IP Helper Address

IP Helper Address
IP helper addresses are needed when the forwarding of UDP broadcast packets is required. Such
instances would be when you need to forward DHCP or DNS requests. In order to enable the
use of IP helper addresses, you need to enter the following command in interface configuration
mode on the interface from which the unicasts should be sourced:
ip helper-address address
address - the address you need to forward the UDP packets to.
You can have multiple IP helper addresses on an interface. By default, the router will unicast
the following UDP broadcast packets to the helper address specified as long as the helper
address is not associated with the interface on which the broadcast was received:
 Trivial File Transfer Protocol (TFTP—port 69)
 Domain Naming System (DNS—port 53)
 Time service (port 37)
 NetBIOS Name Server (port 137)
 NetBIOS Datagram Server (port 138)
 Bootstrap Protocol (BootP) client and server datagrams (ports 67 and 68)
 TACACS service via the login host protocol (port 49)
 IEN-116 Name Service (port 42—obsolete)
66 Chapter 2  IP Addressing
You can add UDP ports to the list of broadcast packets that will be forwarded by using the
following command:
ip forward-protocol udp port
port - the destination UDP port number
You can add additional UDP broadcast packets that will be unicast-forwarded by using the ip
forward-protocol udp port command in global configuration mode, where port is the UDP
port number or related keyword for which to enable forwarding. You can also turn off the default
and additionally configured UDP broadcast packets that will be sent by using the no version of the
command. The following is an example of turning off the TFTP and TACACS service from being
forwarded to the helper address, turning on the Citrix client locator service, and configuring the
IP address of 182.16.45.9 as the helper address reachable through interface Ethernet0:
Router#conf t
Router(config)#no ip forward-protocol udp tftp
Router(config)#no ip forward-protocol udp 49
Router(config)#ip forward-protocol udp 1604
Router(config)#interface Ethernet0
Router(config-if)#ip helper-address 182.16.45.9
Router(config-if)#exit
Router(config)#exit
Router#