms

Interval to wait between renewal attempts. The range is from 1 to 1000 ms. The default is 50 ms.

Command Default

count number : 2interval ms : 50

Command Modes

Interface configuration

Command History

Release

Modification

12.3(14)T

This command was introduced.

Usage Guidelines

Mobile DHCP clients automatically attempt to renew an existing IP address in response to certain events, such as moving between wireless access points. The number of renewal attempts, and the interval between those attempts, depending on network conditions, can be modified by using the ip dhcp client mobile renew command.

Examples

In the following example, the DHCP client will make four attempts to renew its current IP address with an interval of 30 milliseconds between attempts : 


interface FastEthernet0
 ip dhcp client mobile renew count 4 interval 30

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from DHCP.

ip dhcp client request

To configure a Dynamic Host Configuration Protocol (DHCP) client to request an option from a DHCP server, use the ip dhcp client request command in interface configuration mode. To remove the request for an option, use the no form of this command.

ip dhcp client request option-name

no ip dhcp client request option-name

Syntax Description

option-name

The option name can be one of the following keywords:

  • tftp-server-address

  • sip-server-address

  • netbios-nameserver

  • vendor-specific

  • vendor-identifying-specific

  • static-route

  • classless -static-route

  • domain-name

  • dns-nameserver

  • router

By default, all these options except sip-server-address , vendor-identifying-specific , and classless -static-route are requested.

Command Default

All the options are requested except sip-server-address , vendor-identifying-specific , and classless -static-route .

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

12.3(2)XF

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

12.4(22)YB

This command was modified. The sip-server-address , vendor-identifying-specific , and classless -static-route keywords were added.

15.0(1)M

This command was integrated into Cisco IOS Release 15.0(1)M.

Usage Guidelines

By default, all options except sip-server-address , vendor-identifying-specific , and classless -static-route are requested, so you must use the no form of the ip dhcp client request command to disable those default options, and explicitly specify any options that are not enabled by default.

Default options that are specified by the no form are removed from the DHCP originated address for the interface. An option can be reinserted in the list of requested options by using the same command without the no keyword. Multiple options can be specified on one configuration line. However, each option will appear on a separate line in the running configuration.

The ip dhcp client request command is checked only when an IP address is acquired from a DHCP server. If the command is specified 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 the DHCP server. This means that the new configuration will take effect only after either the ip address dhcp command or a DHCP lease renewal or termination that is not initiated by a release dhcp or a renew dhcp command.

Examples

The following example shows how to configure the DHCP client to remove the DNS name server from the options requested from the DHCP server: 


no ip dhcp client request dns-nameserver

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an interface from DHCP.

ip dhcp-client forcerenew

Enables forcerenew-message handling on the DHCP client when authentication is enabled.

ip dhcp client authentication key-chain

Specifies the authentication key used for the DHCP protocol on the interface.

ip dhcp client authentication mode

Specifies the type of authentication to be used in DHCP messages on the interface.

release dhcp

Performs an immediate release of a DHCP lease for an interface.

renew dhcp

Performs an immediate renewal of a DHCP lease for an interface.

ip dhcp client route

To configure the Dynamic Host Configuration Protocol (DHCP) client to associate any added routes with a specified tracked object number, use the ip dhcp client command in interface configuration mode. To restore the default setting, use the no form of this command.

ip dhcp client route track number

no ip dhcp client route track

Syntax Description

route track number

Associates a tracked object number with the DHCP-installed static route. Valid values for the number argument range from 1 to 500.

Command Default

No routes are associated with a track number.

Command Modes

Interface configuration (config-if)

Command History

Release

Modification

12.3(2)XE

This command was introduced.

12.3(8)T

This command was integrated into Cisco IOS Release 12.3(8)T.

12.2(33)SXH

This command was integrated into Cisco IOS Release 12.2(33)SXH.

12.2(33)SRE

This command was integrated into Cisco IOS Release 12.2(33)SRE.

Usage Guidelines

The ip dhcp client command must be configured before the ip address dhcp command is configured on an interface. The ip dhcp client command is checked only when an IP address is acquired from DHCP. If the ip dhcp client command is specified after an IP address has been acquired from DHCP, the ip dhcp client command will not take effect until the next time the router acquires an IP address from DHCP.

