IP Addressing: DHCP Configuration Guide, Cisco IOS XE Gibraltar 16.12.x
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Cisco IOS XE Dynamic Host Configuration Protocol (DHCP) client software provides the flexibility to include various configuration
options for the DHCP client. A DHCP client is defined as an Internet host using DHCP to obtain configuration parameters such
as an IP address. This module describes the concepts and tasks needed to configure the Cisco IOS XE DHCP client.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module,
and to see a list of the releases in which each feature is supported, see the feature information table.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature
Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Information for the Cisco IOS XE DHCP Client
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for the Cisco IOS XE DHCP Client
Feature Name
Releases
Feature Configuration Information
DHCP Client
Cisco IOS XE Release 2.3
Cisco IOS XE Release 3.9S
A DHCP client is defined as an Internet host using DHCP to obtain configuration parameters such as an IP address.
The following command was introduced by this feature:
ipaddressdhcp
Configurable DHCP Client
Cisco IOS XE Release 2.3
Cisco IOS XE Release 3.9S
The configurable DHCP client functionality allows a DHCP client to use a user-specified client identifier, class identifier,
or suggested lease time when requesting an address from a DHCP server.
The following commands were introduced by this feature:
ipdhcpclientclass-id,
ipdhcpclientclient-id,
ipdhcpclienthostname,
ipdhcpclientlease,
ipdhcpclientrequest
DHCPv4 Client Options
Cisco IOS XE Fuji 16.9.1
The DHCP Client supports configuration of all 1-254 options.
DHCP Client Options using unicast mode
Cisco IOS XE Amsterdam 17.2.1
Introduces support for unicast mode on DHCP. This helps with splitting the horizon therefore improving security of the network.
Information About the DHCP Client
DHCP Client Operation
When a Dynamic Host Configuration Protocol (DHCP) client requests an IP address from a DHCP server on a Cisco IOS XE platform,
the default process includes:
DHCPDISCOVERY (broadcast)
DHCPOFFER (broadcast)
DHCPREQUEST (broadcast)
DHCPACK (unicast)
The DHCP on Cisco IOS XE platform supports only broadcast mode with the DHCPOFFER. From Cisco IOS XE Amsterdam Release 17.2,
the DHCP on IOS XE platform also supports unicast mode. The DHCP unicast mode helps to split the horizon for security consideration.
The DHCP broadcast mode is enabled by default. To enable the DHCP unicast mode, configure the ip dhcp client broadcast-flag clear command on the DHCP client. After configuring the command, the DHCPOFFER is sent as a unicast message.
The DHCP provides a framework for passing configuration information to hosts on a TCP/IP network. A DHCP client is an Internet
host using DHCP to obtain configuration parameters such as an IP address. The following figure shows the basic steps that
occur when a DHCP client requests an IP address from a DHCP server. The client, Host A, sends a DHCPDISCOVER broadcast message
to locate a DHCP server. A DHCP server offers configuration parameters (such as an IP address, a MAC address, a domain name,
and a lease for the IP address) to the client in a DHCPOFFER unicast/broadcast message.
Figure 1. DHCP Request for an IP Address from a DHCP Server
A DHCP client may receive offers from multiple DHCP servers. However, it can accept any one of the offers; the client usually
accepts the first offer it receives. The offer from the DHCP server is not a guarantee that the IP address is allocated to
the client. However, the server usually reserves the address until the client has had a chance to formally request the address
.
The client returns a formal request for the offered IP address to the DHCP server in a DHCPREQUEST broadcast message. The
DHCP server confirms that the IP address is allocated to the client by returning a DHCPACK unicast message to the client.
DHCP Client Overview
The configurable dynamic host configuration protocol client functionality allows a DHCP client to use a user-specified client
identifier, class identifier, or suggested lease time when requesting an address from a DHCP server.
Configuration parameters and other control information are carried in tagged data items that are stored in the options field
of the DHCP message. The DHCP client provides flexibility by allowing the following options to be configured for a DHCP client:
Option 12—This option specifies the name of the client. The name may or may not be qualified with the local domain.
Option 51—This option is used in a client request (DHCPDISCOVER or DHCPREQUEST) to allow the client to request a lease time
for the IP address.
Option 55—This option allows the DHCP client to request certain options from the DHCP server. The
ipdhcpclientrequest command allows the system administrator to turn off some of the requested options, thus removing them from the request list.
Option 60—This option allows the user to configure the vendor class identifier string to use in the DHCP interaction.
Option 61—This option is used by DHCP clients to specify their unique identifier. DHCP servers use this value to index their
database of address bindings. This value is expected to be unique for all clients in an administrative domain.
Option 77—This option is used by a DHCP clients to optionally identify the type or category of user or applications it represents.
The information contained in this option represents the user class of which the client is a member. Based on this class, a
DHCP server selects the appropriate address pool to assign an address to the client and the appropriate configuration parameters.
Option 120—This option is used to specify a 32-bit (binary) IPv4 address to be used by the Session Initiation Protocol (SIP)
client to locate a SIP server.
Option 121—This option is used to configure classless static routes by specifying classless network destinations; that is,
each routing table entry includes a subnet mask. Upto ten classless static routes are supported using option 121 on the DHCP
client.
Note
If a request includes both static routes and classless static routes, the client uses only the classless static routes. If
the DHCP server returns both a classless static route option and a router option, the DHCP client ignores the router option.
