How to cluster DHCP

[Clayton Sutton]

Hi everyone,

We are running a Windows 2003 domain and we have two DHCP servers. Anyone
know the pros and cons of clustering DHCP? Can you point me to any white
papers that provide a step-by-step "how-to"? We do have an EMC SAN on our
network too.


TIA,


Clayton

 

[Todd J Heron [MVP]]

It doesn't make sense to use clustering when you can simply use two servers
and split the zone over both servers which is much less expensive.

 

[Clayton Sutton]

Hey Todd,

The problem we have is the 95% of our addresses given out are by reservation
ONLY! So spliting the DHCP wont work for us that's way we were looking at
clustering.


Clayton

 

[Ryan Sokolowski [MVP]]

Hmmm...pros and cons, huh? Some of each...you can achieve greater
availability over a stand-alone DHCP Server by splitting scopes between two
stand-alone DHCP servers. Clustering the DHCP servers and doing the same
gives you more complexity but higher fault-tolerance...

If you want a "How-To", here it is - an awesome checklist for this...

Checklist: Installing a DHCP Service resource
http://technet2.microsoft.com/WindowsServer/en/Library/05e39968-9732-4aa9-a0a9-6d9290a24ca71033.mspx

Hope this helps,
--Ryan

--
Ryan Sokolowski
MVP - Windows Server - Clustering
MCSE, CCNA, CCDA, BCFP
Clustering101.com - Coming Soon!

This posting is provided "AS IS" with no warranties, and confers no rights.

 

[Ryan Sokolowski [MVP]]

Hmmm...pros and cons, huh? Some of each...you can achieve greater
availability over a stand-alone DHCP Server by splitting scopes between two
stand-alone DHCP servers. Clustering the DHCP servers and doing the same
gives you more complexity but higher fault-tolerance...

If you want a "How-To", here it is - an awesome checklist for this...

Checklist: Installing a DHCP Service resource
http://technet2.microsoft.com/WindowsServer/en/Library/05e39968-9732-4aa9-a0a9-6d9290a24ca71033.mspx

Hope this helps,
--Ryan

--
Ryan Sokolowski
MVP - Windows Server - Clustering
MCSE, CCNA, CCDA, BCFP
Clustering101.com - Coming Soon!

This posting is provided "AS IS" with no warranties, and confers no rights.

 

[Andrei Ungureanu]

Why it will not work? Just define the ip reservations on both dhcp servers.


--
Regards,
Andrei Ungureanu
www.eventid.net
Test our new EventReader!
http://www.altairtech.ca/eventreader/default2.asp?ref=au

 

[Jmnts]

If you are worried about DHCP reservations just make sure that you backup
your dhcp servers regulari. Than in case of failure you always can transfer
the backup to another server.
Clustering is hardware expensive enough to justify DHCP service, in my
opinion.

 

[Herb Martin]

Hey Todd,

The problem we have is the 95% of our addresses given out are by
reservation ONLY! So spliting the DHCP wont work for us that's way we
were looking at clustering.

There is nothing wrong with clustering, but when
95% of the addresses are given out by RESERVATION
you do not have the usual problem of trying to synchronize
the majority of the leases -- only %5 are at issue.

Reservations can be duplicated fairly easily in most
cases to multiple servers -- as long as they are identical
it's not an issue since one DHCP server or the other
will never be able to create a conflict for such machines.

But setting up a Server Cluster is fine if you don't want
to manually duplicate all that (tedious if it's large) or
move the files (difficult to keep up to date if you make
changes still.)

 

[Clayton]

We don't want to split the scoops across two servers because there is too
much manual updating that will have to be done to keep both servers in sync.
We are a small Div. II University with over 2000 workstations. There is too
much room for human error. And we don't what to have to manually bring the
second server online if the first one goes down.

We have an EMC CX500 SAN already. The university is willing to buy HBA
cards for the two servers and willing to buy the upgrade to Windows 2003
Enterprise Edition.

If anyone else has any other ideas please let me know. I'm not bent on
clustering, I just want to find the best solution for our environment that
does not involve human intervention.


Clayton

 

[Clayton]

Thanks Ryan,

I will take a look at it tomorrow.


Clayton

 

[Russ Kaufmann [MVP]]

We don't want to split the scoops across two servers because there is too
much manual updating that will have to be done to keep both servers in
sync.

There is no manual updating involved. I think you misunderstand what it
means to split scopes.

We are a small Div. II University with over 2000 workstations. There is
too much room for human error. And we don't what to have to manually
bring the second server online if the first one goes down.

There would be no requirement to manually bring the second DHCP server
online if you have split scopes. Both servers can be online at the same
time.

We have an EMC CX500 SAN already. The university is willing to buy HBA
cards for the two servers and willing to buy the upgrade to Windows 2003
Enterprise Edition.

That is a great deal of money to spend when you can achieve an HA DHCP
environment without using Server Clustering.


--
Russ Kaufmann
MVP - Windows Server - Clustering
ClusterHelp.com, a Microsoft Certified Gold Partner
Web http://www.clusterhelp.com
Blog http://msmvps.com/clusterhelp

 

[Clayton Sutton]

Thanks everyone, your input was invaluable! I think that we will just try
to maintain all the reservations on BOTH DHCP servers. So if we add a
reservation to one server we will also add it to the other server. So both
servers will share the same scopes. Does that sound like it will work?


Clayton

 

[kj]

A much more practical approach than building, deploying and maintaining a
cluster just for DHCP.

