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| 2.8 |  | Helper Addresses |
| 2.8.3 |  | IP helper address example |

Consider this complex sample helper address configuration. Assume it is desired that Host A must automatically obtain its IP configuration from the DHCP server at 172.24.1.9. Because RTA will not forward the Host A DHCPDISCOVER broadcast, RTA must be configured to help Host A.

To configure RTA e0, the interface that receives the Host A 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 IP address of the DHCP server. 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 will also 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 the Host A UDP broadcasts.

Finally, some devices on the Host A segment need to broadcast to the TACACS server, which does not reside in the server farm. Configure the RTA e0 to make it work by adding the command

ip helper-address 172.16.1.2.

Verify the correct helper configuration with the

show ip interface command.

Notice that the RTA interface e3, which connects to the server farm, is not configured with helper addresses. However, the output 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, e3 will need to be configured to forward directed broadcasts with the following command:

> RTA(config)#interface e3

> RTA(config-if)#ip directed-broadcast