John Twohig's Library tagged dhcp →  View Popular

IP Helper-Address

The ip helper-address command is a static command that is configured on a router interface to direct broadcasts to desired destinations within your internetwork. Configuring the ip helper-address command is simple and straightforward. You just have to be aware of which interface the broadcasts are being received on and where you want to forward these broadcasts to.

For example, in Figure 7.21, the 2501A router Ethernet0 interface receives a Dynamic Host Configuration Protocol (DHCP) client request from HostA, which is a broadcast looking for a DHCP server.

Figure 7.21: An ip helper-address single server example

The 2501A router needs to be configured as follows:

2501A#config t
2501A(config)#interface ethernet0
2501A(config-if)#ip helper-address 172.16.30.2

The 2501A router now receives the DHCP client broadcast and forwards this broadcast to the DHCP server, which then provides a DHCP address to the client.

If you have more than one DHCP server for redundancy purposes, as shown in Figure 7.22, you can configure a subnet broadcast address instead of a single server address.

Figure 7.22: An ip helper-address multiple server example

The 2501A router would be configured as follows:

2501A#config t
2501A(config)#interface ethernet0
2501A(config-if)#ip helper-address 172.16.30.255

The 2501 router now takes a DHCP client broadcast and sends this to any DHCP server on the 172.16.30.0 network.

Remember that when you enable the ip helper-address command, you are forwarding more than just DHCP requests. Let’s discuss all the ports that are being forwarded

The entire DHCP server
configuration consists of three stages.




• 
  

  If you are running a
  firewall (iptables), allow the DHCP
  

• 
  

  You must configure the /etc/dhcpd.conf
  

• 
  

  A running dhcpd daemon
  (which can be started at boot time)