Implementing the Dynamic Host Configuration Protocol

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


Note

For a complete description of the DHCP commands listed in this module, refer to the IP Addresses and Services Command Reference for Cisco NCS 5500 Series, Cisco NCS 540 Series, and Cisco NCS 560 Series Routers publication.

Introduction to DHCP Relay

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 agent to forward DHCP packets to a remote server by configuring a DHCP relay profile and configure one or more helper addresses in it. You can assign the profile to an interface or a VRF.

The figure below 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 DHCP client’s packets are received 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

Prerequisites for Configuring DHCP Relay Agent

The following are the prerequisites 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

Limitations for DHCP Relay Feature

These are the limitations for implementing DHCP relay feature:

  • The multicast addresses are not supported. The helper-address command in DHCP relay profile submode supports valid unicast IP address as the helper address.


    Note

    Configuring the helper-address command directly (not using profile) under a interface (such as BVI interface) is not supported.

  • Only interface-id and remote-id DHCP option code are added by a relay agent while forwarding the packet to a DHCP server.


    Note
    Configuring DHCP option code is not supported in DHCP relay profile submode.

How to Configure and Enable DHCP Relay Agent

This section contains the following tasks:

Configuring and Enabling the DHCP Relay Agent

Configuration Example 

Router# configure
/* Enters the global configuration mode */

Router(config)# dhcp ipv4
/* Configures DHCP for IPv4 and enters the DHCPv4 configuration submode. */

Router(config-dhcpv4)# profile r1 relay
/* Enables DHCP relay profile */

Router(config-dhcpv4-relay-profile)# helper-address vrf A 10.10.10.1 giaddr 40.1.1.2
Router(config-dhcpv4-relay-profile)# broadcast-flag policy check
/* Configures VRF addresses for forwarding UDP broadcasts, including DHCP. */

Router(config-dhcpv4-relay-profile)# relay information option vpn
Router(config-dhcpv4-relay-profile)# relay information option vpn-mode rfc
/* Inserts the DHCP relay agent information option (option-82 field) in forwarded BOOTREQUEST messages to a DHCP server. */


Router(config-dhcpv4-relay-profile)# relay information option allow-untrusted
/* (Optional) Configures the DHCP IPv4 Relay not to discard BOOTREQUEST packets 
that have an existing relay information option and the giaddr set to zero. */

Router(config-dhcpv4-relay-profile)# exit
Router(config-dhcpv4)# interface BVI 1 relay profile r1
Router(config-dhcpv4)# commit
/* Configures DHCP relay on a BVI interface and commits the configuration */

Running Configuration 

Router#show running-config
Tue May 23 10:56:14.463 IST
Building configuration...
!! IOS XR Configuration 0.0.0
!! Last configuration change at Tue May 23 10:56:08 2017 by annseque
!
dhcp ipv4
 vrf vrf1 relay profile client
 profile r1 relay
  helper-address vrf A 10.10.10.1 giaddr 40.1.1.2
  broadcast-flag policy check
  relay information option vpn
  relay information option vpn-mode rfc
  relay information option allow-untrusted
 !

Enabling DHCP Relay Agent on an Interface

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

Configuration Example

The DHCP relay agent is disabled by default. 

router#configure

router(config)#dhcp ipv4
/* Configures DHCP for IPv4 and enters the DHCPv4 configuration submode. */

router(config-dhcpv4)#interface HundredGigE 0/2/0/2 relay profile client
/* Attaches a relay profile to an interface. 
To disable the DHCP relay on the interface, use the 'no interface HundredGigE 0/2/0/2 none' command. */

router(config-dhcpv4-if)#commit

Running Configuration 

Router#show running-config dhcp ipv4
dhcp ipv4
interface HundredGigE 0/2/0/2 relay profile client
!

Disabling DHCP Relay on an Interface

This task describes how to disable the DHCP relay on an interface by using the no keyword on the interface. 

