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Can you give me some clarification on helper-addresses?

You recently answered the question below:
"I have two subnets and a Cisco 2600 switch. I want to install a DHCP server on each subnet and use the switch to block DHCP requests getting from one subnet to the other. How can I do this on the 2600? It only has two fast Ethernet interfaces configured.

And you responded:
When a DHCP client sends a request the server -- the server will respond to the one that has a better priority. So you can set priorities so that for one subnet only one server will respond. If you are configuring your router as DHCP server than you need to configure two different DHCP pools and block addresses using Mac address. You can also set priority. You can use access-lists also to block. Also what I think is you might not need two servers. You may only need to have one server with two scopes and use helper address on the other subnet."

Here's my concern:
I don't agree with your answer. From my experience, a router (he states switch in the question but I believe he means router) will not forward DHCP requests across subnets unless explicitly requested to with the "ip helper-address" command. Which means that all DHCP queries in a subnet will only be serviced by the server within its subnet. You yourself said in your answer that you could have one server in one subnet and use a helper address to forward the DHCP messages to the other subnet. Not so?
That's right. You need to have helper-address before you can forward the DHCP request. But you can have six of them on an interface and at the same time you can have secondary IP addresses on you interfaces. So you can have one more secondary IP on that interface with a single or double helper address. You can choose what is good for you.

This was last published in March 2004

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