Examples

The following example configures DHCP on an Ethernet interface and associates tracked object 123 with routes generated from this interface: 


interface ethernet 0/0
 ip dhcp client route track 123
 ip address dhcp

Related Commands

Command

Description

ip address dhcp

Acquires an IP address on an Ethernet interface from the DHCP.

ip dhcp client update dns

To enable Dynamic Domain Name System (DDNS) updates of address (A) Resource Records (RRs) using the same hostname passed in the hostname and fully qualified domain name (FQDN) options by a client, use the ip dhcp client update dns command in interface configuration mode. To disable dynamic updates of A RRs, use the no form of this command.

ip dhcp client update dns [server {both | none}]

no ip dhcp client update dns [server {both | none}]

Syntax Description

server

(Optional) Specifies that the client will include an FQDN option specifying the “N” flag. The server will not perform any DDNS updates for the client. The server can, of course, override this configuration and do the updates anyway.

  • both --Enables the DHCP client to perform DDNS updates on both A (forward) and PTR (reverse) RRs in the primary DNS server unless the DHCP server has specified in the DHCP ACK FQDN option that it has overridden the client request and has updated the information previously.

Note

 

If the both keyword is specified, it means that the client will include an FQDN option specifying the S flag. This keyword instructs the server that it should attempt to dynamically update both the A and PTR RRs.

  • none --On the client side, specifies that the DHCP client should include the FQDN option; however, it should not attempt any DDNS updates.

Note

 

If the none keyword is not specified, the FQDN option will result in the server updating the PTR RR and neither the server nor the client will update the A RR.

Command Default

No default behavior.

Command Modes

Interface configuration

Command History

Release

Modification

12.3(8)YA

This command was introduced.

12.3(14)T

This command was integrated into Cisco IOS Release 12.3(14)T.

Usage Guidelines

Commands that are configured in interface configuration mode override the commands configured using global configuration mode. The ip dhcp-client update dns command (hyphenated) is the global configuration command.

If you specify the both and none keywords in separate configurations, the DHCP client will update both the A and PTR RRs, and the DHCP server will not perform any updates. If you specify the none and both keywords (in this order), the DHCP client will not perform any updates and the server will update both the A and PTR RRs.

There are two parts to the DDNS update configuration on the client side. First, if the ip ddns update method command is configured on the client, which specifies the DDNS-style updates, then the client will be trying to generate or perform A updates. If the ip ddns update method ddns both command is configured, then the client will be trying to update both A and PTR RRs.

Second, the only way for the client to communicate with the server, with reference to what updates it is generating or expecting the server to generate, is to include an FQDN option when communicating with the server. Whether or not this option is included is controlled on the client side by the ip dhcp-client update dns command in global configuration mode or the ip dhcp client update dns command in interface configuration mode.

Even if the client instructs the server to update both or update none, the server can override the client request and do whatever it was configured to do anyway. If there is an FQDN option in the DHCP interaction as above, then the server can communicate to the client that it was overridden, in which case the client will not perform the updates because it knows that the server has done the updates. Even if the server is configured to perform the updates after sending the ACK (the default), it can still use the FQDN option to instruct the client what updates it will be performing and thus the client will not do the same types of updates.

If the server is configured with the update dns command with or without any keywords, and if the server does not see an FQDN option in the DHCP interaction, then it will assume that the client does not understand DDNS and will automatically act as though it were configured to update both A and PTR RRs on behalf of the client.

Examples

The following example shows how to configure the DHCP client to perform A and PTR RR updates, but the DHCP server will not perform the updates: 


ip dhcp client update dns server none

Related Commands

Command

Description

ip ddns update method

Specifies a method of DDNS updates of A and PTR RRs and the maximum interval between the updates.

ip dhcp compatibility lease-query client

To configure the Dynamic Host Configuration Protocol (DHCP) client to send a lease query according to RFC 4388, use the ip dhcp compatibility lease-query client command in global configuration mode. To disable this configuration, use the no form of this command.

ip dhcp compatibility lease-query client {cisco | standard}

no ip dhcp compatibility lease-query client

Syntax Description

cisco

Configures the DHCP client to use the Cisco standard lease-query message type. This is the default value.

standard

Configures the DHCP client to use the RFC 4388 standard lease-query message type.

