2.8 Helper Addresses
2.8.3 IP helper address example
Consider this complex sample helper address configuration (see Figure ). Assume you want Host A to automatically obtain its IP configuration from the DHCP server at 172.24.1.9. Because RTA will not forward Host A's DHCPDISCOVER broadcast, you must configure RTA to help Host A.

To configure RTA's E0 (the interface that receives Host A's broadcasts) to relay DHCP broadcasts as a unicast to the DHCP server, use the following commands:

RTA(config)#interface e0

RTA(config-if)#ip helper-address 172.24.1.9


With this simple configuration, Host A broadcasts using any of the eight default UDP ports will be relayed to the DHCP server's IP address. However, what if Host A also needs to use the services of the NetBIOS server at 172.24.1.5? As configured, RTA will forward NetBIOS broadcasts from Host A to the DHCP server. Moreover, if Host A sends a broadcast TFTP packet, RTA also will forward this to the DHCP server at 172.24.1.9. What is needed in this example is a helper address configuration that relays broadcasts to all servers on the segment. The following commands configure a directed broadcast to the IP subnet that is being used as a server farm:

RTA(config)#interface e0

RTA(config-if)#ip helper-address 172.24.1.255

Configuring a directed broadcast to the server segment (172.24.1.255) is more efficient than entering the IP address of every server that could potentially respond to Host A's UDP broadcasts.

Finally, some devices on Host A's segment need to broadcast to the TACACS server, which does not reside in the server farm. You can configure RTA's E0 to make it work by adding the command
ip helper-address 172.16.1.2.

You can verify the correct helper configuration with the
show ip interface command, as shown in Figure .

Notice in Figure that RTA's interface E3 (which connects to the server farm) is not configured with helper addresses. However, the output in Figure also shows that, for this interface, directed broadcast forwarding is disabled. This means that the router will not convert the logical broadcast 172.24.1.255 into a physical broadcast (with a Layer 2 address of FF-FF-FF-FF-FF-FF). To allow all the nodes in the server farm to receive the broadcasts at Layer 2, you will have to configure E3 to forward directed broadcasts with the following command:

RTA(config)#interface e3

RTA(config-if)#ip directed-broadcast
Interactive Lab Activity (Flash, 387 kB)
  In this lab activity you will configure SanJose2 to act as a DHCP server. Then you will configure SanJose1 to forward UDP broadcasts for DHCP requests.

Finally, you will test your configuration using a DHCP client.