Router# configure terminal
Router(config)# dhcp ipv6
Router(config-dhcpv6)# no interface type name relay profile profile-name
Router(config-dhcpv6-if)# commit

Enabling DHCP Relay on a VRF

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

/CPU0:router# configure terminal
Router(config)# dhcp ipv6
Router(config-dhcpv6)# vrf vrf-name relay profile profile-name
Router(config-dhcpv6-if)# commit

Configure a DHCP Relay Profile with Multiple Helper Addresses

You can configure up to 16 helper IPv4 and IPv6 addresses for a DHCPv4 or DHCpv6 relay profile.

  1. Enter the DHCPv4 or DHCPv6 configuration mode.

    Router(config)# dhcp ipv6

  2. Configure the DHCPv4 or DHCPv6 relay profile. 

    Router(config-dhcpv6)# profile helper relay

  3. Configure helper addresses.


    Note

    You can configure up to 16 IPv4 and IPv6 addresses.

    		 
    Router(config-dhcpv6-relay-profile)# helper-address vrf default 2001:1:1::2

  4. Confirm your configuration.

    Router(config-dhcpv6-relay-profile)# show configuration

!! IOS XR Configuration 0.0.0
dhcp ipv6
 profile helper relay
  helper-address vrf default 2001:1:1::2
 !
!
end

  5. Commit your configuration.

    Router(config-dhcpv6-relay-profile)# commit

  6. Exit the configuration mode and verify the configured helper addresses.

    Router# show dhcp ipv6 relay profile name helper
...
!
Profile: helper
Helper Addresses:
        2001:1:1::2, vrf default
Information Option: Disabled
Information Option Allow Untrusted: Disabled
Information Option VPN: Disabled
Information Option VPN Mode: RFC
Information Option Policy: Replace
Information Option Check: Disabled
GIADDR Policy: Keep
Broadcast-flag Policy: Ignore
VRF References:
Interface References:

    You have successfully configured the DHCPv6 relay helper address.

DHCP Relay Agent Notification for Prefix Delegation

DHCP relay agent notification for prefix delegation allows the router working as a DHCPv6 relay agent to find prefix delegation options by reviewing the contents of a DHCP RELAY-REPLY packet that is being relayed by the relay agent to the client. When the relay agent finds the prefix delegation option, the relay agent extracts the information about the prefix being delegated and inserts an IPv4 or IPv6 subscriber route matching the prefix delegation information onto the relay agent. Future packets destined to that prefix via relay are forwarded based on the information contained in the prefix delegation. The IPv4 or IPv6 subscriber route remains in the routing table until the prefix delegation lease time expires or the relay agent receives a release packet from the client releasing the prefix delegation.

The relay agent automatically does the subscriber route management.

The Ipv4 or IPv6 routes are added when the relay agent relays a RELAY-REPLY packet, and the IPv4 or IPv6 routes are deleted when the prefix delegation lease time expires or the relay agent receives a release message. An Ipv4 or IPv6 subscriber route in the routing table of the relay agent can be updated when the prefix delegation lease time is extended.

This feature leaves an IPv4 or IPv6 route on the routing table of the relay agent. This registered IPv4 or IPv6 address allows unicast reverse packet forwarding (uRPF) to work by allowing the router doing the reverse lookup to confirm that the Tpv4 or IPv6 address on the relay agent is not malformed or spoofed. The IPv6 route in the routing table of the relay agent can be redistributed to other routing protocols to advertise the subnets to other nodes. When the client sends a DHCP_DECLINE message, the routes are removed.

Configuring DHCP Stateful Relay Agent for Prefix Delegation

Perform this task to configure Dynamic Host Configuration Protocol DHCP relay agent notification for prefix delegation.

Configuration Example

  1. Configure a DHCP profile.

  2. Configure the DHCP relay agent.

  3. Enable IPv4 or IPv6 DHCP on an interface that acts as an IPv4 or IPv6 DHCP stateful relay agent.

  4. Configure the profile name.


Note

Prefix Delegation is supported with both DHCP Proxy and DHCP Relay configuration.

