How to use IP helper-address to connect remote DHCP server

Once in a while, I see "IP helper-address" command for broadcast applications such as DHCP client(BOOTP), NetBIOS and so on. As you know IP helper-address is required a network is connected certain application at remote. For example, communicating between old computers that is using NetBIOS protocols, connecting remote DHCP server. In this article, we will mainly discuss about how to use IP helper-address to connect a remote DHCP server. The below example might not be realistic, b/c I just made up a case to better understand this technology.

 

 

[Configuration layout]

– As you can see below diagram, there are a DHCP server and two DHCP clients that one is in local and other is in remote. Where do you think the "IP helper-address" command needs to be inplace for DHCP client in remote? Answer is Ethernet interface of New York router. By default, router drop all the broadcast packtes sent thru the router. Because DHCP clients use bootp packets that are broadcasted to all hosts(255.255.255.255) and it will be all dropped by the router. The "ip helper-address" command makes the router to forward BOOTP broadcast packets to the specific remote DHCP server.

 

[New York Router]

version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname New_York
!
boot-start-marker
boot-end-marker
!
no aaa new-model
memory-size iomem 5
!
ip cef
no ip domain lookup
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
interface FastEthernet0/0
no ip address
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial1/0
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/1
ip address 10.20.1.1 255.255.255.252
load-interval 30
serial restart-delay 0
!
interface Serial1/2
no ip address
load-interval 30
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
interface FastEthernet2/0
ip address 100.100.100.129 255.255.255.128
ip helper-address 100.100.100.2 <— Pointing to Remote DHCP Server
duplex auto
speed auto
!
router bgp 10
no synchronization
bgp log-neighbor-changes
network 100.100.100.128 mask 255.255.255.128
redistribute connected
neighbor 10.20.1.2 remote-as 20
neighbor 10.20.1.2 timers 3 20
no auto-summary
!
ip http server
no ip http secure-server
ip forward-protocol nd
!
control-plane
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
end

 

[LA Router]

version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname LA
!
boot-start-marker
boot-end-marker
!
no aaa new-model
memory-size iomem 5
!
ip cef
no ip domain lookup
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
interface FastEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial1/0
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/1
ip address 10.20.1.2 255.255.255.252
load-interval 30
serial restart-delay 0
!
interface Serial1/2
no ip address
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
interface FastEthernet2/0
ip address 100.100.100.1 255.255.255.0
duplex auto
speed auto
!
router bgp 20
no synchronization
bgp log-neighbor-changes
redistribute connected
neighbor 10.20.1.1 remote-as 10
no auto-summary
!
ip http server
no ip http secure-server
ip forward-protocol nd
!
control-plane
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
end

[DHCP Server / mimic Cisco 3660]

version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname DHCP_Server
!
boot-start-marker
boot-end-marker
!
no aaa new-model
memory-size iomem 5
!
ip cef
no ip domain lookup
no ip dhcp use vrf connected
ip dhcp excluded-address 100.100.100.0 100.100.100.129
!
ip dhcp pool dhcp_pool <——— Name of DHCP pool
network 100.100.100.0 255.255.255.0 <—- IP pool for DHCP client
default-router 100.100.100.129 <———— Default route for DHCP client
dns-server 4.2.2.1 <————- DNS server info will be sent
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
interface FastEthernet0/0
ip address 100.100.100.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
ip http server
no ip http secure-server
ip forward-protocol nd
ip route 0.0.0.0 0.0.0.0 100.100.100.1
!
control-plane
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
end

** To make easy configuration, Cisco router 3660 are simulated as DHCP Server and Clients.

[Dynamips file / GNS3]

I hope now you get more understand about IP-helper address