2.8 Helper Addresses
2.8.1 Using helper addresses
DHCP is not the only critical service that uses broadcasts. Cisco routers and other devices may use broadcasts to locate TFTP servers. Some clients may need to broadcast to locate a TACACS (security) server. Typically, in a complex hierarchical network, clients reside on the same subnet as key servers. Such remote clients will broadcast to locate these servers, but routers, by default, will not forward client broadcasts beyond their subnet. Because some clients are dead in the water without services such as DHCP, you are faced with two choices: to place servers on all subnets, or to use the Cisco IOS helper address feature. Running services such as DHCP or DNS on several computers creates overhead and administrative headaches, so the first option is not very appealing. When possible, administrators use the ip helper-address command to relay broadcast requests for these key UDP services.

By using the helper address feature, a router can be configured to accept a broadcast request for a UDP service and then forward it as a unicast to a specific IP address. Alternately, the router can forward these requests as directed broadcasts to a specific network or subnetwork.