Configuration

/* Enter the global configuration mode and then enter the DHCPv6 configuration mode. */
Router# config
Router(config)# dhcp ipv6
Router(config-dhcpv6)#

/* Enter the proxy profile configuration mode and configure the DHCPv6 relay agent. */
Router(config-dhcpv6)# profile downstream proxy
Router(config-dhcpv6-profile)# helper-address 2001:db8::1 GigabitEthernet 0/1/0/1

/* Exits from the proxy profile configuration mode and enable IPv6 DHCP on an interface. */
Router(config-dhcpv6-profile)# exit
Router(config-dhcpv6-if)# interface GigabitEthernet 0/1/0/0 proxy

/* Configure a profile name. */

Router(config-dhcpv6-if)# profile downstream
Router(config-dhcpv6-if)# commit

DHCPv6 Relay Over BVI for IANA Address Allocation

DHCPv6 Relay agents relay all packets that are coming from DHCPv6 clients over the access-interfaces towards external DHCPv6 servers to request IP addresses (::/128) through IANA allocation for the DHCPv6 clients. DHCPv6 Relay agents also receive response packets from the DHCPv6 servers and forward the packets towards DHCPv6 clients over BVI interfaces. DHCPv6 Relay agents acts as stateless, by default, for DHCPv6 clients by not maintaining any DHCPv6 binding and respective route entry for the allocated IP addresses. You can enable a DHCPv6 client to get a particular IPv6 address assigned by the DHCPv6 server over a Bridge Virtual Interface (BVI) through Internet Assigned Numbers Authority (IANA) address allocation. Thereby, the DHCPv6 relay agent acts as a stateful relay agents and maintains DHCPv6 binding and respective route entry for the allocated IPv6 addresses.

Restrictions

  • You can configure up to 500 client sessions over a BVI interface for DHCP relay.

  • Each DHCPv6 relay profile can be configured with upto 8 DHCPv6 server addresses.

Configuration Example

To configure DHCPv6 Relay Over BVI for IANA Address Allocation, use the following steps.

  1. Enter the interface configuration mode and configure a BVI interface.

  2. Assign an IPv6 address to the BVI interface.

  3. Route the L2 access interface to the L3 BVI interface of the relay agent.

  4. Enter the DHCP IPv6 configuration mode and then create a DHCP IPv6 Stateful relay profile.

  5. Attach the relay profile to a server address.

  6. Configure a stateful relay agent by enabling route allocation through IANA.

  7. Attach the BVI Interface to the DHCPv6 relay profile.

Configuration

/* Enter the interface configuration mode and configure a BVI interface. */
Router# configure
Router(config)# interface BVI1

Assign an IPv6 address to the BVI interface.
Router(config-if)# ipv6 address 2001:db8::2/64
Router(config-if)# commit
Router(config-if)# exit

/* Route the L2 access interface to the L3 BVI interface of the relay agent. */
Router(config)# l2vpn bridge group 1
Router(config-l2vpn-bg)# bridge-domain 1
Router(config-l2vpn-bg-bd)# interface hundredGigE 0/0/0/1.100
Router(config-l2vpn-bg-bd-ac)# commit
Router(config-l2vpn-bg-bd-ac)# exit
Router(config-l2vpn-bg-bd)# routed interface BVI1
Router(config-l2vpn-bg-bd)# exit
Router(config-l2vpn-bg)# exit
Router(config-l2vpn-bg)# exit
Router(config-l2vpn)# exit
Router(config)# 

/* Enter the DHCP IPv6 configuration mode and then create a DHCP IPv6 Stateful relay profile. */
Router(config)# dhcp ipv6
Router(config-dhcpv6)# profile RELAY1 relay

/* Attach the relay profile to a server address. */
Router(config-dhcpv6-relay-profile)# helper-address vrf default 2001:DB8::1

