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Cisco IOS XR IP Addresses and Services Configuration Guide for the Cisco XR 12000 Series Router, Release 4.3.x

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Book Title

Cisco IOS XR IP Addresses and Services Configuration Guide for the Cisco XR 12000 Series Router, Release 4.3.x

Chapter Title

Implementing the Dynamic Host Configuration Protocol

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Updated:
September 19, 2016

Chapter: Implementing the Dynamic Host Configuration Protocol

Implementing the Dynamic Host Configuration Protocol

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

Release

Modification

Release 3.2

This feature was introduced.

Release 3.4.0

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

Release 3.7.0

The DHCP CLI was modified.

Prerequisites for Configuring DHCP Relay Agent

The following prerequisites are required to configure a DHCP relay agent:

  • You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is 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 DHCP server

Information About DHCP Relay Agent

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

DHCP clients use User Datagram Protocol (UDP) broadcasts to send DHCPDISCOVER messages when they lack 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 network segment to ensure that DHCP packets reach the servers on another network segment. UDP broadcast packets are not forwarded, because most routers are not configured to forward broadcast traffic. You can configure a DHCP relay profile and configure one or more helper addresses in it. You can assign the profile to an interface or a VRF.

Figure 1 demonstrates the process. The DHCP client broadcasts a request for an IP address and additional configuration parameters on its local LAN. Acting as a DHCP relay agent, Router B picks up the broadcast, changes the destination address to the DHCP server's address and sends the message out on another interface. The relay agent inserts the IP address of the interface, on which the relay profile into the gateway address (giaddr) field of the DHCP packet, which enables the DHCP server to determine which subnet should receive the offer and identify the appropriate 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 Agent

This section contains the following tasks:

Configuring and Enabling the DHCP Relay Agent

Configuring a DHCP Relay Profile

This task describes how to configure and enable the DHCP relay agent.

SUMMARY STEPS

    1.    configure

    2.    dhcp ipv4

    3.    profile profile-name relay

    4.    helper-address [vrf vrf- name ] address

    5.    commit


DETAILED STEPS
     Command or ActionPurpose
    Step 1 configure
          		 
    Step 2 dhcp ipv4


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

    Enters DHCP IPv4 configuration mode.

     
    Step 3 profile profile-name relay


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

    Enters DHCP IPv4 profile relay submode.

     
    Step 4 helper-address [vrf vrf- name ] address


    Example: 
    RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# helper-address vrf 
foo 10.10.1.1
     

    Forwards UDP broadcasts, including BOOTP and DHCP.

    • The value of the address argument can be a specific DHCP server address or a network address (if other DHCP servers are on the destination network segment). Using the network address enables other servers to respond to DHCP requests.

    • For multiple servers, configure one helper address for each server.

     
    Step 5 commit
          		 

    Configuring the DHCPv6 (Stateless) Relay Agent

    Perform this task to specify a destination address to which client messages are forwarded and to enable Dynamic Host Configuration Protocol (DHCP) for IPv6 relay service on the interface.

    SUMMARY STEPS

      1.    configure

      2.    dhcp ipv6

      3.    interface type interface-path-id relay

      4.    destination ipv6-address

      5.    commit


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 configure
              		 
      Step 2dhcp ipv6


      Example:
      
RP/0/0/CPU0:router(config) # dhcp ipv6
RP/0/0/CPU0:router(config-dhcpv6)#
       

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

       
      Step 3interface type interface-path-id relay


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

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

       
      Step 4destination ipv6-address


      Example:
      
RP/0/0/CPU0:router(config-dhcpv6-if) # destination 10:10::10
       

      Specifies a destination address to which client packets are forwarded.

      When relay service is enabled on an interface, a DHCP for IPv6 message received on that interface is forwarded to all configured relay destinations. The incoming DHCP for IPv6 message may have come from a client on that interface, or it may have been relayed by another relay agent.

       
      Step 5 commit
              		 

      Enabling DHCP Relay Agent on an Interface

      This task describes how to enable the Cisco IOS XR DHCP relay agent on an interface.


      Note

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

      SUMMARY STEPS

        1.    configure

        2.    dhcp ipv4

        3.    interface type name relay profile profile-name

        4.    commit


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 configure
                  		 
        Step 2 dhcp ipv4


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

        Enters DHCP IPv4 configuration submode.

