ios xr

Implementing the Dynamic Host ConfigurationProtocol

This module describes the concepts and tasks you will use to configure Dynamic Host Configuration Protocol(DHCP).

Feature History for Implementing the Dynamic Host Configuration Protocol

ModificationRelease

This feature was introduced.Release 2.0

The DHCP IPv6 Information Pool configuration procedure wasadded and DCHP relay information description was updated.

Release 3.4.0

The DHCP CLI was modified.Release 3.7.0

Prerequisites for Configuring DHCP Relay Agent , page 1

Information About DHCP Relay Agent, page 2

How to Configure and Enable DHCP Relay Agent, page 2

Configuring a DHCP Proxy Profile, page 13

DHCPv4 Client, page 15

Information About Configuring DHCP IPv6 Information Pools, page 16

How to Configure DHCP IPv6 Information Pools, page 17

Configuration Examples for the DHCP Relay Agent, page 18

Additional References, page 20

Prerequisites for Configuring DHCP Relay AgentThe following prerequisites are required to configure a DHCP relay agent:

Cisco IOS XR IP Addresses and Services Configuration Guide for the Cisco CRS Router, Release 4.2.x OL-26066-04 1

Youmust be in a user group associated with a task group that includes the proper task IDs. The commandreference guides include the task IDs required for each command. If you suspect user group assignmentis preventing you from using a command, contact your AAA administrator for assistance.

A configured and running DHCP client and DHCP server

Connectivity between the relay agent and DCHP server

Information About DHCP Relay AgentA DHCP relay agent is a host that forwards DHCP packets between clients and servers that do not reside ona shared physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router whereIP datagrams are switched between networks transparently.

DHCP clients use User Datagram Protocol (UDP) broadcasts to send DHCPDISCOVERmessages when theylack information about the network to which they belong.

If a client is on a network segment that does not include a server, a relay agent is needed on that networksegment to ensure that DHCP packets reach the servers on another network segment. UDP broadcast packetsare not forwarded, because most routers are not configured to forward broadcast traffic. You can configure aDHCP relay profile and configure one or more helper addresses in it. You can assign the profile to an interfaceor a VRF.

Figure 1: Forwarding UDP Broadcasts to a DHCP Server Using a Helper Address, on page 2 demonstratesthe process. The DHCP client broadcasts a request for an IP address and additional configuration parameterson its local LAN. Acting as a DHCP relay agent, Router B picks up the broadcast, changes the destinationaddress to the DHCP server's address and sends the message out on another interface. The relay agent insertsthe IP address of the interface, on which the relay profile into the gateway address (giaddr) field of the DHCPpacket, which enables the DHCP server to determine which subnet should receive the offer and identify theappropriate IP address range. The relay agent unicasts the messages to the server address, in this case 172.16.1.2(which is specified by the helper address in the relay profile).

Figure 1: Forwarding UDP Broadcasts to a DHCP Server Using a Helper Address

How to Configure and Enable DHCP Relay AgentThis section contains the following tasks:

Cisco IOS XR IP Addresses and Services Configuration Guide for the Cisco CRS Router, Release 4.2.x2 OL-26066-04

Implementing the Dynamic Host Configuration ProtocolInformation About DHCP Relay Agent

Configuring and Enabling the DHCP Relay Agent

Configuring a DHCP Relay ProfileThis task describes how to configure and enable the DHCP relay agent.

SUMMARY STEPS

1. configure2. dhcp ipv43. profile profile-name relay4. helper-address [vrf vrf- name ] address5. Use the commit or end command.

DETAILED STEPS

PurposeCommand or Action

Enters global configuration mode.configure

Example:

RP/0/RP0/CPU0:router# configure

Step 1

Enters DHCP IPv4 configuration mode.dhcp ipv4

Example:

RP/0/RP0/CPU0:router(config)# dhcp ipv4

Step 2

Enters DHCP IPv4 profile relay submode.profile profile-name relay

Example:

RP/0/RP0/CPU0:router(config-dhcpv4)# profile client relay

Step 3

ForwardsUDP broadcasts, includingBOOTPand DHCP.

helper-address [vrf vrf- name ] address

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# helper-address

Step 4

The value of the address argumentcan be a specific DHCP server addressor a network address (if other DHCPvrf

foo 10.10.1.1 servers are on the destination networksegment). Using the network addressenables other servers to respond toDHCP requests.

