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ip helper-address

I’m in the last stage of my studies, preparing for the BSCI exam. The big picture is clear but the amount of details…

I was playing with ip helper-address commands and want to review this simple yet useful command, that BSCI present when DHCP is in question but has other uses. I was reading the relevant parts from the Advanced Routing Companion Guide book by CiscoPress.

In short, broadcast is blocked on routers (remember the broadcast domain from your CCNA exam?) but sometimes we still need to forward this traffic. While BSCI focus on forwarding DHCP traffic across routers, DNS, TFTP or BOOTP are commonly used protocols that require the same solution. Let’s say you have a time-server at the HQ office and you want your branch office to use it or maybe a DNS server at HQ that branch offices use to resolve IPs.
ip helper-address present a solution to those scenarios.

The command syntax is simple:

ip helper-address [vrf name | global] address [redundancy vrg-name]

The important option is address = Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.

By default the helper option is disabled.
When enabled the following UDP services will be available: 37, 49, 53, 67-69, 137, 138.
The default services can be locked using the global no ip forward-protocol command.

The ip forward-protocol command (syntax), used in global mode to specify which protocols and ports the router forwards when forwarding broadcast packets.

ip forward-protocol {udp [port] | nd | sdns}

  1. James
    March 9, 2010 at 12:58 pm

    Yeah, the joke was always that a CCNA is trained to believe that routers do not forward broadcasts ever in a million years. And Cisco will test you on that on the CCNA. A CCNP is trained to know that a router can do anything you tell it to do, including forwarding broadcasts.

    Love the blog. Renewing my CCNA in preparation for the NEW CCNP tests.

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