Command Default

The DHCP client is configured to use the Cisco standard lease-query message type.

Command Modes

Global configuration (config)

Command History

Release

Modification

12.4(22)T

This command was introduced.

12.2(33)SRC

This command was integrated into Cisco IOS Release 12.2(33)SRC.

12.2(33)SCE1

This command was integrated into Cisco IOS Release 12.2(33)SCE1.

Usage Guidelines

Some DHCP servers support only the RFC 4388 standard of lease query. If the DHCP server supports only the RFC 4388 standard, then you must configure the DHCP client to send a lease query according to the RFC 4388 standard.

The Cisco IOS DHCP client sends a lease query with the message type set to 13 and receives either an ACK (acknowledge) or NAK (deny) from the DHCP server. This is the behavior of the DHCP client as per the Cisco standard.

As per the RFC 4388 standard, if a DHCP server receives a lease query with the message type set to 10, it will reply with one of the following message types:

  • DHCPLEASEUNASSIGNED 11

  • DHCPLEASEUNKNOWN 12

  • DHCPLEASEACTIVE 13

By using the ip dhcp compatibility lease-query client command, you can switch between the Cisco standard and the RFC 4388 standard implementation.

Examples

The following example shows how to configure the DHCP client to switch from the Cisco standard implementation to the RFC 4388 standard implementation: 


Router(config)# ip dhcp compatibility lease-query client standard

Related Commands

Command

Description

ip dhcp compatibility suboption

Configures DHCP compatibility for a relay-agent suboption.

ip dhcp compatibility suboption link-selection

To configure the Dynamic Host Configuration Protocol (DHCP) client to use private as well as the Internet Assigned Numbers Authority (IANA) standard relay agent suboption numbers, use the ip dhcp compatibility suboption link-selection command in global configuration mode. To disable this configuration, use the no form of this command.

ip dhcp compatibility suboption link-selection {cisco | standard}

no ip dhcp compatibility suboption link-selection

Syntax Description

cisco

Configures the DHCP client to use the private Cisco suboption numbers.

standard

Configures the DHCP client to use the standard IANA suboption numbers.

Command Default

Disabled. (The DHCP client is configured to use the private relay agent suboption numbers.)

Command Modes

Global configuration (config)

Command History

Release

Modification

12.4(20)T

This command was introduced.

12.2(33)SRC

This command was integrated into Cisco IOS Release 12.2(33)SRC.

Usage Guidelines

Sometimes new features are implemented in advance of standardization. That is, features are developed before the IANA numbers are assigned to the relay agent suboptions. In these cases, the DHCP client uses the private Cisco relay agent suboption numbers. When the IANA numbers are assigned later, the DHCP client must be able to use both the private as well as the IANA relay suboption numbers. You can use the ip dhcp compatibility suboption link-selection command to configure the DHCP client to use the IANA relay agent suboption numbers.

Examples

The following example shows how to configure the DHCP client to support the relay agent with the IANA standard suboption numbers: 


Router(config)# ip dhcp compatibility suboption link-selection standard

Related Commands

Command

Description

ip dhcp compatibility lease-query client

Configures the DHCP client to send a lease query according to the RFC 4388 standard.

ip dhcp conflict logging

To enable conflict logging on a Dynamic Host Configuration Protocol (DHCP) server, use the ip dhcp conflict logging command in global configuration mode. To disable conflict logging, use the no form of this command.

ip dhcp conflict logging

no ip dhcp conflict logging

Syntax Description

This command has no arguments or keywords.

Command Default

Conflict logging is enabled.

Command Modes

Global configuration

Command History

Release

Modification

12.0(1)T

This command was introduced.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2SX

This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.

Usage Guidelines

A DHCP server database agent should be used to store automatic bindings. If a DHCP server database agent is not used, specify the no ip dhcp conflict logging command to disable the recording of address conflicts. By default, the DHCP server records DHCP address conflicts in a log file.

