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DHCP design

Posted on 2011-03-10
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I need to build a new  DHCP environment to handle about 350 users.
I want to divide the users into seperate vlans and create multiple scopes in my DHCP server accordingly.  I want to ensure DHCP redundancy in case of the primary server failing.  I am going to be setting up two windows 2008 servers and would like to utilize the spit scope feature. My question is what would be the optimal number of users I should have in each vlan and how should the split scope be setup.  I hear a lot of recommendations for the 80/20 split but I don't understand how that would work.

thanks
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Question by:FREDARCE
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by:Burns2007
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The 80 20 works like this:
If you have 100 address usable in a vlan:
Configure DHCP on 1 server to serve 80 addresses. Configure DHCP on the other to serve 20 address.
So one server has 80% of the addresses, and if it fails, you use the other 20% on the other server.


For specific setup details for this scenario see: http://technet.microsoft.com/en-us/library/ee405264%28WS.10%29.aspx

Just replace their subnet with yours as appropriate.
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by:araberuni
araberuni earned 250 total points
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You can configure DHCP cluster. DHCP cluster is advantagious than Split-Scope DHCP in terms of fault tolerance and redundancy. Here is a complete guide for DHCP cluster http://microsoftguru.com.au/2011/02/27/configure-microsoft-fail-over-cluster-for-dhcp-servicesstep-by-step/
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by:araberuni
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In addition to my previous post, 350 clients are not much. I reckon, a single vlan is ok for you.
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by:FREDARCE
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It was recommended to me to not have more than 150 to 200 hosts in a vlan so I will most likely split up the 350 user among 3 vlans.  But I still don't quite grasp the concept of the 80/20 split.  If the primary DHCP server is serving 80 percent of the IP's and goes down then only 20 percent of the hosts will be able to obtain or renew IP's for that particular vlan.  How does that provide redundancy?
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Thomas_Roes earned 250 total points
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The main problem of lot's of clients in one (V)LAN is more and more broadcast traffic. 350 is not a problem in that sense.

What might be a problem is the commonly used /24 subnets (subnet mask 255.255.255.0). There you can have 253 adresses (2^8 - 3 (broadcast/network and router)).

I personally am not happy with the 80/20 split, but it all depends a lot on your configuration and network use. You should pay attention that your 20% scope is not used as long as the 80% server is online. But even then, a client that somehow get's served by your 20% server will keep on trying to use it's last (20% server). That is why I am more in favour of 50%/50%, and only use half the available adresses. In your case 2x 350 = 700, so you need a subnetmask of /22 (255.255.252.0).

Best practise is to configure the whole subnet range on both DHCP controllers and put exclude ranges so the 2 DHCP servers can't serve the same IP addresses.

Added bonus is that you can create specific reservations in both servers (you don't want that printer to get a different IP address if one of your servers is down).

And one last point to pay attention to: UPDATE your DHCP server. The out-of-the-box 2008r2 dhcp server has a bug:
- in the configuration I just described
- printer is turned on
- both servers offer an IP address
- Printer selects server A
- Server B delete's the reservation.

You don't want that.

Good luck

Thomas



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by:FREDARCE
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Thanks for all your feedback.  I am now deciding between using  50/50 split or DHCP cluster.  
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