2.8 Helper Addresses
2.8.2 Configuring IP helper addresses

To configure the helper address, identify the router interface that will be receiving the broadcasts for UDP services. In interface configuration mode, use the ip helper-address command to define the address to which UDP broadcasts for services should be forwarded.

By default, the
ip helper-address command forwards the eight UDP services shown in Figure .

What if Company XYZ needs to forward requests for a service not on this list? The Cisco IOS provides the global configuration command
ip forward-protocol to allow an administrator to forward any UDP port in addition to the default eight. In order to forward UDP on port 517, you would use the global configuration command, ip forward-protocol udp 517. This command is used not only to add a UDP port to the "default eight" (see Figure ), but also to subtract an unwanted service from the default group. For instance, if you wanted to forward DHCP, TFTP, and DNS, and, for some reason, not Time, TACACS, and NetBIOS, the Cisco IOS requires that you configure the router according to Figure .