         
        Step 3 interface type name relay profile profile-name


        Example: 
        RP/0/0/CPU0:router(config-dhcpv4)# interface 
FastEthernet0/0 relay profile client
         

        Attaches a relay profile to an interface.

         
        Step 4 commit
                  		 

        Disabling DHCP Relay on an Interface

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

        SUMMARY STEPS

          1.    configure

          2.    dhcp ipv4

          3.    interface type name none

          4.    commit


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 configure
                      		 
          Step 2 dhcp ipv4


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

          Enters DHCP IPv4 configuration submode.

           
          Step 3 interface type name none


          Example: 
          RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# interface 
pos 0/1/4/1 none
           

          Disables the DHCP relay on the interface.

           
          Step 4 commit
                      		 

          Enabling DHCP Relay on a VRF

          This task describes how to enable DHCP relay on a VRF.

          SUMMARY STEPS

            1.    configure

            2.    dhcp ipv4

            3.    vrf vrf-name relay profile profile-name

            4.    commit


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 configure
                          		 
            Step 2 dhcp ipv4


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

            Enters DHCP IPv4 configuration submode.

             
            Step 3 vrf vrf-name relay profile profile-name


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

            Enables DHCP relay on a VRF.

             
            Step 4 commit
                          		 

            Configuring the Relay Agent Information Feature

            This 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 relay information. By default, the relay information from the previous relay agent is replaced (using the replace option).

            SUMMARY STEPS

              1.    configure

              2.    dhcp ipv4

              3.    profile profile-name relay

              4.    relay information option

              5.    relay information check

              6.    relay information policy {drop | keep}

              7.    relay information option allow-untrusted

              8.    commit


            DETAILED STEPS
               Command or ActionPurpose
              Step 1 configure
                              		 
              Step 2 dhcp ipv4


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

              Enters DHCP IPv4 configuration mode.

               
              Step 3 profile profile-name relay


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

              Enters DHCP IPv4 profile relay mode.

               
              Step 4 relay information option


              Example: 
              RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# relay information option
               

              Enables the system to insert the DHCP relay agent information option (option-82 field) in forwarded BOOTREQUEST messages to a DHCP server.

              • This option is injected by the relay agent while forwarding client-originated DHCP packets to the server. Servers recognizing this option can use the information to implement IP address or other parameter assignment policies. When replying, the DHCP server echoes the option back to the relay agent. The relay agent removes the option before forwarding the reply to the client.

              • The relay agent information is organized as a single DHCP option that contains one or more suboptions. These options contain the information known by the relay agent.

                The supported suboptions are:

                • Remote ID

                • Circuit ID

              Note   

              This function is disabled by default.

               
              Step 5 relay information check


              Example: 
              RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# relay information check
               

              (Optional) Configures DHCP to check that the relay agent information option in forwarded BOOTREPLY messages is valid.

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

              Note   

              Use the relay information check command to reenable this functionality if the functionality has been disabled.

               
              Step 6 relay information policy {drop | keep}


              Example: 
              RP/0/0/CPU0:router(config)# dhcp relay information policy drop
               

              (Optional) Configures the reforwarding policy for a DHCP relay agent; that is, whether the relay agent will drop or keep the relay information.

               
              Step 7 relay information option allow-untrusted


              Example: 
              RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# relay information check
               

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

               
              Step 8 commit
                              		 

              Configuring Relay Agent Giaddr Policy

              This 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.    configure

                2.    dhcp ipv4

                3.    profile profile-name relay

                4.    giaddr policy {replace | drop}

                5.    commit


              DETAILED STEPS
                 Command or ActionPurpose
                Step 1 configure
                                  		 
                Step 2 dhcp ipv4


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

                Enables the DHCP IPv4 configuration submode.

                 
                Step 3 profile profile-name relay


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

                Enables profile relay submode.

                 
                Step 4 giaddr policy {replace | drop}


                Example: 
                RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# giaddr policy drop
                 

                Specifies the giaddr policy.

                • replace—Replaces the existing giaddr value with a value that it generates.

