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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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biggynet
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biggynet
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2 Solutions
 
atechnicnateCommented:
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.
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atechnicnateCommented:
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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biggynetAuthor Commented:
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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BDC-NetCommented:
When a computer needs and IP address it send a broadcast out (which is contained within that vlan). If the DHCP server is a member of that vlan then you don't need a helper address because it will see the request. If the DHCP server is on another vlan then you will need the helper address to direct the broadcast to the specific server (unicast). If you have multiple vlans and only one DHCP server then you will need to put a helper address on each vlan pointing to that server.
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Premkumar YogeswaranCommented:
I have confirmed in our lab that both Cat2950 and Cat2960 are capable of running as DHCP Relay Agents. The prerequisites are:

 

    Have a SVI (i.e. the interface Vlan X) created for the particular VLAN in which the clients reside. This SVI must be configured with an appropriate IP address, netmask and the ip helper-address command, and it must be activated using the no shutdown command
    Have the correct default gateway configured for the switch using the ip default-gateway command
    This switch itself must be capable of successfully pinging the DHCP server specified in the ip helper-address command

 

There is a major limitation on the 2950 switches: as there can be at most one SVI in the "no shutdown" state at any time, the DHCP Relay feature can be activated only for a single VLAN whose SVI is currently activated. There is, I believe, no workaround available. The 2960 switches do not have this limitation.

 

IOSes used in my experiment:

 

2950: c2950-i6k2l2q4-mz.121-22.EA13.bin

2960: c2960-lanbasek9-mz.122-58.SE1.bin


Refered this link:
https://supportforums.cisco.com/thread/2124839

Regards,
Prem
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biggynetAuthor Commented:
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
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BDC-NetCommented:
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.
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