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| 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. In a complex hierarchical network, clients may not 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. Some clients are unable to make a connection without services such as DHCP. For this reason, the administrator must provide DHCP and DNS servers on all subnets, or use the Cisco IOS helper address feature. Running services such as DHCP or DNS on several computers creates overhead and administrative problems, 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

ip helper-address command, a router can be configured to accept a broadcast request for a User Datagram Protocol (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.

| Web Links UDP Broadcast Flooding http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ics/cs006.htm