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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 Server cannot 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.
The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. The DHCP server confirms that the IP address has been allocated to the client by returning a DHCPACK unicast message to the client.
The Cisco IOS DHCP Server feature offers the following benefits:
This Cisco IOS DHCP Server feature is supported on the following routers and access servers:
No new or modified standards are supported by this feature.
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.
Before you configure the Cisco IOS DHCP Server feature, complete the following tasks:
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.
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] | Configures the database agent and the interval between database updates and database transfers. |
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 | Disables DHCP address conflict logging. |
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. |
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.
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). |
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] | 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 (/). |
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. |
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. |
Windows Internet Naming Service (WINS) is a name resolution service that Microsoft DHCP clients use to correlate host names to IP addresses within a general grouping of networks. To configure the NetBIOS WINS servers that are available to a Microsoft DHCP client, use the following command in DHCP pool configuration mode:
| 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. |
The NetBIOS node type for Microsoft DHCP clients can be one of four settings: broadcast, peer-to-peer, mixed, or hybrid. To configure 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. |
After a DHCP client has booted, the client begins sending packets to 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. |
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} | Specifies the duration of the lease. The default is a a one-day lease. |
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.
Automatic bindings are IP addresses that have been automatically mapped to the MAC addresses 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.
To configure manual bindings, use the following commands beginning in global configuration mode:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# ip dhcp pool name | Creates a name for the a DHCP server address pool and places you in DHCP pool configuration mode---identified by the (config-dhcp)# prompt. | ||
| Router(config-dhcp)# host address [mask | /prefix-length] | 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 alternative way of specifying the network mask of the client. The prefix length must be preceded by a forward slash (/). | ||
| Router(config-dhcp)# hardware-address hardware-address type or Router(config-dhcp)# client-identifier unique-identifier | 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. | ||
| Router(config-dhcp)# client-name name | (Optional) Specifies the name of the client 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. |
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. To specify a 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. |
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. |
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. |
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. |
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 |
Router# clear ip dhcp conflict address | * | Clears an address conflict from the DHCP database. Specifying 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. |
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 server. |
Router> show ip dhcp conflict [address] | Displays a list of all address conflicts recorded by a specific DHCP server. |
Router# show ip dhcp database [url] | Displays recent activity on the DHCP database. Note Use this command in privileged EXEC mode. |
Router> show ip dhcp server statistics | Displays count information about server statistics and messages sent and received. |
This section provides the following configuration examples:
ip dhcp database ftp://user:password@172.16.4.253/router-dhcp write-delay 120
In the following example, three DHCP address pools are created: one in network 172.16.0.0, one in subnetwork 172.16.1.0, and one in subnetwork 172.16.2.0. Attributes from network 172.16.0.0, 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.
| 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 | Default routers | 172.16.2.100 |
DNS server | 172.16.1.102 --- --- --- --- | ||||
NetBIOS name server | 172.16.1.103 --- --- --- --- | ||||
NetBIOS node type | h-node --- --- --- --- | ||||
ip dhcp database ftp://user:password@172.16.4.253/router-dhcp write-delay 120ip dhcp excluded-address 172.16.1.100 172.16.1.103ip dhcp excluded-address 172.16.2.100 172.16.2.103!ip dhcp pool 0network 172.16.0.0 /16domain-name cisco.comdns-server 172.16.1.102 172.16.2.102netbios-name-server 172.16.1.103 172.16.2.103netbios-node-type h-node!ip dhcp pool 1network 172.16.1.0 /24default-router 172.16.1.100 172.16.1.101lease 30!ip dhcp pool 2network 172.16.2.0 /24default-router 172.16.2.100 172.16.2.101lease 30
ip dhcp pool Mars host 172.16.2.254 hardware-address 02c7.f800.0422 ieee802 client-name Mars
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
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.
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.
To specify the name of the default boot 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
filename | Specifies the name of the file that is used as a boot image. |
No default behavior or values.
DHCP pool configuration
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
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 | *
address | The address of the binding you want to clear. |
* | Clears all automatic bindings. |
Privileged EXEC
Typically, the address denotes the client's IP address. If the asterisk (*) character is used as the address parameter, DHCP clears all automatic bindings.
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
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 | *
address | The IP address of the host that contains the conflicting address you want to clear. |
* | Clears all address conflicts. |
Privileged EXEC
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
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 statisticsThis command has no arguments or keywords.
Privileged EXEC
This command first appeared in Cisco IOS Release 12.0(1)T.
The show ip dhcp server statistics command displays DHCP counters. All counters are cumulative. The counters will be initialized, or set to zero, with this command.
The following example resets all DHCP counters to zero:
clear ip dhcp server statistics
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
unique-identifier | The distinct identification of the client in dotted-hexadecimal notation, for example, 01b7.0813.8811.66. |
None
DHCP pool configuration
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
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
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
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
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.
default-router address [address2 ... address8]
address | Specifies the IP address of a router. One IP address is required, although you can specify up to eight addresses in one command line. |
None
DHCP pool configuration
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
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]
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.