For multiple servers, configure onehelper address for each server.


Implementing the Dynamic Host Configuration ProtocolConfiguring and Enabling the DHCP Relay Agent


commitSaves the configuration changesand remains within the configuration session.

Use the commit or end command.Step 5

endPrompts user to take one of theseactions:

Yes Saves configuration changes andexits the configuration session.

NoExits the configuration sessionwithout committing the configurationchanges.

CancelRemains in the configurationmode, without committing theconfiguration changes.

Configuring the DHCPv6 (Stateless) Relay AgentPerform this task to specify a destination address to which client messages are forwarded and to enableDynamic Host Configuration Protocol (DHCP) for IPv6 relay service on the interface.

SUMMARY STEPS

1. configure2. dhcp ipv63. interface type interface-path-id relay4. destination ipv6-address5. Use the commit or end command.

DETAILED STEPS



Example:


Step 1

Enables DHCP for IPv6 and enters the DHCP IPv6 configuration mode.dhcp ipv6

Example:

Step 2

RP/0/RP0/CPU0:router(config) # dhcp


Implementing the Dynamic Host Configuration ProtocolConfiguring the DHCPv6 (Stateless) Relay Agent


ipv6RP/0/RP0/CPU0:router(config-dhcpv6)#

Specifies an interface type and interface-path-id, places the router ininterface configuration mode, and enables DHCPv6 relay service on theinterface.

interface type interface-path-id relay

Example:

Step 3

RP/0/RP0/CPU0:router(config-dhcpv6) #interface tenGigE 0/5/0/0 relay

Specifies a destination address to which client packets are forwarded.destination ipv6-addressStep 4

Example:When relay service is enabled on an interface, a DHCP for IPv6 messagereceived on that interface is forwarded to all configured relay destinations.The incoming DHCP for IPv6 message may have come from a client onthat interface, or it may have been relayed by another relay agent.RP/0/RP0/CPU0:router(config-dhcpv6-if)

# destination 10:10::10

commitSaves the configuration changes and remains within theconfiguration session.


endPrompts user to take one of these actions:

Yes Saves configuration changes and exits the configurationsession.

NoExits the configuration session without committing theconfiguration changes.

CancelRemains in the configuration mode, without committingthe configuration changes.

Enabling DHCP Relay Agent on an InterfaceThis task describes how to enable the Cisco IOS XR DHCP relay agent on an interface.

On Cisco IOS XR software, the DHCP relay agent is disabled by default.Note

SUMMARY STEPS

1. configure2. dhcp ipv43. interface type name relay profile profile-name4. Use the commit or end command.


Implementing the Dynamic Host Configuration ProtocolEnabling DHCP Relay Agent on an Interface

DETAILED STEPS



Example:


Step 1

Enters DHCP IPv4 configuration submode.dhcp ipv4

Example:

RP/0/RP0/CPU0:router(config-if)# dhcp ipv4

Step 2

Attaches a relay profile to an interface.interface type name relay profile profile-name

Example:

RP/0/RP0/CPU0:router(config-dhcpv4)# interface

Step 3

FastEthernet0/0 relay profile client

commitSaves the configuration changes and remains withinthe configuration session.



Yes Saves configuration changes and exits theconfiguration session.

NoExits the configuration session without committingthe configuration changes.

CancelRemains in the configuration mode, withoutcommitting the configuration changes.

Disabling DHCP Relay on an InterfaceThis task describes how to disable the DHCP relay on an interface by assigning the none profile to the interface.

SUMMARY STEPS

1. configure2. dhcp ipv43. interface type name none4. Use the commit or end command.


Implementing the Dynamic Host Configuration ProtocolDisabling DHCP Relay on an Interface

DETAILED STEPS



Example:


Step 1


Example:


Step 2

Disables the DHCP relay on the interface.interface type name none

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# interface

Step 3

pos 0/1/4/1 none

commitSaves the configuration changes andremains within the configuration session.