Examples

The following example disables the recording of DHCP address conflicts: 


no ip dhcp conflict logging

Related Commands

Command

Description

clear ip dhcp conflict

Clears an address conflict from the Cisco IOS DHCP server database.

ip dhcp database

Configures a Cisco IOS DHCP server to save automatic bindings on a remote host called a database agent.

show ip dhcp conflict

Displays address conflicts found by a Cisco IOS DHCP server when addresses are offered to the client.

ip dhcp conflict resolution

To configure Dynamic Host Configuration Protocol (DHCP) address conflict resolution, use the ip dhcp conflict resolution command in global configuration mode. To disable the configuration, use the no form of this command.

ip dhcp conflict resolution [interval minutes]

no ip dhcp conflict resolution

Syntax Description

interval minutes

(Optional) Specifies the time interval, in minutes. Range: 5 to 1440. Default: 60.

Command Default

DHCP address conflict resolution is disabled by default.

Command Modes

Global configuration (config)

Command History

Release

Modification

12.2(33)SRE

This command was introduced.

Usage Guidelines

DHCP addresses added to the conflicted address list may become available after some time. This behavior will eventually cause a major chunk of the IP addresses that are actually available to be blocked.

You can use the ip dhcp conflict resolution command to configure the DHCP server to periodically audit the conflicted address list and clear the inactive IP addresses.

Examples

The following example shows how to configure address conflict resolution on a DHCP server to take place after 65 minutes: 


Router # configure terminal
Router(config)# ip dhcp conflict resolution interval 65

Related Commands

Command

Description

ip dhcp conflict logging

Enables conflict logging on a DHCP server.

ip dhcp database

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

ip dhcp database url [timeout seconds | write-delay seconds | write-delay seconds timeout seconds]

no ip dhcp database url


Note

When using the ip dhcp database command, ensure the correct URL is entered. An incorrect URL may cause the ip dhcp pool command to hang the console as the DHCP service attempts to reach the URL multiple times before returning a failure. This is expected behavior from the DHCP side. Additionally, it is crucial to ensure that the file name is included as part of the ftp/tftp URL to prevent this issue.

Syntax Description

url

Specifies the remote file used to store the automatic bindings. The following are acceptable URL file formats:

  • tftp://host/filename

  • ftp://user:password@host/filename

  • rcp://user@host/filename

  • flash://filename

  • disk0://filename

timeout seconds

(Optional) Specifies how long (in seconds) the DHCP server should wait before aborting a database transfer. Transfers that exceed the timeout period are aborted. By default, DHCP waits 300 seconds (5 minutes) before aborting a database transfer. Infinity is defined as 0 seconds.

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.

Command Default

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

Command Modes

Global configuration

Command History

Release

Modification

12.0(1)T

This command was introduced.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2SX

This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.

Usage Guidelines

A DHCP database agent is any host (for example, an FTP, TFTP, or rcp server) or storage media on the DHCP server (for example, disk0) 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.

The DHCP relay agent can save route information to the same database agents to ensure recovery after reloads.

In the following example, the timeout value and write-delay are specified in two separate command lines: 


ip dhcp database disk0:router-dhcp timeout 60
ip dhcp database disk0:router-dhcp write-delay 60

However, the second configuration overrides the first command line and causes the timeout value to revert to the default value of 300 seconds. To prevent the timeout value from reverting to the default value, configure the following on one command line: 


ip dhcp database disk0:router-dhcp write-delay 60 timeout 60

Examples

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


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

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


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

Related Commands

Command

Description

show ip dhcp database

Displays Cisco IOS DHCP Server database agent information.

ip dhcp debug ascii-client-id

To display the client ID in ASCII format in Dynamic Host Configuration Protocol (DHCP) debug output, use the ip dhcp debug ascii-client-id command in global configuration mode. To disable To disable display of the client ID in ASCII format in Dynamic Host Configuration Protocol (DHCP) debug output, use the no form of this command.

ip dhcp debug ascii-client-id

no ip dhcp debug ascii-client-id

Syntax Description

This command has no arguments or keywords.