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
To specify the domain name for a Dynamic Host Configuration Protocol (DHCP) client, use the domain-name DHCP pool configuration command. Use the no form of this command to remove the domain name.
domain-name domain
domain | Specifies the client's domain name string. |
None.
DHCP pool configuration
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
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
hardware-address | Specifies the MAC address of the client's hardware platform. |
type | Indicates the protocol of the hardware platform. Strings and values are acceptable. The string options are:
The value options are:
If no type is specified, the default protocol is Ethernet. |
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
To specify 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]
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
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.
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
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 loggingThis command has no arguments or keywords.
Conflict logging is enabled.
Global configuration
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
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]
url | Specifies the remote file used to store the automatic bindings. Following are the acceptable URL file formats:
|
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 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. |
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.
The administrator 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
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
show ip dhcp database
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]
low-address | The excluded IP address, or first IP address in an excluded address range. |
high-address | (Optional) The last IP address in the excluded address range. |
All IP pool addresses are assignable.
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)
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
count | Indicates the number of ping packets that are sent before assigning the address to a requesting client. The default value is two packets. |
Two packets
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
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
milliseconds | The amount of time in milliseconds that the DHCP server waits for a 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. |
500 milliseconds
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
To configure a Dynamic Host Configuration Protocol (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
name | Can either be a symbolic string (such as "engineering") or an integer (such as 0). |
DHCP address pools are not configured.
Global configuration
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)
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 checkThis command has no arguments or keywords.
The DHCP server checks relay information. Invalid messages are dropped.
Global configuration
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
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 optionThis command has no arguments or keywords.
The DHCP server does not insert 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. 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
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}
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. |
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 dropip dhcp relay information policy keepip dhcp relay information policy replace
ip dhcp relay information check
ip dhcp relay information option
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}
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. |
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
ip dhcp pool
To configure NetBIOS Windows Internet Naming Service (WINS) name servers that are available to Microsoft Dynamic Host Configuration Protocol (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]
address | Specifies the IP address of the NetBIOS WINS name server. |
None
DHCP pool configuration
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
To configure 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
type | Specifies the NetBIOS node type. Valid types are:
|
None
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
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]
network-number | The IP address of the DHCP address pool. |
mask | (Optional) The bit combination that renders which portion of the address of the DHCP address pool refers to the network or subnet and which part refers to the host. |
/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
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.
The following example configures 172.16.0.0/16 as the DHCP pool's subnetwork number and mask:
network 172.16.0.0 /16
host
ip dhcp excluded-address
ip dhcp pool
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]
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.
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
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}
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. |
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
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
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 dhcpThis command has no keywords or arguments.
The feature is enabled.
Global configuration
This command first appeared in Cisco IOS Release 12.0(1)T.
The following example enables DHCP services on the DHCP server:
service dhcp
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]
address | (Optional) Specifies the IP address of the DHCP client for which bindings will be displayed. |
None
EXEC
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 172.16.1.11IP address Hardware address Lease expiration Type172.16.1.11 00a0.9802.32de Feb 01 1998 12:00 AM AutomaticRouter>show ip dhcp binding 172.16.3.254IP address Hardware address Lease expiration Type172.16.2.254 02c7.f800.0422 Infinite Manual
| 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. |
clear ip dhcp binding
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]
address | (Optional) Specifies the IP address of the conflict found. |
None
EXEC
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 DHCP server has offered that have conflicts with other devices. Table 3 lists descriptions of the fields in the example.
Router>show ip dhcp conflictIP address Detection Method Detection time172.16.1.32 Ping Feb 16 1998 12:28 PM172.16.1.64 Gratuitous ARP Feb 23 1998 08:12 AM
| 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. |
clear ip dhcp conflict
ip dhcp ping packets
ip dhcp ping timeout
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]
url | (Optional) Specifies the remote file used to store automatic DHCP bindings. Following are the acceptable URL file formats:
|
If a URL is not specified, all database agent records are shown. Otherwise, only information about the specified agent is displayed.
Privileged EXEC
This command first appeared in Cisco IOS Release 12.0(1)T.
The following example shows all DHCP server database agent information. Table 4 lists descriptions 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
| Field | Description |
|---|---|
URL | Specifies the remote file used to store automatic DHCP bindings. Following are the acceptable URL file formats:
|
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
To display Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server statistics, use the show ip dhcp server statistics EXEC command.
show ip dhcp server statisticsThis command has no arguments or keywords.
None
EXEC
This command first appeared in Cisco IOS Release 12.0(1)T.
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
| 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. |
clear ip dhcp server statistics
This section describes the following new debug command:
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}
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 relationships in a radix tree). |
DHCP server debugging is not enabled.
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 eventsRouter#debug ip dhcp server packetsDHCPD: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 linkageDHCPD: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.
show ip dhcp bindings
show ip dhcp database
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 pool by the DHCP server (automatic address 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.
database agent---Any host storing the DHCP bindings database, for example, a Trivial File Transfer 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 assigned IP address to a host. Manual allocation allows DHCP to be used to eliminate the error-prone process of manually configuring hosts with IP addresses.
server---Any host providing configuration parameters.
Posted: Fri Mar 17 09:31:09 PST 2000
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