Help Understanding IP Helper

There is obviously something about ip helper I do not understand. Perhaps someone could explain.

My understanding is that when ip helper is placed on an interface it will forward the following upd brodcasts to the address specified:

TFTP (69) DNS(53) Time(37) Netbios name Service (137) Netbios Datagram (138) Bootp Server (67) Bootp Client (68) TACAS (49)

I did not think that dns or tftp, for example, are broadcasts. Could someone explain what a dns broacast is? In both dns and tftp the client specifies an ip address. doesn't that make them unicast?


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I think Macs use broadcast DNS as part of their "Rendezvous" mechanism.

Broadcast TFTP is used by netbooting devices. In fact, Cisco routers themselves will send out a broadcast TFTP request for their config file.

That's a question about what applications use broadcast. What question did you have about the "ip helper-address" configuration? :)

Reply to
Barry Margolin

Thanks for the insight Barry. I think I understand how to configure ip-helper. I have an application that required it. I need to allow dhcp broadcasts to forward from one subnet to another. I configured ip-helper address on one interface of the router pointing to the dhcp server on the other subnet. Since I had an acl on the same interface as the ip-helper address that only permitted telnet, I had to add permit upd any any eq 67 to the acl.

Thanks again.

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