Command Default

DHCP debug outputs do not display the client ID in ASCII format.

Command Modes

Global configuration (config)

Command History

Release

Modification

15.2(1)T

This command was introduced.

Usage Guidelines

Use the ip dhcp debug ascii-client-id command to display the client ID in ASCII format in Dynamic Host Configuration Protocol (DHCP) debug output.

Examples

The following example shows how to display the client ID in ASCII format in Dynamic Host Configuration Protocol (DHCP) debug output: 


Router(config)# ip dhcp debug ascii-client-id

Related Commands

Command

Description

odap client

Configures ODAP client parameters.

ip dhcp excluded-address

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

ip dhcp excluded-address [vrf vrf-name] ip-address [last-ip-address]

no ip dhcp excluded-address [vrf vrf-name] ip-address [last-ip-address]

Syntax Description

vrf

(Optional) Excludes IP addresses from a virtual routing and forwarding (VRF) space.

vrf-name

(Optional) The VRF name.

ip-address

The excluded IP address, or first IP address in an excluded address range.

last-ip-address

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

Command Default

The DHCP server can assign any IP address to the DHCP clients.

Command Modes

Global configuration (config)

Command History

Release

Modification

12.0(1)T

This command was introduced.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

Cisco IOS XE Release 2.6

This command was modified. The vrf keyword and vrf-name argument were added.

Usage Guidelines

Use the ip dhcp excluded-address command to exclude a single IP address or a range of IP addresses.

The DHCP server assumes that all pool addresses can be assigned to the clients. You cannot use the ip dhcp excluded-address command to stop the DHCP server from assigning the pool addresses (assigned to an interface using the ip address pool command) to the clients. That is, the ip dhcp excluded-address command is not supported for the addresses assigned using the ip address pool command.

Examples

The following example shows how to configure an excluded IP address range from 172.16.1.100 through 172.16.1.199: 


Router> enable
Router# configure terminal
Router(config)# 
ip dhcp excluded-address vrf vrf1 172.16.1.100 172.16.1.199

Related Commands

Command

Description

ip dhcp pool

Configures a DHCP address pool on a Cisco IOS DHCP server and enters DHCP pool configuration mode.

network (DHCP)

Configures the subnet number and mask for a DHCP address pool on a Cisco IOS DHCP server.

ip address pool

Enables the IP address of an interface to be automatically configured when a DHCP pool is populated with a subnet from IPCP negotiation.

ip dhcp global-options

To enter DHCP global options configuration mode, which is used to configure DHCP-related global configurations, use the ip dhcp global-options command in global configuration mode. To remove DHCP-related global configurations, use the no form of this command.

ip dhcp global-options

no ip dhcp global-options

Syntax Description

This command has no arguments or keywords.

Command Default

DHCP-related global options are not configured.

Command Modes

Global configuration (config)
      

Command History

Release

Modification

15.1(3)S

This command was introduced.

Cisco IOS XE Release 3.5S

This command was integrated into Cisco IOS XE Release 3.5S.

Usage Guidelines

You can configure DHCP options that are common for all pools in DHCP global options configuration mode.

Examples

The following example shows how to enter DHCP global options configuration mode: 


Router(config)# ip dhcp global-options 
Router(config-dhcp-global-options)#

Related Commands

Command

Description

dns-server (config-dhcp-global-options)

Configures the DNS IP servers that are available to DHCP clients on request.

ip dhcp limit lease log

To enable DHCP lease violation logging when a DHCP lease limit threshold is exceeded, use the ip dhcp limit lease log command in global configuration mode. To disable the lease violation logging of DHCP lease violations, use the no form of this command.

ip dhcp limit lease log

no ip dhcp limit lease log

Syntax Description

This command has no arguments or keywords.

Command Default

DHCP lease violation logging is disabled.

Command Modes

Global configuration (config)

Command History

Release

Modification

12.2(33)SRC

This command was introduced.

Usage Guidelines

The ip dhcp limit lease log command logs violations for global- and interface-level lease violations. If this command is configured, any lease limit violations will display in the output of the show ip dhcp limit lease command.

