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.
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