Enabling DHCP Relay on a VRFThis task describes how to enable DHCP relay on a VRF.

SUMMARY STEPS

1. configure2. dhcp ipv43. vrf vrf-name relay profile profile-name4. Use the commit or end command.


Implementing the Dynamic Host Configuration ProtocolEnabling DHCP Relay on a VRF

DETAILED STEPS



Example:


Step 1


Example:


Step 2

Enables DHCP relay on a VRF.vrf vrf-name relay profile profile-name

Example:

RP/0/RP0/CPU0:router(config-dhcpv4)#vrf default relay profile client

Step 3







Configuring the Relay Agent Information FeatureThis task describes how to configure the DHCP relay agent information option processing capabilities.

A DHCP relay agent may receive a message from another DHCP relay agent that already contains relayinformation. By default, the relay information from the previous relay agent is replaced (using the replaceoption).


Implementing the Dynamic Host Configuration ProtocolConfiguring the Relay Agent Information Feature

SUMMARY STEPS

1. configure2. dhcp ipv43. profile profile-name relay4. relay information option5. relay information check6. relay information policy {drop | keep}7. relay information option allow-untrusted8. Use the commit or end command.

DETAILED STEPS



Example:


Step 1


Example:


Step 2

Enters DHCP IPv4 profile relay mode.profile profile-name relay

Example:


Step 3

Enables the system to insert the DHCP relay agentinformation option (option-82 field) in forwardedBOOTREQUEST messages to a DHCP server.

relay information option

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# relayinformation option

Step 4

This option is injected by the relay agent whileforwarding client-originated DHCP packets to theserver. Servers recognizing this option can use theinformation to implement IP address or otherparameter assignment policies. When replying,the DHCP server echoes the option back to therelay agent. The relay agent removes the optionbefore forwarding the reply to the client.

The relay agent information is organized as asingle DHCP option that contains one or moresuboptions. These options contain the informationknown by the relay agent.

The supported suboptions are:




Remote ID

Circuit ID

This function is disabled bydefault.

Note

(Optional) Configures DHCP to check that the relayagent information option in forwarded BOOTREPLYmessages is valid.

relay information check

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# relayinformation check

Step 5

By default, DHCP checks that the option-82 fieldin DHCP reply packets, received from the DHCPserver, is valid. If an invalid message is received,the relay agent drops the message. If a validmessage is received, the relay agent removes theoption-82 field and forwards the packet.

Use the relay information check commandto reenable this functionality if the functionalityhas been disabled.

Note

(Optional) Configures the reforwarding policy for aDHCP relay agent; that is, whether the relay agent willdrop or keep the relay information.

relay information policy {drop | keep}

Example:

RP/0/RP0/CPU0:router(config)# dhcp relay informationpolicy drop

Step 6

(Optional) Configures the DHCP IPv4 Relay not todiscard BOOTPREQUEST packets that have an existingrelay information option and the giaddr set to zero.

relay information option allow-untrusted

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# relayinformation check

Step 7

commitSaves the configuration changes and remainswithin the configuration session.



Yes Saves configuration changes and exits theconfiguration session.

NoExits the configuration session withoutcommitting the configuration changes.

CancelRemains in the configuration mode,without committing the configuration changes.



Configuring Relay Agent Giaddr PolicyThis task describes how to configure BOOTPREQUEST packets for Dynamic Host Configuration Protocol(DHCP) IPv4 Relay processes, that already contain a nonzero giaddr attribute.

SUMMARY STEPS

1. configure2. dhcp ipv43. profile profile-name relay4. giaddr policy {replace | drop}5. Use the commit or end command.

DETAILED STEPS



Example:


Step 1

Enables the DHCP IPv4 configuration submode.dhcp ipv4

Example:


Step 2

Enables profile relay submode.profile profile-name relay

Example:


Step 3

Specifies the giaddr policy.giaddr policy {replace | drop}Step 4

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# giaddrpolicy drop

replaceReplaces the existing giaddr valuewith a value that it generates.

dropDrops the packet that has an existingnonzero giaddr value.

commitSaves the configuration changes andremains within the configuration session.