Some scripting should make that concept much easier to maintain. Here's a
snippet I found (stole) from elsewhere.
You probably have some talented scripting resources around to put together a
nice light weight interface to maintain reservations on both DHCP servers.

===========
With this you can insert a list of a reservation in a dhcp scope.
The information are taken from the input.txt

for /f "tokens=1-4 delims=;" %%a in (input.txt) do (
netsh dhcp server DHCPSERVERIP scope SCOPEIP add reservedip %%a %%b %%c %%d
dhcp)

The structure of the input.txt is:

IPADDRESS;MACADDRESS;MACHINENAME;DESCRIPTION

EXAMPLE:

172.17.169.100;018017462DD0;MYPCNAME;MYPCNAME
===========

 

[Russ Kaufmann [MVP]]

Thanks everyone, your input was invaluable! I think that we will just try
to maintain all the reservations on BOTH DHCP servers. So if we add a
reservation to one server we will also add it to the other server. So
both servers will share the same scopes. Does that sound like it will
work?

No, it won't work. You can't share the same scope, and thus the reservations
between two servers.

Each server must have its own scope, which can be a segment of the overall
address space available. For example, if your network is a 192.168.2.0
network with a 255.255.255.0, that leaves you 192.168.2.1-254 for your
addresses. DHCP1 can host 192.168.2.1-128 and DHCP2 can host
192.168.2.129-254. The scope is split. In the event either DHCP server
fails, the other is available to provide addresses.

--
Russ Kaufmann
MVP - Windows Server - Clustering
ClusterHelp.com, a Microsoft Certified Gold Partner
Web http://www.clusterhelp.com
Blog http://msmvps.com/clusterhelp

 

[kj]

Reservations and leases have different rules when it comes to scopes.

http://support.microsoft.com/kb/196066/en-us

==============


The DHCP server must service the subnet where the reservation is being
created.

These three rules guide you in creating reservations: 1. You may create a
reservation in any scope range.

This can include Excluded ranges.
2. You may create a reservation in a subnet range even if the
reservation falls outside the actual scope Start and End addresses for that
subnet.
3. You cannot create reservations outside the subnet range of any of
your existing scopes.
As a simple example, take a DHCP server that has no existing scopes: . Let's
say you have a standard Class C subnet for 192.168.1.0 with mask
255.255.255.0.
. You can create a scope for the first half of the subnet so that the
scope has a starting address of 192.168.1.1 and an ending address of
192.168.1.150.
. You can now create a reservation for the address 192.168.1.25. (Rule
1)
. You can also create a reservation that lies outside the scope but
still lies in the subnet range.

For example, reserve 192.168.1.200. (Rule 2)
. You cannot create a reservation for the address 192.168.2.4, which
lies outside the subnet of any existing scope. (Rule 3)

 

[Jmnts]

I'm sorry but I desegree with Russ

Actually you can Share the same scope and reservations.

Of course if you do this you can have problems. The problems are that you
probably end up with different DHCP servers adding the same IP address to
different machines. You can try to minimize this with
conflict detection attempts under server properties, but this isn't a good
practice.

If I were you.....

Configure the 1 Dhcp Server. (Create the scope and Reservations, etc)
Backup the 1 Dhcp Server.

Go to the 2 Dhcp Server
Import the backup that you did from the 1Dhcp server to a local drive, right
click on 2Dhcp and select restore option, then select the backup that you'd
copied.

Now all you 've to do is something like what Russ told you to (of course
what Russ told is right, but having to manually configure both Dhcp servers
you'll have more admin work).


For example on a subnet 192.168.0.x/24

Reservation are being issued between 192.168.0.1 - 54(Once reservations are
defined in both server neither one will give to other machines)

Now we have to configure 192.168.0.55 - 254

To do this MS recomends 80% - 20%, which means that 1 Dhcp server will give
80% of the available addresses and the other one will give 20%.
In this case we'll con figure 50% 50%, and we have to make sure that none of
the both Dhcp server will have the same address to give to clients.

Server 1Dhcp Add Exclusion range 192.168.0.156 to 254 (because the 2Dhcp is
responsible for this range of address)
Server 2Dhcp Add Exclusion range 192.168.0.55 to 155 (because the 1Dhcp is
responsible for this range of address)

Voulá and you're done.

--
Best Regards
Systems Administrator
MCSA + Exchange

 

[kj]

Everyone keeps dwelling on leases, but quoting the original post;

"The problem we have is the 95% of our addresses given out are by
reservation
ONLY! "

Reservations do NOT have to be within the DHCP scope, only within the subnet
of one of the DHCP server's scopes.

Each DHCP server can have the same reservation defined equally and there
will be no conflict so long as the reservation is not within the range of
leased address of either server.

 

[Jmnts]

I just give you the fastest solution to configure both DHCP servers.
And of course you can only add Reservations based on IP/Subnet defined on
the Dhcp.
But once again we're only try to avoid other problems and comum mistakes
made when people configure Dhcp.

--
Best Regards
Systems Administrator
MCSA + Exchange

 

[kj]

Do you have issues with;

"You may create a reservation in a subnet range even if the reservation
falls outside the actual scope Start and End addresses for that subnet."

or;

A duplicate reservation on a two seperate DHCP servers that have split
scopes that do not overlap or contain the reservation IP address.

?

The solution for this thread is Reservations, not the standard split scope -
80\20 leases.

 

[Phillip Windell]

I usually make the Scopes the entire Address Range regaurdless, and then
"cut them down" by using the Exclusions. Beyond that I do it identical to
what you describe.