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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.
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