Configuring DHCP pools for Multiple VLANs

Unanswered Question
Oct 9th, 2009

I have a DHCP server (configured on a Windows Server). I'd like to know how I would go about configuring my Cisco Routers/Switches to allow my separate subnets/VLANs to receive the appropriate addresses from the correct pools on my server.


What prompted this effort was the fact that my network has grown beyond the point of what my /24 scope can facilitate. I've decided that breaking up the network into separate /24 subnets will solve that problem...but unsure on how to get it done.


A little additional information - I'm running Windows Server 2003, Cisco 2950 Switches on the LAN and a 2800 as my core router.

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mattwilsonuk Fri, 10/09/2009 - 08:20

Create your vlans on the switch, apply the default gateway for each subnet to each vlan and add the helper address to each vlan as the windows server.


Put the relevant ports in the relevant vlan and any devices which send a dhcp request will always be assinged from the dhcp pool on your windows box which is relevant to the vlan the request comes from.

jbrenesj Fri, 10/09/2009 - 11:59

If that'e the only hardware you have then creating VLANs on the switch works but in order for the devices in that VLAN to receive an Ip address from the windows server that is on another VLAN you need the help from a layer 3 device that can move the DHCP discovery from the vlan of the host to the server.

The 2950 can't do thos task so you need your "inter-vlan routing" done by the router.

Take a look at the following example called "router on a stick".


Then you need the command:

ip helper address x.x.x.x (where x.x.x.x is the IP if the DHCP server)


This command will be applied on the NEW sub-interface that will route for each of the new VLANs.


http://www.cisco.com/en/US/products/hw/switches/ps628/products_configuration_example09186a00800ef797.shtml

neville.warner Mon, 10/12/2009 - 16:27

Thanks.....I've tried your suggestions, but still unable to get traffic flowing through the subnets.


A little bit more details on the network...


We recently put in a CUCM and with that created 2 subnets (VLAN10 for data and VLAN20 for voice). The switches (2950) are configured with spanning tree PVST. I assumed that given that those 2 VLANs are pretty easily distributed along the switches that adding a third data VLAN wouldn't be too difficult. But I guess the requirement of separate DHCP scopes for each VLAN complicates it.


What I've done so far...



As suggested I've created the VLAN on the switches, and on the router (dot1q encapsulation). On the sub-interface on the router, I added the ip helper-address command, pointing it to the DHCP server.


Please let me know what I'm missing and if there's any additional info I can provide to make it any more clearer.


Thanks again


iyde Tue, 10/13/2009 - 01:01

Neville,

As far as I read your response the only thing missing is to create the different scopes on the DHCP server.

One other thing to check out, though: You mention CUCM - I believe that in the voice VLAN you have to put in DHCP option 150 (I believe it's that option) in order to make the phones work.

On the ports of the Cat2950 you'll need something like this

interface fastethernet0/1

switchport

switchport mode access

switchport access vlan

switchport voice vlan

spanning-tree portfast


HTH.

neville.warner Tue, 10/13/2009 - 08:03

OK...Here's where I'm at.


What works....Getting my laptop to be assigned an address from the respective DHCP scope. Thats a plus.


However, that only works if i'm connected to the same switch that the DHCP server is connected to. I tried that as a control measure. If I connect it to any other switch that branches off that main switch, I get nothing. This would look like a trunk problem, but all other switches are connected via trunk ports.


I notice though that after creating the additional VLans on the other switches, the interfaces are all down/down, except on the switch that I've assigned a port to that VLan. In other words, if I've only just created the VLan interface, and assigned an IP address to it, it remains down seemingly until I assign a switch port to it. Should this be the case?


...and thanks a lot for the assistance thus far.


Let me know what you all think.

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