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DHCP split scope

Posted on 2010-09-21
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Last Modified: 2012-05-10
HI all,
I would like to split scope in DHCP. I has the IP range 192.168.0.100-192.168.0.199. We have aobut 70-80 computers which use DHCP. We have two DHCP servers. How do we configure split scope? Do I use 50/50 or something else?
Thank you
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Question by:dongocdung
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by:jorlando66
jorlando66 earned 50 total points
ID: 33729212
Generally an 80/20 rule is used in establishing split dhcp scopes.  Here are two good articles to get you going.

What operating system?  Windows 2008 r2 handles dhcp failover differently.. I will post both :)


http://technet.microsoft.com/en-us/library/dd296651(WS.10).aspx
http://technet.microsoft.com/en-us/library/cc757346(WS.10).aspx

2008 r2
http://technet.microsoft.com/en-us/library/ee405264(WS.10).aspx

http://blogs.technet.com/b/teamdhcp/archive/2009/01/22/how-to-configure-split-scope-using-wizard.aspx



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by:HunterPine
HunterPine earned 100 total points
ID: 33729225
You need a DHCP pool twice the size of your computer account to enable split-scope DHCP.

The idea is that if one of the servers goes down, the other server can hand out enough addresses to satisfy the needs of the network devices.

So for 80 computers, you'd want to be handing out somewhere along the lines of 192.168.0.2-101 on one server, and 192.168.0.102-201 on the second, which leaves you expansion space for 20 more computers, and gives you 192.168.0.202-254 for static addresses.

When configuring your scopes, however, you want to configure the ENTIRE range in both DHCP servers, so all 200 IP addresses, then exclude the 100 addresses the other server will be responsible for. You do this because DHCP reservations take precedence over exclusions. So if you have a DHCP reservation, you can add it to both servers and in the event one server goes down, or the wrong server responds to the DHCP request, the reservation will still be properly handled.

Honestly though, with that many computers, I'd probably up my IP network to a /23 (192.168.0.0 through 192.168.1.255, using a subnet mask of 255.255.254.0) and make the DHCP scopes 200 IPs each or somewhere in that area so you're future-proof.
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by:Matthew England
ID: 33729266
You could use 50/50 given your quantity of addresses in the scope and the number of systems you have currently. (The general rule is 80/20)

Just create the same scope on both DCHP server, then create an exclusion for the range you want to omit on each.

So with the 50/50 split. Create an exclusion for 192.168.0.100 - 150 on DHCP Server 1, then create an exclusion for 151 - 199 on DHCP 2.

Given that you have about 80 computers and your scope only allows for 99 host's currently, I'd look in to expanding that, (especially if you use VPN which draws from the same DHCP scope).

You could also just create the scopes on each server to be lmited to the IP's you want them to serve rather than using exclusions, but the benefit of having them the same with an exclusion, is that you can easily delete/add exclusions, without modifying the scope. If you want to change the scope later, then you have to delete/recreate on most systems.
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by:dongocdung
ID: 33729358
I just have that IP Range 192.168.85.100-199 so i don't think I have enough IP Address if I use 50/50 rule because I have around 70-80 computers using DHCP. I  would like to have 50/50 rule before but don't think it is good idea. If we use 80/20 rule and the primary DHCP server down, the second DHCP server willhave around 20 IP addresses and it will not provide enough IP address.
Other IP address we use for route, switches, reservation and special PCs.
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by:Matthew England
ID: 33729473
PC's don't care which DHCP server they get their IP from, so you'll see leases on both servers all the time.

Keep in mind that the lease length plays a part in this too. If your lease duration is set to a short period (and your not able to recover your second DHCP server within half that time) then you'll end up running out of addresses. If you have longer leases (1 week or 1 month) which is good for stable networks, then the second fail over scope becomes a bit less important as it would only need to service PC's which haven't been online since the failure.

If you have short leases and lots of laptops (like I have), then you'll want to have a larger secondary scope, so that it can service all or a majority of your existing clients, which serves as the justification of your 50/50 split, but you would need 80-100 addresses in each scope.
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Author Comment

by:dongocdung
ID: 33735364
I am thinking to configure the second DHCP server as DHCP delay. We have 72 PCs which are active now. what do you think?
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by:Matthew England
ID: 33736844
Sounds like a good plan. So then your setup would look something like:

SERVER A
  Start - 192.168.85.100
  End - 192.168.0.85.199
  Lease Duration: 7 Days

  Exclusions:
     Start- 192.168.85.180
     End- 192.168.85.199

SERVER B
  Start - 192.168.85.100
  End - 192.168.0.85.199
  Lease Duration: 1 Days

  Exclusions:
     Start- 192.168.85.100
     End- 192.168.85.179

The lease duration's can be adjusted to suit your environment. As shown above, the 7 Day's on Server A, allows PC's to retain leases obtained from Server A, for a week with it offline, before they'll pull one from SERVER B. The shorter duration on SERVER B, will help facilitate quicker circulation of IP's and fail back to the primary DHCP server, when it comes back online.

The duplication of the scope, and subsequent use of Exclusions to limit the available IP's, will enable you to create reservations (as required), and ensure that both DHCP servers are able to honor those reservations, provided you configure the reservation on both DHCP servers.

It also allows you to more easily shift available IP's in the pool from one server to the other, without recreating your Scope. (For example if you needed to have SERVER A offline for an extended period, you could easily reduce or delete the reservation on SERVER B, to increase the available pool of IP's.

I'd also reccomend reviewing the Best Practices guide for MS DHCP, in addition to the good articles jorlando66 provided above. The Best Practices are less technical, but provide a series of things to consider when planning DHCP for your network.
http://technet.microsoft.com/en-us/library/cc780311(WS.10).aspx
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by:dongocdung
ID: 33737711
Does server B have enough IP addresses if the server A is down? As your recommendation, server B will lease 20% IP Addresses.
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Matthew England earned 350 total points
ID: 33738321
That will depend on your lease duration, how long the server is down, and the stability of your network. (i.e. lot's of desktops, alway's on, versus laptops that get unplugged / turned off.)

Personally I would opt for a larger IP range for your DHCP scope, in order to allow for growth and fail over, but the config above will provide a reasonable amount of coverage. (As a personal preference I like to keep my average available IP's in my address pool, around 25% on the primary server, and because I operate in networks with many RAS clients and laptops, I keep my failover pool closer to about 40% of the average utilized addresses for each scope. For the Scope servicing my VoIP phones, I have a 50/50 setup with an available pool of addresses equal to or greater than my number of VoIP devices.)

Keep in mind the point of Server B is temporary failover. You want to get Server A back online as quickly as possible. If you have an environment with mostly desktops that stay online, then they won't try to renew their lease for 3.5 days (50% of their lease duration). If it takes you longer than that to recovery a DHCP server, then you likely have a larger issue to deal with. Still, you could always remove the Exclusion range on Server B at that point, and continue to function as normal, just with no fail over capacity.After you got Server A back online  you could re-set the exclusion on Server B.
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Author Comment

by:dongocdung
ID: 33739292
I understand now.
Thank you for your helps
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Author Closing Comment

by:dongocdung
ID: 33739301
I understand the situation now
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