Option 124—This option is used by DHCP clients and servers to exchange vendor-class information.
Option 125—This option is used by DHCP clients and servers to exchange vendor-specific information.
How to Configure the DHCP Client
Configuring the DHCP Client
Cisco devices running Cisco software include the Dynamic Host Configuration Protocol (DHCP) server and relay agent software,
which are enabled by default. Your device can act as both the DHCP client and the DHCP server. Use the
ip address dhcp command to obtain IP address information for the configured interface.
SUMMARY STEPS
enable
configure terminal
interfacetype number
ip address dhcp
end
debug dhcp detail
debug ip dhcp server packets
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configure terminal
Example:
Device# configure terminal
Enters global configuration mode.
Step 3
interfacetype number
Example:
Device(config)# interface GigabitEthernet 0/0/1
Configures an interface type and enters interface configuration mode.
Step 4
ip address dhcp
Example:
Device(config-if)# ip address dhcp
Acquires an IP address on an interface from DHCP.
Step 5
end
Example:
Device(config-if)# end
Returns to privileged EXEC mode.
Step 6
debug dhcp detail
Example:
Device# debug dhcp detail
Displays the DHCP packets that were sent and received.
Step 7
debug ip dhcp server packets
Example:
Device# debug ip dhcp server packets
Displays the server side of the DHCP interaction.
Troubleshooting Tips
To verify the configuration, you can use the debugdhcpdetail EXEC command to display the DHCP packets that were sent and received. To display the server side of the DHCP interaction,
use the debugipdhcpserverpackets command.
Configuration Examples for the DHCP Client
Configuring the DHCP Client Example
The figure below shows a simple network diagram of a DHCP client on an Ethernet LAN.
Figure 2. Topology Showing DHCP Client with GigabitEthernet Interface
On the DHCP server, the configuration is as follows:
ip dhcp pool 1
network 10.1.1.0 255.255.255.0
lease 1 6
On the DHCP client, the configuration is as follows on interface GigabitEthernet 0/0/0:
interface GigabitEthernet 0/0/0
ip address dhcp
This configuration allows the DHCP client to acquire an IP address from the DHCP server through GigabitEthernet interface
0/0/0.
Customizing the DHCP Client Configuration Example
The following example shows how to customize the DHCP client configuration with various options on GigabitEthernet interface
0/0/1:
interface GigabitEthernet 0/0/1
ip dhcp client client-id ascii my-test1
ip dhcp client class-id my-class-id
ip dhcp client lease 0 1 0
ip dhcp client hostname sanfran
no ip dhcp client request tftp-server-address
ip address dhcp
The following example shows DHCP Client configuration on GigabitEthernet 0/0/1 to generically request options:
The following example shows how to configure DHCP Client options with parameters, IP address and string:
!
interface GigabitEthernet 0/0/1
ip dhcp client option 1 ip 1.1.1.1
ip dhcp client option 13 ascii test13
ip dhcp client option 14 ascii test14
ip dhcp client option 16 ip 1.1.1.16
ip dhcp client option 46 ascii test46
ip dhcp client option 47 ascii test47
ip dhcp client option 50 ip 1.1.1.50
ip dhcp client option 51 ascii test51
ip dhcp client option 52 ascii test52
ip dhcp client option 54 ascii test54
ip dhcp client option 58 ascii test58
ip dhcp client option 59 ascii test59
ip dhcp client option 60 ascii test60
ip dhcp client option 61 ascii test61
ip dhcp client option 62 ascii test62
ip dhcp client option 63 ip 1.1.1.63
ip dhcp client option 64 ascii test64
ip dhcp client option 65 ip 1.1.1.65
ip dhcp client option 67 ascii test67
ip dhcp client option 90 ascii test90
ip dhcp client option 116 ascii test116
ip dhcp client option 118 ip 1.1.1.118
ip dhcp client option 220 ip 1.1.1.220
ip dhcp client option 221 ascii test221
ip address dhcp
shutdown
!
The following example shows how to configure DHCP Client options with class::
Example: Configuring the DHCP Client in Unicast Mode
The following example shows how to configure DHCP Client in unicast mode:
The figure below shows a simple network diagram of a DHCP client in unicast mode.
Figure 3. Topology Showing DHCP Client with GigabitEthernet Interface
Client:
interface GigabitEthernet6
ip address dhcp
ip dhcp client broadcast-flag clear
Relay:
interface GigabitEthernet6
ip address 1.1.1.10 255.255.255.0
ip helper-address 111.111.111.111
!
interface GigabitEthernet5
ip address 10.10.10.1 255.255.255.0
ip route 111.111.111.111 255.255.255.255 GigabitEthernet5
Server:
interface Loopback10
ip address 111.111.111.111 255.255.255.255
no shutdown
!
ip route 0.0.0.0 0.0.0.0 GigabitEthernet5
ip dhcp pool Cisco
network 11.11.11.0 255.255.255.0
default-router 11.11.11.1
“Configuring the DHCP Server On-Demand Address Pool Manager” module
DHCP relay agent configuration
“Configuring the Cisco IOS XE DHCP Relay Agent” module
DHCP advanced features
“Configuring DHCP Services for Accounting and Security” module
RFCs
RFCs
Title
RFC 2131
Dynamic Host Configuration Protocol
RFC 2132
DHCP Options and BOOTP Vendor Extensions
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