IP Helper-Address Command on CISCO Router/Switch

Command

IP Helper-Address

Use

This command is used to enable the router to forward BOOTP broadcast (and other)packets to a specific host.

Syntax

Router(config-if)#ip helper-address <ip address>

Example

In this example, we will configure R1 to provide DHCP services. After that we will configure R3’s Fa0/0 interface to receive an IP address from R1’s DHCP pool.

R1(config)#ip dhcp pool VLAN_A
R1(dhcp-config)#network 10.2.2.0 /24
R1(dhcp-config)#default-router 10.2.2.3
R1(dhcp-config)#dns 4.2.2.2

We see below that, even we configure R3 to get its interface via DHCP, because the DHCP server is more then one hop away, we will need to use a helper-address.

R3(config-if)#do show ip interface brief | ex unass
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.2.2.3 YES NVRAM up up
Serial1/1 192.168.13.2 YES manual up up
Loopback0 3.3.3.3 YES NVRAM up up
Loopback1 33.33.33.33 YES NVRAM up up
Loopback2 133.133.133.133 YES NVRAM up up

R3(config)#int fa0/0
R3(config-if)#ip add dhcp
R3(config-if)#
*Mar 2 03:06:43.437: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

Now we configure a helper-address on R2’s Fa1/0 because it will be the interface that receives the broadcast from R3.

R2(config-if)#int fa1/0
*Mar 2 03:07:20.441: %OSPF-5-ADJCHG: Process 1, Nbr 133.133.133.133 on FastEthernet1/0 from FULL to DOWN, Neighbor Down: Dead timer expired

R2(config-if)#ip helper-address 10.1.1.1
R2(config-if)#
*Mar 2 03:07:54.805: %OSPF-5-ADJCHG: Process 1, Nbr 133.133.133.133 on FastEthernet1/0 from LOADING to FULL, Loading Done

Now we will flap the interface on R3 to resend the DHCP information.

R3(config-if)#shut
R3(config-if)#
*Mar 2 03:07:39.385: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Mar 2 03:07:40.385: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
R3(config-if)#no shut
R3(config-if)#
*Mar 2 03:07:45.573: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Mar 2 03:07:46.573: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
R3(config-if)#
*Mar 2 03:07:50.057: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 10.2.2.1, mask 255.255.255.0, hostname R3

R3(config-if)#
*Mar 2 03:07:54.161: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from LOADING to FULL, Loading Done

R3(config-if)#do sh ip int brief | ex unass
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.2.2.1 YES DHCP up up

R1(config)#do sh ip dhcp bind
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
10.2.2.1 0063.6973.636f.2d63. Mar 03 2002 03:07 AM Automatic
6330.332e.3134.3138.
2e30.3030.302d.4661.
302f.30

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