                • drop—Drops the packet that has an existing nonzero giaddr value.

                 
                Step 5 commit
                                  		 

                Configuring the Broadcast Flag Policy

                This task describes how to configure DHCP IPv4 Relay to broadcast BOOTPREPLY packets only if the DHCP IPv4 broadcast flag is set in the DHCP IPv4 header.


                Note

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

                SUMMARY STEPS

                  1.    configure

                  2.    dhcp ipv4

                  3.    profile profile-name relay

                  4.    broadcast-flag policy check

                  5.    commit


                DETAILED STEPS
                   Command or ActionPurpose
                  Step 1 configure
                                      		 
                  Step 2 dhcp ipv4


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

                  Configures DHCP IPv4 mode.

                   
                  Step 3 profile profile-name relay


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

                  Enables profile relay mode.

                   
                  Step 4 broadcast-flag policy check


                  Example: 
                  RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# broadcast-flag policy check
                   

                  Enables checking of the broadcast flag in packets.

                   
                  Step 5 commit
                                      		 

                  Configuring a DHCP Proxy Profile

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

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

                  SUMMARY STEPS

                    1.    configure

                    2.    dhcp ipv4

                    3.    profile profile-name proxy

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

                    5.    commit


                  DETAILED STEPS
                     Command or ActionPurpose
                    Step 1 configure
                                          		 
                    Step 2 dhcp ipv4


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

                    Enters DHCP IPv4 configuration mode.

                     
                    Step 3 profile profile-name proxy


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

                    Enters DHCP IPv4 profile proxy submode.

                     
                    Step 4 helper-address [vrf vrf- name ] address [ giaddr gateway-address ]


                    Example: 
                    RP/0/0/CPU0:router(config-dhcpv4-proxy-profile)# helper-address vrf 
foo 10.10.1.1
                     

                    Forwards UDP broadcasts, including BOOTP and DHCP.

                    • The value of the address argument can be a specific DHCP server address or a network address (if other DHCP servers are on the destination network segment). Using the network address enables other servers to respond to DHCP requests.

                    • For multiple servers, configure one helper address for each server.

                     
                    Step 5 commit
                                          		 

                    DHCPv4 Client

                    The Dynamic Host Configuration Protocol (DHCP) client functionality enables the router interfaces to dynamically 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, and deliver 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 usually runs on the router, and is required because the client device does not know its own IP address initially. The agent sends out a Layer 2 broadcast to find a server that has this information. The router relays these broadcasts to the DHCP server, and forwards the responses back to the correct Layer 2 address so that the correct device gets 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 period must be renewed before the IP address expires. The client renews the lease based on configuration that was sent from the server. The client unicasts a REQUEST message using the IP address of the server. When a server receives the REQUEST message and responds with an ACK message. The lease period of the client is 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 Interface

                    The DHCP client can be enabled at an interface level. The DHCP component receives a notification when DHCP is enabled or disabled on an interface.

                    SUMMARY STEPS

                      1.    configure

                      2.    interface MgmtEth rack/slot/CPU0/port

                      3.    interface <interface_name> ipv4 address dhcp


                    DETAILED STEPS
                       Command or ActionPurpose
                      Step 1 configure
                                              		 
                      Step 2 interface MgmtEth rack/slot/CPU0/port


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

                      Enters interface configuration mode.

                       
                      Step 3interface <interface_name> ipv4 address dhcp


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


                      Example: 
                       dhcp Enable IPv4 DHCP client
                                              		Configure DHCP on the interface.  
                      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 Mask
dhcp Enable IPv4 DHCP client

                      Information About Configuring DHCP IPv6 Information Pools

                      A DHCP IPv6 configuration information pool is a named entity that includes information about available configuration parameters and policies that control assignment of the parameters to clients from the pool. A pool is configured independently of the DHCP service and is associated with the DHCP service through the command 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 and associated preferred and valid lifetimes

                      • Domain name service (DNS) servers—List of IPv6 addresses of DNS servers

                      • Domain search list—String containing domain names for DNS resolution

                      • SIP server address—List of IPv6 addresses of SIP server

                      • SIP server domain list—String containing domain names for SIP server

                      How to Configure DHCP IPv6 Information Pools

                      This section contains the following task:

                      Configuring Cisco IOS XR DHCP IPv6 Information Pool Option

                      This task describes how to enable support for the DHCP IPv6 information pool option with the name pool1.