/* Configure a stateful relay agent by enabling route allocation through IANA. */
Router(config-dhcpv6-relay-profile)# iana-route-add
Router(config-dhcpv6-relay-profile)# exit


/* Attach the BVI Interface to the DHCPv6 relay profile. */
Router(config-dhcpv6-relay-profile)# interface BVI1 relay profile RELAY1
Router(config-dhcpv6-relay-profile)# commit

Running Configuration

Router# show running configuration
interface BVI1
 ipv6 address 2001:db8::2/64
!
l2vpn
 bridge group 1
  bridge-domain 1
   interface HundredGigE0/0/0/1.100
   !
   routed interface BVI1
   !
  !
 !
!
dhcp ipv6
 profile RELAY1 relay
  helper-address vrf default 2001:db8::1
  iana-route-add
  !
 interface BVI1 relay profile RELAY1
!

Verification

Use the following command to verify that more than one DHCP client is bridged over BVI:

Router# show dhcp ipv6 relay binding
Thu Nov 21 05:48:38.463 UTC
 
Summary:
Total number of clients: 500
 
IPv6 Address: 2000::418f/128 (BVI31)
    Client DUID: 000100015dcf28de001094003295
    MAC Address: 0010.9400.3295
    IAID: 0x0
    VRF: default
    Lifetime: 600 secs (00:10:00)
    Expiration: 533 secs (00:08:53)
    L2Intf AC: Bundle-Ether3.1
    SERG State: NONE
    SERG Intf State: SERG-NONE
IPv6 Address: 2000::4190/128 (BVI31)
    Client DUID: 000100015dcf28de001094003296
    MAC Address: 0010.9400.3296
    IAID: 0x0
    VRF: default
    Lifetime: 600 secs (00:10:00)
    Expiration: 531 secs (00:08:51)
    L2Intf AC: Bundle-Ether3.1
    SERG State: NONE
    SERG Intf State: SERG-NONE
IPv6 Address: 2000::4191/128 (BVI31)
    Client DUID: 000100015dcf28de001094003297
    MAC Address: 0010.9400.3297
    IAID: 0x0
    VRF: default
    Lifetime: 600 secs (00:10:00)
    Expiration: 448 secs (00:07:28)
    L2Intf AC: Bundle-Ether3.1
    SERG State: NONE
    SERG Intf State: SERG-NONE
IPv6 Address: 2000::4192/128 (BVI31)
    Client DUID: 000100015dcf28de001094003298
    MAC Address: 0010.9400.3298
    IAID: 0x0
    VRF: default
    Lifetime: 600 secs (00:10:00)
    Expiration: 439 secs (00:07:19)
    L2Intf AC: Bundle-Ether3.1
    SERG State: NONE
    SERG Intf State: SERG-NONE

Use the following command to verify that unique IPv6 address is assigned to a client due to IANA allocation:

Router# show route ipv6
Mon Oct 21 06:16:43.617 UTC
 
Codes: C - connected, S - static, R - RIP, B - BGP, (>) - Diversion path
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - ISIS, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, su - IS-IS summary null, * - candidate default
       U - per-user static route, o - ODR, L - local, G  - DAGR, l - LISP
       A - access/subscriber, a - Application route
       M - mobile route, r - RPL, t - Traffic Engineering, (!) - FRR Backup path
 
Gateway of last resort is not set
 
A    2000::/64
      [1/0] via fe80::1, 00:00:37, BVI700
A    2000::1/128
      [1/0] via fe80::210:94ff:fe00:8, 00:00:12, BVI700
C    2007:3019::/64 is directly connected,
      00:00:37, Loopback1
L    2007:3019::1/128 is directly connected,
      00:00:37, Loopback1
C    7001:6018::/64 is directly connected,
      00:00:37, BVI700
L    7001:6018::1/128 is directly connected,
      00:00:37, BVI700
C    7001:6019::/64 is directly connected,
      00:00:37, TenGigE0/0/0/2.2
L    7001:6019::1/128 is directly connected,
      00:00:37, TenGigE0/0/0/2.2

DHCP Relay Profile: Example

The following example shows how to configure the DHCP 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 option

 !
!

