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DHCP and VLAN  Problems

Posted on 2010-11-11
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Last Modified: 2012-05-10
Hi ;

Currently all my users is using this IP range 10.0.0.x 255.255.0.0 .
We have 7 floors in this one building . Each floor have a 24 ports switch 3Com 2924-SFP Plus which serves all the user at that particular floor .
Each switch will connect back to our main switch which is 3Com 2924-SFP Plus through fiber.
This main switch will connect to a firewall thru UTP cable for the gateway.
Gateway IP is 10.0.0.1/16

Now what I'm trying to do is :-
1) To allocate each floor with different ip segment(using DHCP) . i.e :-
     Level 1 = 10.0.1.x /16
     Level 2 = 10.0.2.x/16 and so on

So my question will be :-
1) Can it be done with our existing switches ? If yes , do I need to configure each LAN port at those switches manually ? Can i just do it at the SPF ports instead?
2) If it CANT be done , what are the hardware that I should procure ( ie switch specs)
3) For DHCP , I cannot create multiple VLAN that have the same subnet .It will throw an alert informing me to change the subnet .
Example :- First I will create a scope of 10.0.1.x/16 . It works well . Then when I try to put in 10.0.2.x/16 , the alert came in. How is the correct way to do this? I want the DHCP server to automatically issue IP address based on the user locations. Else every user will have 10.0.1.x despite their locations.

Thx
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Question by:chongbenkee
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by:Trackhappy
ID: 34117865
First issue is you nee to use /24 not /16. 10.0.1.x 255.255.255.0, 10.0.2.x 255.255.255.0 etc. You could set the VLAN at the core switch on each port without changing the floor level switches, but that is probably not best practice.

Each vlan will need to have a DHCP helper address on it pointing back at a central DHCP server wih all the ranges defined.

As long as those switches are layer 3 switches, you should be fine.
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Expert Comment

by:Trackhappy
ID: 34117882
Looks like that switch is not Layer 3 capable, ie it will not perform the inter-vlan routing. You might have to replace the core switch with a Layer 3 switch to make it all work.
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Author Comment

by:chongbenkee
ID: 34118283
Ok. Therefore , I will need to buy :-
a) A layer 3 switch for all floors
b) And those switch must come with DHCP helper address . Is this module exist on all L3 switches ?

Then, assume I got the switch :-
If I change all the PCs address to use /24 , will it be able to find our gateway that's been using /16 ? Or I need to change it as well?
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Expert Comment

by:Trackhappy
ID: 34132601
You don't have to buy a Layer 3 switch for each floor (but you could if you wanted to and have the budget), you could use one central one as a Layer 3 router and concentrator and keep the layer two ones at each floor. DHCP Helper addredd is a term for an IP address you configure on the switch where it knows to send DHCP requests. On a central Layer 3 switch you would configure a VLAN for each floor and make a particular port a member of that vlan, to which the particular floor switch is connected. The Vlan itself will have an IP address, which becomes the default gateway for that segment. The DHCP Helper will listen for client requests on the VLAN's and forward them to your DHCP server, on which you configure ranges for each segment.

Hope that helps.
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Expert Comment

by:Trackhappy
ID: 34132608
Your default gateway (Internet firewall?) will be configured in the central switch as its default gateway and any traffic not destined for the configured valn's will be sent there.
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Accepted Solution

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Trackhappy earned 500 total points
ID: 34132612
Man, I gotta learn to type or get a spell checker on this site....
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Author Closing Comment

by:chongbenkee
ID: 34142483
Thanks.It help a lot.
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