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2 DHCP servers for failover?

Posted on 2004-09-15
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I have a new network with 2 Domain Controllers for redundancy.
These domain controllers run WINS which replicates and DNS which replicates with the Active Directory.

One domain controller currently runs DHCP and I wanted a way of configuring this for failover as I have with the other services.

I have read the following Microsoft article which mentions the 80/20 rule:

http://www.microsoft.com/windows2000/techinfo/reskit/en-us/default.asp?url=/windows2000/techinfo/reskit/en-us/cnet/cncb_dhc_ogjw.asp

I do not understand how this provides failover. Why not just configure 2 DHCP servers with active scopes of half each?

I will have both DHCP servers on the same small subnet but I will be using about 75% of the addresses so if i just use 50/50 on each DHCP server if one goes down there is a possibility that not all machines could get an IP.

Has anyone set this up? and can they explain to me how it works?
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Question by:mistaj
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Debsyl99 earned 115 total points
ID: 12070523
Hi
Hopefully these will explain it better than the article you referenced,

The Simplicity and Serenity of DHCP Fault Tolerance
http://www.enterpriseitplanet.com/networking/features/article.php/11315_3327231_3
Ensuring DHCP Server Availability
http://www.windowsdevcenter.com/pub/a/windows/2004/04/13/DHCP_Server.html

The problem is that dhcp isn't clever enough to look at the available leases on any other server other than it's own. It doesn't say - ok now you need a lease - let's check out the other dhcp server and see if that's allocated the same lease before it allocates a free one of it's own. It just says, ok, this lease is free, here you go have this one. As dhcp server no.2 will do the same, that is why it's possible to have duplicate ip's on the network, and also why it's necessary to have exclusion zones that do not allow either server to allocate the same ip addresses. Your idea of 50/50 is fine and will work also. The idea behind 80/20 is that if one server goes down - ie your main 80% server, the time to take you to get it back up is going to be relatively short so that your 20% server can pick up the re-allocation of any leases that have expired over that period. It really depends on how many leases that you require on that subnet, the time that you anticipate getting a server back up will take and your average lease expiry time. The ideal is to have a 50/50 split where either server can allocate all required addresses within that subnet. But none of us live in such a world so you have to do the math that best covers your situation,

Hope that helps

Deb :))

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