Relay Agent Giaddr Policy: Example

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


dhcp ipv4
 profile client relay
  giaddr policy drop
 !
!

Configure 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.

Configuration Example

  1. Enter DHCP IPv4 or DHCP IPv6 profile proxy submode.

  2. Forward UDP broadcasts, including DHCP.


Note

  • 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.

Configuration

/* Enter the DHCP IPv4 profile proxy submode. */
Router(config)# dhcp ipv4
Router(config-dhcpv4)# profile client proxy

/* Forward UDP broadcastrs, including DHCP */
Router(config-dhcpv4-proxy-profile)# helper-address vrf vrf1 foo 10.10.1.1
Router(config-dhcpv4-proxy-profile)# commit

DHCP Server

A DHCP server accepts address assignment requests and renewals, and assigns the IP addresses from predefined groups of addresses contained within Distributed Address Pools (DAPS). DHCP servers can also be configured to supply additional information to the requesting client such as subnet mask, domain-name, the IP address of the DNS server, the default router, and other configuration parameters. DHCP servers can accept broadcasts from locally attached LAN segments or from DHCP requests that have been forwarded by other DHCP relay agents within the network.

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.

When the DHCP server receives an address assignment request, it assigns the IP addresses from groups of IP addresses for DHCP in Distributed Address Pools (DAPS). The IP address used by the DHCP server to complete such requests is automatically excluded from the DAPS so that the DHCP server can safely assume that all the IP addresses available for its use in the DAPS are free.

DHCP Service-based Mode Selection

As part of DHCP service-based mode selection feature, a new mode called DHCP base is introduced. If an interface is configured in the DHCP base mode, then the DHCP selects either the DHCP proxy or the DHCP server mode to process the client request by matching option 60 (class-identifier) value of the client request with the configured value under the DHCP base profile.

The pool is configured under server-profile mode and server-profile-class submode. The class-based pool selection is always given priority over profile pool selection.

The DHCPv6 server-profile-class submode supports configuring DHCP options except few (0, 12, 50, 52, 53, 54, 58, 59, 61, 82, and 255 ). 

dhcp ipv6
profile DHCP_BASE base
  match option 60 41424344 profile DHCPv6_PROXY proxy
  match option 60 41424355 profile DHCPv6_SERVER server
  default profile DEFAULT_PROFILE server
  relay information authenticate inserted
   !
profile DHCPv6_PROXY proxy
  helper-address vrf default 10.10.10.1 giaddr 0.0.0.0
!
profile DHCPv6_SERVER server
  lease 1 0 0
  pool IP_POOL
!
profile DEFAULT_PROFILE server
  lease 1 0 0
  pool IP_POOL
!
!
interface gigabitEthernet 0/0/0/0 base profile DHCP_BASE

Configuring DHCP Server Profile

You can configure routers with DHCPv4 or DHCPv6 server profile.

Perform this task to configure the DHCPv6 server profile. 

Router# configure
Router(config)# dhcp ipv6
Router(config-dhcpv6)# profile profile-name server
Router(config-dhcpv6-server-profile)# bootfile boot-file-name
Router(config-dhcpv6-server-profile)# broadcast-flag policy unicast-always
Router(config-dhcpv6-server-profile)# class class-name
Router(config-dhcpv6-server-profile-class)# exit
Router(config-dhcpv6-server-profile)# default-router address1 address2 ... address8 
Router(config-dhcpv6-server-profile)# lease {infinite | days minutes seconds }
Router(config-dhcpv6-server-profile)# limit lease {per-circuit-id | per-interface| per-remote-id} value 
Router(config-dhcpv6-server-profile)# netbios-name server address1 address2 ... address8 
Router(config-dhcpv6-server-profile)# netbios-node-type {number |b-node|h-node |m-node |p-node} 
Router(config-dhcpv6-server-profile)# option option-code {ascii string | hex string |ip address} 
Router(config-dhcpv6-server-profile)# pool pool-name 
Router(config-dhcpv6-server-profile)# requested-ip-address-check disable
Router(config-dhcpv6-server-profile)# commit

Configuring Multiple Classes with a Pool

Perform this task to configure multiple classes with a pool.