Yes Saves configuration changes and exitsthe configuration session.


Implementing the Dynamic Host Configuration ProtocolConfiguring Relay Agent Giaddr Policy


NoExits the configuration session withoutcommitting the configuration changes.

CancelRemains in the configuration mode,without committing the configuration changes.

Configuring the Broadcast Flag PolicyThis task describes how to configure DHCP IPv4 Relay to broadcast BOOTPREPLY packets only if theDHCP IPv4 broadcast flag is set in the DHCP IPv4 header.

By default, the DHCP IPv4 Relay always broadcasts BOOTPREPLY packets.Note

SUMMARY STEPS

1. configure2. dhcp ipv43. profile profile-name relay4. broadcast-flag policy check5. Use the commit or end command.

DETAILED STEPS



Example:


Step 1

Configures DHCP IPv4 mode.dhcp ipv4

Example:


Step 2

Enables profile relay mode.profile profile-name relay

Example:


Step 3


Implementing the Dynamic Host Configuration ProtocolConfiguring the Broadcast Flag Policy


Enables checking of the broadcast flag inpackets.

broadcast-flag policy check

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# broadcast-flagpolicy check

Step 4

commitSaves the configuration changesand remains within the configurationsession.



Yes Saves configuration changesand exits the configuration session.


CancelRemains in theconfiguration mode, withoutcommitting the configurationchanges.

Configuring a DHCP Proxy ProfileThe DHCP proxy performs all the functions of a relay and also provides some additional functions. The DHCPproxy conceals DHCP server details from DHCP clients. The DHCP proxy modifies the DHCP replies suchthat the client considers the proxy to be the server. In this state, the client interacts with the proxy as if it isthe DHCP server.

This task describes how to configure and enable the DHCP proxy profile.

SUMMARY STEPS

1. configure2. dhcp ipv43. profile profile-name proxy4. helper-address [vrf vrf- name ] address [ giaddr gateway-address ]5. Use the commit or end command.


Implementing the Dynamic Host Configuration ProtocolConfiguring a DHCP Proxy Profile

DETAILED STEPS



Example:


Step 1


Example:


Step 2

Enters DHCP IPv4 profile proxy submode.profile profile-name proxy

Example:

RP/0/RP0/CPU0:router(config-dhcpv4)# profile client proxy

Step 3

ForwardsUDP broadcasts, includingBOOTPand DHCP.

helper-address [vrf vrf- name ] address [ giaddr gateway-address ]

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-proxy-profile)# helper-address

Step 4

The value of the address argumentcan be a specific DHCP server addressor a network address (if other DHCPvrf

foo 10.10.1.1 servers are on the destination networksegment). Using the network addressenables other servers to respond toDHCP requests.

For multiple servers, configure onehelper address for each server.

commitSaves the configuration changesand remains within the configuration session.







Implementing the Dynamic Host Configuration ProtocolConfiguring a DHCP Proxy Profile

DHCPv4 ClientThe Dynamic Host Configuration Protocol (DHCP) client functionality enables the router interfaces todynamically acquire the IPv4 address using DHCP.

The DHCP provides configuration parameters to Internet hosts. DHCP consists of two components:

a protocol to deliver host-specific configuration parameters from a DHCP server to a host.

a mechanism to allocate network addresses to hosts.

DHCP is built on a client-server model, where designated DHCP server hosts allocate network addresses, anddeliver configuration parameters to dynamically configured hosts.

A relay agent is required if the client and server are not on the same Layer 2 network. The relay agent usuallyruns on the router, and is required because the client device does not know its own IP address initially. Theagent sends out a Layer 2 broadcast to find a server that has this information. The router relays these broadcaststo the DHCP server, and forwards the responses back to the correct Layer 2 address so that the correct devicegets the correct configuration information.

DHCP has the ability to allocate IP addresses only for a configurable period of time, called the lease period.If the client is required to retain this IP address for a longer period beyond the lease period, the lease periodmust be renewed before the IP address expires. The client renews the lease based on configuration that wassent from the server. The client unicasts a REQUEST message using the IP address of the server. When aserver receives the REQUEST message and responds with an ACK message. The lease period of the clientis extended by the lease time configured in the ACK message.

