Cisco CCNP / BCSI Exam Tutorial: Broadcasts And The IP Helper-Address Command
Learn how to allow certain broadcasts to be forwarded by a router in this tutorial from Chris Bryant, CCIE #12933.
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While routers accept and generate broadcasts, they do not forward them. This can be quite a problem when a broadcast needs to get to a device such as a DHCP or TFTP server that's on one side of a router with other subnets on the other side.
If a PC attempts to locate a DNS server with a broadcast, the broadcast will be stopped by the router and will never get to the DNS server. By configuring the ip helper-address command on the router, UDP broadcasts such as this will be translated into a unicast by the router, making the communication possible. The command should be configured on the interface that will be receiving the broadcasts.
R1(config-if)#ip helper-address ?
A.B.C.D IP destination address
R1(config-if)#ip helper-address 18.104.22.168
Now, you may be wondering if this command covers all UDP services. Sorry, you're not getting off that easy! The command does forward eight common UDP service broadcasts, though.
TIME, port 37
TACACS, port 49
DNS, port 53
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