RP/0/RSP0/CPU0:router# configure
RP/0/RSP0/CPU0:router(config)# dhcp ipv6
RP/0/RSP0/CPU0:router(config-dhcpv6)# profile profile-name server
RP/0/RSP0/CPU0:router(config-dhcpv6-server-profile)# pool pool-name
RP/0/RSP0/CPU0:router(config-dhcpv6-server-profile)# class class-name
RP/0/RSP0/CPU0:router(config-dhcpv6-server-class)# pool pool-name
RP/0/RSP0/CPU0:router(config-dhcpv6-server-class)# match option option [ sub-option sub-option] [ ascii asciiString | hex hexString ]
RP/0/RSP0/CPU0:router(config-dhcpv6-server-class)# exit 
RP/0/RSP0/CPU0:router(config-dhcpv6-server-profile)# class class-name
RP/0/RSP0/CPU0:router(config-dhcpv6-server-class)# pool pool-name
RP/0/RSP0/CPU0:router(config-dhcpv6-server-class)# match vrf vrf-name 
RP/0/RSP0/CPU0:router(config-dhcpv6-server-class)# commit

Configuring a Server Profile DAPS with Class Match Option

This section discusses configuring a server profile DAPS with class match option.

Configuration Example 

router#configure

router(config)#dhcp ipv4
/* The 'dhcp ipv6' command configures DHCP for IPv6 and enters the DHCPv6 configuration submode. */

router(config-dhcpv4)#profile ISP1 server
/* Enters the server profile configuration mode. */

router(config-dhcpv4-server-profile)#pool ISP1_POOL
/* Configures the DAPS pool name. */

router(config-dhcpv4-server-profile)#class ISP1_CLASS
/* Creates and enters server profile class configuration submode. */

router(config-dhcpv4-server-profile-class)#pool ISP1_CLASS_POOL
/* Configures the pool name. */

router(config-dhcpv4-server-profile-class)#match option 60 hex PXEClient_1
/* DHCP server selects a pool from a class by matching options in the received DISCOVER packet with the match option. */

router(config-dhcpv4-server-profile-class)#exit

router(config-dhcpv4-server-profile)#exit

router(config-dhcpv4)#profile ISP2 server
/* Enters the server profile configuration mode. */

router(config-dhcpv4-server-profile)#dns-server 10.20.3.4
/* Configures the name of the DNS server or the IP address. */

router(config-dhcpv4-server-profile)#pool ISP2_POOL
/* Configures the pool name. */

router(config-dhcpv4-server-profile)#class ISP2_CLASS
/* Creates and enters the server profile class. */

router(config-dhcpv4-server-profile-class)#pool ISP2_CLASS_POOL
/* Configures the pool name. */

router(config-dhcpv4-server-profile-class)#match option 60 hex PXEClient_2
/* DHCP server selects a pool from a class by matching options in the received DISCOVER packet with the match option. */

router(config-dhcpv4-server-profile-class)#exit

router(config-dhcpv4-server-profile)#exit

router(config-dhcpv4)#commit

Running Configuration 

Router#show running-config dhcp ipv4
dhcp ipv4
profile ISP1 server
pool ISP1_POOL
class ISP1_CLASS
pool ISP1_CLASS_POOL
match option 60 hex PXEClient_1
exit
exit
profile ISP2 server
dns-server 10.20.3.4
pool ISP2_POOL
class ISP2_CLASS
pool ISP2_CLASS_POOL
match option 60 hex PXEClient_2
exit
exit
!

Configuring Server Profile without DAPS Pool Match Option

This section discusses configuring a server profile without DAPS pool match option.

