Suggestions about network design - DHCP multiple subnets

Posted on 2008-10-27
Last Modified: 2010-08-05
Hi Everyone,

I'm designing a network for around 3 servers (The DC is the DHCP Server), 25 Workstation, 25 IP Phones, 1 router, 1 NAS and a few other network devices.

I'd like to setup the DHCP Server to hand-out IP's depending on which switch it's attached to.
I'd also like setup each group of devices to be in a specific IP range (see below).

Servers: - 255
NAS/SAN Enironment:
Other Network Devicess ( Printers etc):
IP Phones and PABX:

Currently we have a number of unmanaged switches for the workstations and IP Phones (these are seperate, meaning the traffic is physically seperated until it reaches the main switch) which run uplinks back to the Comms Room into the main switch (unmanaged).
The servers use a HP Managed Switch which also connects to the main switch.
And finally we have all other network devices patched back into main switch.
The router connects directely into the main too. And it connects out to our modem as well.
All workstations/servers use the router as a gateway, and use our DC as our DNS address (It has a DNS running already).

I would like our DHCP Server to distribute IP based on which switch I plug the device into.  
I'm not sure exactly how to set this up with multiple subnets etc.  Currently all IP are using a subnet mask of so everything can see everything (which is what I want it to stay).

Could someone please provide any suggestions/options as to whether this is possible and how to acheive this?

Also if you could provide subnetting suggestions to, that would be greatly appreciated.

Thank you everyone in advance.
Question by:james_daley
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LVL 11

Expert Comment

ID: 22819674
In order to hand out different DHCP subnets from the same server, you'll need to VLAN your network to isolate the various subnets. This isn't very hard to do at all, especially with the flood of cheap used Cisco Layer-3 switches, such as the Cisco 3550, which can function as a multiport VLAN router.

So the question is, before I or somebody else spends time on a VLAN tutorial, are you prepared to go that route (as it were ;). You'll need one Layer-3 switch (an HP will do, but Cisco is better because it's more plentiful and there is more talent readily available), and you'll need to replace all your unmanaged switches with managed VLAN-capable switches (used Cisco 2948s can be had for $100 on eBay; they're perfect for this application).

To reliably deploy VoIP you generally must have VLAN traffic isolation. Otherwise simple YouTube videos and the like will overrun your voice traffic and kill voice quality.

Once you have a solid VLAN network in place, then I am happy to explain the details of using the DHCP Helper settings in Cisco routers, and how to configure multiple DHCP scopes on your DC. It's straightforward, but you have to have a good foundation or you'll just end up with a garbage network.

Author Comment

ID: 22827115
Hi Packetguy,

Firstly, thank you for your reply.
I had a quick look at our existing router, and it already supports vlans.  Its a Snapgear SG580 router.
I'd be interested in using this router before spending any money.  I has 4 LAN ports which can be seperated into a per port/vlan.  Perhaps this will do?

You were saying that we'd need to replace all unmanaged switches with managed ones, could we use our unmanaged switches to connect the same devices, then physically seperate the uplink and run it into a managed switch (which can be used as the backbone)? <- We could purchase this managed switch.
I'm thinking this will reduce cost and setup etc.  We've currently got our workstations and IP Phones physically connected to different unmanaged switches which runs to the backbone managed switch.

Thanks for all your help,

LVL 11

Accepted Solution

packetguy earned 500 total points
ID: 22827480
According to the SG820 Administrator's Guide, the device supports DHCP relay:

 DHCP Relay page
 Use this page to configure a DHCP relay on the selected interface. A DHCP
 relay allows you to forward DHCP requests to a DHCP server on another
 network. This allows you to use a single DHCP server to handle multiple
 networks. The DHCP proxy allows the SnapGear appliance to forward DHCP
 requests from the LAN to an external server for resolution. This allows both
 static and dynamic addresses to be given out on the LAN just as running a
 DHCP server would.

So it appears that you could use this device to route up to four separate VLANs using the Port VLAN approach you mentioned. You would need to configure separate DHCP scopes for each of your VLANs. When the SG sees a DHCP request on a port, it will forward it to the designated DHCP server, passing along the IP network address and mask. Your DHCP server will interpret this information and use the correct scope for granting IP assignments.

You could use your unmanaged ports with independent home runs to a single managed switch. You would have to configure the VLANs as all untagged ports, and run three cables (one for each of the SG's Port-VLAN ports) to the SG.

The downside of this method is that unmanaged switches don't give you any visibility into traffic storms, port negotiation problems, or other LAN failures. In any kind of business network, you're much, much better off with managed over unmanaged. Using unmanaged switches is like trying to drive at night with no headlights. You can sort of do it, but eventually you're going to get hammered. Given that you can buy very high quality used Cisco managed switches for the same price as a new unmanaged switch (and even buy Cisco maintenance in the bargain), I can't see any reason to stick with the unmanaged misfits other than the inconvenience of physically swapping them.

I say misfits deservedly: unmanaged switches have caused more network headaches than any other device I know. They create bridge loops, cause port negotiation problems, and induce all manner of bad things. I've had clients spend DAYS tracking down a single errant unmanaged switch throwing packet storms into their LAN. Unmanaged switches were never designed to be used in a multi-switch LAN fabric. They're simply not designed for it. They work great at home on the desktop, but they ultimately cost more in wasted time than they save in capital expense.

LVL 11

Expert Comment

ID: 22926600
james_daley, did this address your issues? Points, man, I need points! ;)


Author Comment

ID: 22927104
SOrry packetguy, yes it sure did.

Thank you for your help :)

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