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Multiple DHCP Scopes for Vlans

Posted on 2011-02-22
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Last Modified: 2012-08-14
Forgive my ignorance on this subject, but here goes.

DHCP Server: Server 2003 SP2
Switches: Cisco 3560's

We have a vlan setup for our voip phones, and currently they get all of their DHCP from the phone server itself since that vlan could not see our DHCP server. However now I have activated the 2nd nic on our DHCP box, and would like to manage DHCP from there. So I have a nic in the box set to 192.168.0.x and one to 192.168.2.x (and the switch set so the 2.x net only sees that vlan). I know how to add a new scope, but how do I link that scope to a specific nic. Here is how I think it works.

PC/PHONE/WHATEVER needs an IP address, so it asks hey is there a dhcp server here. In which case regardless of subnet this server will respond yes and give it an address.

I found this basic definition.

A DHCP scope is a valid range of IP addresses which are available for assignments or lease to client computers on a particular subnet.

That sounds all good, except the client machine is not on a subnet until the DHCP server assigns one right? So how does it know which scope to use?

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Question by:bhieb
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by:pitchford
pitchford earned 50 total points
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Why not reconsider your thinking. You can run multiple VLANs with a single DHCP server. We have man VLANs all connecting back to the same DHCP server with 1 NIC. On the VLAN interface you'd set something like the following:

int vlan 100
ip helper-address x.x.x.x (this is your DHCP server)

This will point all notes sending a DHCP request on VLAN 100 to point to the DHCP server. We do this with our local VLANs and our remote VLANs across the WAN.
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by:bhieb
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Regardless of how the request gets to the server, how does the server know which scope to use? In your case how does your server know to use scope A for vlan1 and scope b for vlan2?
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Krzysztof Pytko earned 400 total points
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Those VLANs are separate networks and each VLAN on switch have configured IP Helper which point them to appropriate DHCP server. Then using network ID, DHCP server located appropriate scope and issued IP lease to the device. So, there is no possibility to get IP adrdess for other VLAN if you are in another :)

Regards,
Krzysztof
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by:bhieb
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By the way your method is exactly what I'm trying to do. Currently we have 2 DHCP servers, I want to get rid of the extra one and just use our main DHCP.
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by:bhieb
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@iSiek

Ok I think I understand because the DHCP relay or helper in the swich only forwards traffic on 2.x the server assigns according to that traffic.
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by:bhieb
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So simply adding the scope is all I need to do, as whatever subnet it is coming from is what the server will use.  Let me try that out.
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by:Krzysztof Pytko
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OK :) Wish you luck. If you configure DHCP Relay Agent/IP Helper properly, should work like a charm :]

Krzysztof
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by:h3nnys
h3nnys earned 50 total points
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In the DHCP console, server properties, Advanced tab, Bindings button. You
should be able to provide DHCP on any and all interfaces that have a static
IP address. Any scopes you create must be consistent with the subnet for
one of the static IPs unless you are using DHCP Relay Agent. Not sure
whether that answers your question.

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