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Four DHCP servers & Four DNS Sever on Four Different Domain Controllers

Posted on 2016-08-11
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Hi All We have one PDC & three other associated with it we have dchp enabled on all Domain Controller can this cause a conflict
thank you all for your support
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Question by:MarK PercY
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by:Etienne Lau
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If all Domain Controllers are on different subnets, should not be an issue. But even if they are all on the same subnets, it really depends on how DHCP is configured on those Domain controllers. So without more details from you not sure how one defines "Cause a conflict".
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by:Mdlinnett
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Multiple DHCP Servers on the same subnet are normally a bad idea, your clients will generate 4 times as much network traffic when searching for a DHCP Server to obtain an address, but as Etienne said, it depends on how DHCP is configured.

DNS on 4 DC's won't cause any issues, only DHCP could provide a problem.
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by:Seth Simmons
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Multiple DHCP Servers on the same subnet are normally a bad idea,

depends on network topology
we don't know how many clients there are
should have at least 2 for redundancy
4 dhcp servers might be overkill, but again don't know the network setup
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by:MarK PercY
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Hi All

I agree we've been having users with windows 7 pc issues whereby  dns/dhcp are an issue.
we been looging onto users machines & running these commands  ipconfig/flushdns plus ipconfig/release/renew in order for them to do any work.
We do have a DC that has 2003 & dhcp/dns are still enabled so the functional level isn't 2012
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by:Etienne Lau
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Functional level of AD is not relevant. It really would help to know how the DHCP is configured if you want help with this issue.
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by:MarK PercY
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Hi Etienne,
Yes id  help please what would you like to know?
Thank you
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by:Mdlinnett
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For each dhcp server, what are the dhcp scopes / pools / exclusion zones for each?

Are all servers providing the correct DNS server and default gateway to the dhcp clients within the dhcp options?

My initial thoughts are that two computers are being given the same ip address by different dhcp servers or possibly the dhcp options aren't configured correctly on one of the servers.
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by:MarK PercY
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Hi Mdlinnett

Please have a look at the SS provided I do agree with what your saying because we keep seeing on two different DC's the same IP/Computer name leased out at the same time & we keep having to do a ipconfig/release & ipconfig/dnsflush I order for the user to work on certain services

Many thanks for your input
DHCP-Pool.docx
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by:MarK PercY
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Hi All
can anyone help
Thank you
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by:Etienne Lau
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Your screen shot only shows one of your DHCP server. But does not show the other DHCP server, given what you describe so far, you should "disable" one of the 2 DHCP servers......
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by:DrDave242
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Are all four of your DHCP servers configured as shown in the screenshot? Are the address ranges and exclusions identical on all of them? Also, I don't believe you've specifically said whether all four servers are on the same subnet.

If all four servers are on the same subnet and have the same DHCP configuration, that's certainly redundancy, but it's not the good kind of redundancy. It can cause problems, as the first server to respond to a client request will be the one that assigns an address to that client. The servers don't share information with each other regarding the addresses they've assigned, so it is possible that the same address will be assigned to different clients by different servers. Conflict detection, if enabled, is supposed to minimize this, but it's better to configure the environment in such a way that it can't happen in the first place.

If at least two of your DHCP servers are running Windows Server 2012 or 2012 R2, you may want to consider implementing DHCP failover. It provides redundancy for DHCP without the potential for conflict.

If your DHCP servers are running older versions and you still want some form of redundancy, you should consider a split scope, in which the address range is divided among two servers, and the second server is configured not to respond to requests right away. This effectively makes it a standby server that will only service clients if the primary server is unable to do so.
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by:noci
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Your problem is in how DHCP operates.
First it does a broadcast (address ff:ff:ff:ff:ff:ff, in a fabricated UDP packet destined for 255.255.255.255, source 0.0.0.0) on the network and it asks for the active DHCP (servers).
Then ALL DHCP servers that receive such a packet will Respond that they are alive and capable to supply services. (if they are)....
Then the client will choose one server (Mostly the one that responds first to the query). and continue talking to it.
All this conversation is still Ethernet level, with faked IP info, not having a valid IP address yet...

So there is DEFINITELY no knowledge on the DHCP client about windows networks, routing topology etc. (until the DHCP sequence is completed).
With multiple DHCP servers on a single broadcast domain (ie. an ethernet (V)LAN)  there should be none that supplies addresses from a range, all should only handout reservations.
(As that will prevent them from responding to others). But that almost defeats the purpose of DHCP.

After IP is up (DHCP is completed) the kerberos & LDAP queries can be started.
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by:MarK PercY
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Hi

Thank you all!. Can I now ask how I would beginning to start phasing at least two of the current DHCP servers?

Thank you
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by:Mdlinnett
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Hi Mark,

It depends on the answers to the questions asked of you since you posted the screenshot of one of your DHCP Servers.

Is your DHCP Pool the same on each of your 4 DHCP Servers?

If so, deactivate the DHCP Servers you don't want to use and, if necessary, remove the DHCP role.

If 2 of your Servers are running Server 2012 or 2012 R2 then true DHCP failover can be configured.  See the following Microsoft Article, specifically the section on 'Configure a Failover Configuration' > https://technet.microsoft.com/en-us/library/hh831385(v=ws.11).aspx

If not 2012, the same article discusses split scope DHCP which I believe is what you would need to use if you intend on keeping 2 DHCP Servers on the same subnet.
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by:MarK PercY
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Hi
When phasing out one dchp server is it worth putting a 0 on the lease time duration & then disabling one then the other?
thank you for you're help
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DrDave242 earned 500 total points
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No, that shouldn't be necessary. A client that has leased an address from a DHCP server that you retire will first attempt to contact that particular DHCP server after half of its lease duration has passed. Since the server has been retired, the client won't receive a response. It'll try to contact the server a few more times, but it'll continue operating normally with the assigned IP address. When 87.5% (7/8) of the lease duration has expired, the client will broadcast a request that any DHCP server can respond to. As long as there is at least one active DHCP server on the network, it'll respond to that request. Depending on the response, the client will either retain its IP address or begin the process of requesting a new one.

If your DHCP servers are all configured identically, retiring one or two (or even three) will have no effect on your clients' network connectivity. They'll simply end up leasing an address from the remaining server(s) when the time comes.
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by:MarK PercY
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id like to give all points to all contributors because all were completely helpful & made me understand dhcp so much better thank you
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