Examples

The following example shows how to enable logging of lease violations: 


Router(config)# ip dhcp limit lease log

Related Commands

Command

Description

ip dhcp limit lease

Limits the number of leases offered to DHCP clients per interface.

show ip dhcp limit lease

Displays the number of times the lease limit threshold has been violated on an interface.

ip dhcp limit lease per interface

To limit the number of leases offered to DHCP clients behind an ATM routed bridge encapsulation (RBE) unnumbered or serial unnumbered interface, use the ip dhcp limit lease per interface command in global configuration mode. To remove the restriction on the number of leases, use the no form of the command.

ip dhcp limit lease per interface lease-limit

no ip dhcp limit lease per interface lease-limit

Syntax Description

lease-limit

Number of leases allowed. The range is from 1 to 65535.

Command Default

The number of leases offered is not limited.

Command Modes

Global configuration (config)

Command History

Release

Modification

12.3(2)T

This command was introduced.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

15.1(1)S

This command was integrated into Cisco IOS Release 15.1(1)S.

Usage Guidelines

This command is not supported on numbered interfaces. The lease limit can be applied only to ATM with RBE unnumbered interfaces or serial unnumbered interfaces.

Examples

The following example shows how to allow three DHCP clients to receive IP addresses. If a fourth DHCP client tries to obtain an IP address, the DHCPDISCOVER messages will not be forwarded to the DHCP server. 


Router(config)# ip dhcp limit lease per interface 3

Related Commands

Command

Description

clear ip dhcp limit lease

Clears the stored lease violation entries.

show ip dhcp limit lease

Displays the number of times the lease limit threshold has been violated.

ip dhcp limited-broadcast-address

To override a configured network broadcast and have the Dynamic Host Configuration Protocol (DHCP) server and relay agent send an all networks, all nodes broadcast to a DHCP client, use the ip dhcp limited-broadcast-address command in global configuration mode. To disable this functionality, use the no form of this command.

ip dhcp limited-broadcast-address

no ip dhcp limited-broadcast-address

Syntax Description

This command has no arguments or keywords.

Command Default

Default broadcast address: 255.255.255.255 (all ones)

Command Modes

Global configuration

Command History

Release

Modification

12.1

This command was introduced.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2SX

This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.

Usage Guidelines

When a DHCP client sets the broadcast bit in a DHCP packet, the DHCP server and relay agent send DHCP messages to clients using the all ones broadcast address (255.255.255.255). If the ip broadcast-address command has been configured to send a network broadcast, the all ones broadcast set by DHCP is overridden. To remedy this situation, use the ip dhcp limited-broadcast-address command to ensure that a configured network broadcast does not override the default DHCP behavior.

Some DHCP clients can only accept an all ones broadcast and may not be able to acquire a DHCP address unless this command is configured on the router interface connected to the client.

Examples

The following example configures DHCP to override any network broadcast: 


ip dhcp limited-broadcast-address

Related Commands

Command

Description

ip broadcast-address

Defines a broadcast address for an interface.

ip dhcp ping packets

To specify the number of packets a Dynamic Host Configuration Protocol (DHCP) server sends to a pool address as part of a ping operation, use the ip dhcp ping packets command in global configuration mode. To prevent the server from pinging pool addresses, use the no form of this command. To return the number of ping packets sent to the default value, use the default form of this command.

ip dhcp ping packets number

no ip dhcp ping packets

default ip dhcp ping packets

Syntax Description

number

The number of ping packets that are sent before the address is assigned to a requesting client. The default value is two packets.

Command Default

Two packets

Command Modes

Global configuration

Command History

Release

Modification

12.0(1)T

This command was introduced.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2SX

This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware.

Usage Guidelines

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.

Setting the number argument to a value of 0 completely turns off DHCP server ping operation .

Examples

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


ip dhcp ping packets 5

Related Commands

Command

Description

clear ip dhcp conflict

Clears an address conflict from the Cisco IOS DHCP server database.

ip dhcp ping timeout

Specifies how long a Cisco IOS DHCP Server waits for a ping reply from an address pool.

show ip dhcp conflict

Displays address conflicts found by a Cisco IOS DHCP server when addresses are offered to the client.