Restrictions and Limitations

DHCP client can be enabled only on management interfaces.

Either DHCP or static IP can be configured on an interface.

Enabling DHCP Client on an InterfaceThe DHCP client can be enabled at an interface level. The DHCP component receives a notification whenDHCP is enabled or disabled on an interface.

SUMMARY STEPS

1. configure2. interface MgmtEth rack/slot/CPU0/port3. interface ipv4 address dhcp


Implementing the Dynamic Host Configuration ProtocolDHCPv4 Client

DETAILED STEPS



Example:


Step 1

Enters interface configuration mode.interface MgmtEth rack/slot/CPU0/port

Example:RP/0/RP0/CPU0:router(config)#interface mgmtEth 0/0/CPU0/0

Step 2

Configure DHCP on the interface.interface ipv4 address dhcp

Example:

Step 3

RP/0/RP0/CPU0:router(config)# interface mgmtEth 0/0/CPU0/0ipv4 address dhcp

Example:dhcp Enable IPv4 DHCP client

The following example shows a sample of using IPv4 address command:RP/0/0/CPU0:ios(config)#interface mgmtEth 0/0/CPU0/0 ipv4 address ?A.B.C.D/prefix IPv4 address/prefix or IPv4 address and Maskdhcp Enable IPv4 DHCP client

Information About Configuring DHCP IPv6 Information PoolsA DHCP IPv6 configuration information pool is a named entity that includes information about availableconfiguration parameters and policies that control assignment of the parameters to clients from the pool. Apool is configured independently of the DHCP service and is associated with the DHCP service through thecommand line interface.

Each configuration pool can contain the following configuration parameters and operational information:

Prefix delegation information, which could include a list of available prefixes for a particular client andassociated preferred and valid lifetimes

Domain name service (DNS) serversList of IPv6 addresses of DNS servers

Domain search listString containing domain names for DNS resolution

SIP server addressList of IPv6 addresses of SIP server

SIP server domain listString containing domain names for SIP server


Implementing the Dynamic Host Configuration ProtocolInformation About Configuring DHCP IPv6 Information Pools

How to Configure DHCP IPv6 Information PoolsThis section contains the following task:

Configuring Cisco IOS XR DHCP IPv6 Information Pool OptionThis task describes how to enable support for the DHCP IPv6 information pool option with the name pool1.

SUMMARY STEPS

1. configure2. dhcp ipv63. pool pool-name4. Use the commit or end command.5. show dhcp ipv6 pool [ pool-name ]

DETAILED STEPS



Example:


Step 1

Enables the DHCP IPv6 configuration mode.dhcp ipv6

Example:

RP/0/RP0/CPU0:router (config)# dhcp ipv6

Step 2

Creates a DHCP pool specified by the pool-name argument for theprefix delegation and the other configurations on the interface.

pool pool-name

Example:

RP/0/RP0/CPU0:router (config-dhcpipv6)# pool pool1

Step 3







Implementing the Dynamic Host Configuration ProtocolHow to Configure DHCP IPv6 Information Pools



(Optional) Displays the DHCP IPv6 pool name.show dhcp ipv6 pool [ pool-name ]

Example:

RP/0/RP0/CPU0:router# show dhcp ipv6pool pool1

Step 5

Configuration Examples for the DHCP Relay AgentThis section provides the following configuration examples:

DHCP Relay Profile: ExampleThe following example shows how to configure the Cisco IOS XR relay profile:

dhcp ipv4profile client relayhelper-address vrf foo 10.10.1.1!! ...

DHCP Relay on an Interface: ExampleThe following example shows how to enable the DHCP relay agent on an interface:

dhcp ipv4interface GigabitEthernet 0/1/1/0 relay profile client!

DHCP Relay on a VRF: ExampleThe following example shows how to enable the DHCP relay agent on a VRF:

dhcp ipv4vrf default relay profile client!


