Increase in conflict detection, causes less addresses to become available in DHCP?!

hey guys,

We occasionally get a load of Bad Addresses' in the 'DHCP' leases. Anyway, whenever I increase the 'conflict detection' in here from around 0 to 2, suddenly the number of DHCP addresses available drops from 35 to around 25?

Something not quite right on our DHCP/

Any points to the right direction would be great.

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Krzysztof PytkoSenior Active Directory EngineerCommented:
That informs you about IP addresses are used in a network and cannot be issued. 0 means do not check IP address availability, 2 or more means ping that IP before you will issue it. If any echo reply answers then IP cannot be issued and DHCP chooses another one

Solarwinds have nice free IP address manager utility . Give that a shot , it tells you about all static/dynamic IPs that you have in your network. it i a good point to start narrowing down the conflicts.
Krzysztof PytkoSenior Active Directory EngineerCommented:
And one more thing. Have you recently moved DHCP from another device (router) or server? If so, that's the issue. You should enable conflict detection attempt first (before DHCP service started). Now, DHCP is serving IPs without check. DHCP server doesn't know anyting about issued IPs so, it pulls them from the beginning anf that's why you have BAD_ADDRESS entry in console.

So, I suggest to enable detection value to 2 and wait until DHCP database will be rebuilt

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YashyAuthor Commented:
Yes, but why does the number of available addresses go from 35 to 25, just because I increased conflict detection?

We don't have any new joiners etc coming on and suddenly taking IP addresses.
Krzysztof PytkoSenior Active Directory EngineerCommented:
No, I don't belive that conflict detection attempt may decrease IP address pool. Its only role is to check if IP address is answering before lease issue. This mechanism uses ping command to determine if it is free or not.

I would check if scope is configured properly (pool size, no IP exclusion etc).
Next, click right mouse button on scope and choose "reconcile" and verify button. Check if you didn't recovered IPs.
If no above steps helped, stop DHCP server service, delete DHCP database from %WINDIR%\SYSTEM32\DHCP folder
start DHCP server serice again and recreate scope (do not activate it at the end!). Then enable Conflict Address Detection first and as the last step activate scope. Check if everything works fine.

If not, reinstall DHCP role. And also before scope activation, enable conflict detection attemp

Then, everything would work fine.


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YashyAuthor Commented:
Thanks a lot Kryszof:). Much appreciated.
Krzysztof PytkoSenior Active Directory EngineerCommented:
You're welcome :)

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Windows Server 2008

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