                      SUMMARY STEPS

                        1.    configure

                        2.    dhcp ipv6

                        3.    pool pool-name

                        4.    commit

                        5.    show dhcp ipv6 pool [ pool-name ]


                      DETAILED STEPS
                         Command or ActionPurpose
                        Step 1 configure
                                                  		 
                        Step 2 dhcp ipv6


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

                        Enables the DHCP IPv6 configuration mode.

                         
                        Step 3 pool pool-name


                        Example: 
                        RP/0/0/CPU0:router (config-dhcp ipv6)# pool pool1
                         

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

                         
                        Step 4 commit
                                                  		 
                        Step 5 show dhcp ipv6 pool [ pool-name ]


                        Example: 
                        RP/0/0/CPU0:router# show dhcp ipv6 pool pool1
                         

                        (Optional) Displays the DHCP IPv6 pool name.

                         

                        Configuration Examples for the DHCP Relay Agent

                        This section provides the following configuration examples:

                        DHCP Relay Profile: Example

                        The following example shows how to configure the Cisco IOS XR relay profile: 

                        dhcp ipv4 
 profile client relay
  helper-address vrf foo 10.10.1.1
 !        
! ...

                        DHCP Relay on an Interface: Example

                        The following example shows how to enable the DHCP relay agent on an interface: 

                        dhcp ipv4
 interface GigabitEthernet 0/1/1/0 relay profile client
!

                        DHCP Relay on a VRF: Example

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

                        dhcp ipv4
 vrf default relay profile client
!

                        Relay Agent Information Option Support: Example

                        The following example shows how to enable the relay agent and the insertion and removal of the DHCP relay information option:

                        dhcp ipv4
 profile client relay
relay information 
check
 !
!

                        Relay Agent Giaddr Policy: Example

                        The following example shows how to configure relay agent giaddr policy:

                        dhcp ipv4
 profile client relay
  giaddr policy drop
 !
!

                        Cisco IOS XR Broadcast Flag Policy: Example

                        This task describes how to configure DHCP IPv4 Relay to broadcast BOOTPREPLY packets only if the DHCP IPv4 broadcast flag is set in the DHCP IPv4 header.


                        Note

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

                        SUMMARY STEPS

                          1.    configure

                          2.    dhcp ipv4

                          3.    profile profile name relay

                          4.    broadcast-flag policy check

                          5.    commit


                        DETAILED STEPS
                           Command or ActionPurpose
                          Step 1 configure
                                                      		 
                          Step 2 dhcp ipv4


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

                          Configures DHCP IPv4 mode.

                           
                          Step 3 profile profile name relay


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

                          Enables profile relay mode.

                           
                          Step 4 broadcast-flag policy check


                          Example: 
                          RP/0/0/CPU0:router(config-dhcpv4-relay-profile)# broadcast-flag policy check
                           

                          Enables checking of the broadcast flag in packets.

                           
                          Step 5 commit
                                                      		 

                          Additional References

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

                          Related Documents

                          Related Topic

                          Document Title

                          Cisco IOS XR

                          DHCP commands

                          DHCP Commands module in the Cisco IOS XR IP Addresses and Services Command Reference for the Cisco XR 12000 Series Router

                          Getting started material

                          Cisco IOS XR Getting Started Guide for the Cisco XR 12000 Series Router

                          Information about user groups and task IDs

                          Configuring AAA Services module in the Cisco IOS XR System Security Configuration Guide for the Cisco XR 12000 Series Router

                          Standards

                          Standards

                          Title

                          No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

                          		—

                          MIBs

                          MIBs

                          MIBs Link

                          —

                          To locate and download MIBs, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http:/​/​cisco.com/​public/​sw-center/​netmgmt/​cmtk/​mibs.shtml

                          RFCs

                          RFC

                          Title

                          RFC 2131

                          RFC 3315

                          Dynamic Host Configuration Protocol

                          Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

                          Technical Assistance

                          Description

                          Link

                          The Cisco Technical Support website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

                          http:/​/​www.cisco.com/​techsupport

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