Implementing the Dynamic Host Configuration ProtocolConfiguration Examples for the DHCP Relay Agent

Relay Agent Information Option Support: ExampleThe following example shows how to enable the relay agent and the insertion and removal of the DHCP relayinformation option:

dhcp ipv4profile client relayrelay informationcheck!!

Relay Agent Giaddr Policy: ExampleThe following example shows how to configure relay agent giaddr policy:

dhcp ipv4profile client relaygiaddr policy drop!!

Cisco IOS XR Broadcast Flag Policy: ExampleThis task describes how to configure DHCP IPv4 Relay to broadcast BOOTPREPLY packets only if theDHCP IPv4 broadcast flag is set in the DHCP IPv4 header.

By default, the DHCP IPv4 Relay always broadcasts BOOTPREPLY packets.Note

SUMMARY STEPS

1. configure2. dhcp ipv43. profile profile name relay4. broadcast-flag policy check5. Use the commit or end command.

DETAILED STEPS



Example:


Step 1


Implementing the Dynamic Host Configuration ProtocolRelay Agent Information Option Support: Example


Configures DHCP IPv4 mode.dhcp ipv4

Example:


Step 2

Enables profile relay mode.profile profile name relay

Example:


Step 3

Enables checking of the broadcast flag inpackets.

broadcast-flag policy check

Example:

RP/0/RP0/CPU0:router(config-dhcpv4-relay-profile)# broadcast-flagpolicy check

Step 4

commitSaves the configuration changesand remains within the configurationsession.



Yes Saves configuration changesand exits the configuration session.


CancelRemains in theconfiguration mode, withoutcommitting the configurationchanges.

Additional ReferencesThe following sections provide references related to implementing the Cisco IOS XR DHCP relay agent.

Related Documents

Document TitleRelated Topic

DHCP Commands module in the Cisco IOS XR IPAddresses and Services Command Reference for theCisco CRS Router

Cisco IOS XR

DHCP commands


Implementing the Dynamic Host Configuration ProtocolAdditional References

Document TitleRelated Topic

Cisco IOS XR Getting Started Guide for theCisco CRS Router

Getting started material

Configuring AAA Services module in theCisco IOS XR System Security Configuration Guidefor the Cisco CRS Router

Information about user groups and task IDs

Standards

TitleStandards

No new or modified standards are supported by thisfeature, and support for existing standards has notbeen modified by this feature.

MIBs

MIBs LinkMIBs

To locate and download MIBs, use the Cisco MIBLocator found at the following URL and choose aplatform under the CiscoAccess Productsmenu: http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

TitleRFC

Dynamic Host Configuration Protocol

Dynamic Host Configuration Protocol for IPv6(DHCPv6)

RFC 2131

RFC 3315

Technical Assistance

LinkDescription

http://www.cisco.com/techsupportThe Cisco Technical Support website containsthousands of pages of searchable technical content,including links to products, technologies, solutions,technical tips, and tools. Registered Cisco.com userscan log in from this page to access evenmore content.





Implementing the Dynamic Host Configuration ProtocolPrerequisites for Configuring DHCP Relay AgentInformation About DHCP Relay AgentHow to Configure and Enable DHCP Relay AgentConfiguring and Enabling the DHCP Relay AgentConfiguring a DHCP Relay Profile

Configuring the DHCPv6 (Stateless) Relay AgentEnabling DHCP Relay Agent on an InterfaceDisabling DHCP Relay on an InterfaceEnabling DHCP Relay on a VRFConfiguring the Relay Agent Information FeatureConfiguring Relay Agent Giaddr PolicyConfiguring the Broadcast Flag Policy

Configuring a DHCP Proxy ProfileDHCPv4 ClientEnabling DHCP Client on an Interface

Information About Configuring DHCP IPv6 Information PoolsHow to Configure DHCP IPv6 Information PoolsConfiguring Cisco IOS XR DHCP IPv6 Information Pool Option

Configuration Examples for the DHCP Relay AgentDHCP Relay Profile: ExampleDHCP Relay on an Interface: ExampleDHCP Relay on a VRF: ExampleRelay Agent Information Option Support: ExampleRelay Agent Giaddr Policy: ExampleCisco IOS XR Broadcast Flag Policy: Example

Additional References