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Cisco ip helper-address command

Can somebody help me out in understanding how ip helper-address works as far as using it in a network design for DHCP? I understand that you use the command on the router and when the DHCP is on another segment. This is to allow the router to send the dhcp broadcast to the other segment. So with that understanding, it is used on a layer 3. But I notice that you can also configure it on a Catalyst 2960 layer 2. What I did was I configured vlan 20, assigned an IP address to it, and configure the ip helper-address command. So how will this work? Can somebody clarify? Thanks
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atechnicnate

The ip helper is going to allow dhcp to be active across that vlan using the echo pool from the attached defined pool.  So anything on vlan 20 will now get ip addresses from the network that the ip is contained in.
Ps it also helps broadcast UNDP services farther throughout the network so things like automatic discovery can pass through the switch. It increases you broadcast domain.
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so you have to use the helper-address command within its specific vlan (vlan20 in this case) so that the users within that vlan can get ip addresses. Correct? How about those users not in that vlan20? How will they get the dhcp ip addresses? Thx
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Premkumar Yogeswaran
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Very good input BDC-Net & premglitz. So you can only use 1 svi on the 2950.
Now, when you use the helper-address, I have to specify the IP address of the DHCP server. My DHCP server has three scopes for my three sets of users. Each set of user has been assigned to its own vlan. So how will the IP addresses assigned to my users.

On my dhcp server, I have:
10.10.10.0/24, 10.20.20.0/24, and 10.30.30.0/24

I have vlan10, vlan20, and vlan30. I configured vlan10 for 10.10.10.1, vlan20 for 10.20.20.1, and vlan30 for 10.30.30.1.

In the helper-address, I specify the ip address of the server (the IP address of the NIC card). But how will this work as the 2900 switches are L2 switches? How will it know how to get to the dhcp server?

Thanks
The helper address need's to be put on a L3 interface. Usually the default gateway interface. (i.e. vlan where you gateway lives) When it sends the unicast request to the server it will include the source address of the subnet so the server will know what scope to use.