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