Configuration Example

router#configure

router(config)#dhcp ipv4
/* The 'dhcp ipv6' command configures DHCP for IPv6 and enters the DHCPv6 configuration submode. */

router(config-dhcpv4)#profile ISP1 server
/* Enters the server profile configuration mode. */

router(config-dhcpv4-server-profile)#dns-server ISP1.com
/* Configures the name of the DNS server or IP address. */

router(config-dhcpv4-server-profile)#exit

router(config-dhcpv4)#profile ISP2 server
/* Enters the server profile configuration mode. */

router(config-dhcpv4-server-profile)#dns-server ISP2.com
/* Configures the name of the DNS server or IP address. */

router(config-dhcpv4-server-profile)#exit

router(config-dhcpv4)#commit

Running Configuration

Router#show running-config dhcp ipv4
dhcp ipv4
 profile ISP1 server   
  dns-server ISP1.com 

  exit
 profile ISP2 server
  dns-server ISP2.com

  exit
!

Configuring an Address Pool for Each ISP on DAPS

This section discusses configuring an address pool for each ISP on DAPS.

Configuration Example 

router#configure

router(config)#pool vrf ISP_1 ipv4 ISP1_POOL
/* Configures an IPv4 pool for the specifed VRF or all VRFs. Use the 'ipv6' keyword for IPv6 pool. */

router(config-pool-ipv4)#network 10.10.10.0
/* Specifies network for allocation. */

router(config-pool-ipv4)#exit

router(config)#pool vrf ISP_2 ipv4 ISP2_POOL
/* Configures an IPv4 pool for the specifed VRF or all VRFs. */

router(config-pool-ipv4)#network 10.20.20.0
/* Specifies network for allocation. */

router(config-pool-ipv4)#exit

router(config-dhcpv4)#commit

Running Configuration 

Router#show running-config pool
pool vrf ISP_1 ipv4 ISP1_POOL 
 network 10.10.10.0 
 exit
pool vrf ISP_2 ipv4 ISP2_POOL
 network 10.20.20.0 
!

DHCP Client

The Dynamic Host Configuration Protocol (DHCP) client functionality enables the router interfaces to dynamically acquire the IPv4 or DHCPv4 or DHCPv6 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

  • DHCPv4 or DHCPv6 client can be enabled only on management interfaces.

  • Either DHCPv4, DHCPv6, static IPv4, or static IPv6 can be configured on an interface.

DHCP Proxy Binding Table Reload Persistency

The Cisco IOS-XR Dynamic Host Configuration Protocol (DHCP) application is responsible for maintaining the DHCP binding state for the DHCP leases allocated to clients by the DHCP application. These binding states are learned by the DHCP application (proxy/relay/snooping). DHCP clients expect to maintain a DHCP lease regardless of the events that occur to the DHCP application.


Note

From Release 6.2.2 onwards, 200K sessions are supported on a proxy or server running DHCPv4 or DHCPv6.
This feature enables the DHCP application to maintain bind state through the above events:

  • Process restart – Local checkpoint

  • RP failover – Hot standby RP through checkpoint

  • LC IMDR – Local checkpoint

  • LC OIR – Shadow table on RP

  • System restart – Bindings saved on local disk

Configuring DHCP Relay Binding Database Write to System Persistent Memory

Perform this task to configure the DHCP relay binding database write to the system persistent memory. This helps to recover the DHCP relay binding table after a system reload. The file names used for a full persistent file write are dhcpv4_srpb_{nodeid}_odd or dhcpv6_srpb_{nodeid}_odd and dhcpv4_srpb_{nodeid}_even or dhcpv6_srpb_{nodeid}_even. The nodeid is the actual node ID of the node where the file is written. The incremental file is named the same way as the full file, with a _inc appended to it. 

Router# configure
Router(config)# dhcp ipv6
Router(config-dhcpv6)# database [relay] [full-write-interval full-write-interval] [incremental-write-interval incremental-write-interval]
Router(config-dhcpv6)# commit