How to cluster DHCP

[Russ Kaufmann [MVP]]

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.

May != Must.

If you have multiple DHCP servers, they will all respond to a request for
address. The process works like this:
1. Client machine (or server) comes online and says, "Help me, I need an IP
to get on the network" using a broadcast message.
2. All DHCP servers that can hear the request send a response with an offer.
A DHCP server that has a reservation for the broadcasting MAC offers the
reservation address and says, "BTW, this one is a reservation just for you.
You should use this one over all other DHCP offers. Also, you have used this
one before, so you really should like it even more."
3. The requesting computer then sends an acceptance message to the DHCP
server that has the address it wants.
4. The DHCP server acknowledges the acceptance and any other that don't get
accepted will just expire.


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

 

[Russ Kaufmann [MVP]]

I'm sorry but I desegree with Russ

Actually you can Share the same scope and reservations.

You can not have the same scopes on multiple servers. You are very wrong
here. I am guessing that you have confused the term. A scope is defined by
the range of addresses provided. You can not have ServerA providing
192.168.2.1-150 and ServerB also providing 192.168.2.1-150. You can't. It
will screw up your network left, right, and in the middle.

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).

There is no requirement for manual changes. However, I do agree that it
would be a good practice to put reservations on all DHCP servers serving the
same network segment just in case the one that servs the actual range of the
reservation is unavailable.

The questions
--
Russ Kaufmann
MVP - Windows Server - Clustering
ClusterHelp.com, a Microsoft Certified Gold Partner
Web http://www.clusterhelp.com
Blog http://msmvps.com/clusterhelp
of the day, why so many reservations? Why not static IP addresses instead
of reservations?

 

[Russ Kaufmann [MVP]]

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! "

For reasons that we do not know. It does not preclude having multiple DHCP
servers.

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

Agreed, and it makes sense. However, there is no requirement that a
reservation be put on all DHCP servers providing addresses for the segment.


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

 

[kj]

For reasons that we do not know. It does not preclude having multiple DHCP
servers.


Agreed, and it makes sense. However, there is no requirement that a
reservation be put on all DHCP servers providing addresses for the
segment.

Only for the OP who wants either DHCP server to provide for the reservation.

 

[Clayton Sutton]

The problem with that Russ is that 95% of our addresses are reservations.
So if one DHCP server goes down we will have half of our users down until we
can come in and move the reservations over to the 2nd DHCP server. That's
what we are trying to avoid.



Clayton

 

[Clayton Sutton]

Thank you jk for bring everyone back on track.

I know what I have to do for the 5% of addresses that are given out by
leases (80% - 20% or something like that). I am looking for a solution for
the 95% of the addresses that are given out via reservations so I can
minimize manually updating BOTH dhcp servers.



Clayton

 

[Jmnts]

I'm sorry Russ but it seems that you misunderstood me.



What I said was:



You can have equal scopes among different servers (There's no question about
this: NOTHING prohibits you to configure Identical scopes between your DHCP
servers).



I also SAID that if you do this you'll have problems.



So in conclusion, the solution that I provided was:

- Configure the 1DHCP server with all options.

- Take advantage of the Dhcp Backup to export the configuration to
others Dhcp servers (saves time and admin work).

- Then MAKE SURE that different Dhcps don't give the same Range of
address.

- How you do that. Simple. Use the Exclusion Range option.



Any way it seems to me that there is to much confusion for a very simple
question.

And it's hard to believe that Clayton doesn't figure out the best solution
for him.

After all these posts, that in my opinion, only differ from some technical
issues and they don't even matter for Clayton's case, that Clayton can't
decide what is best for him.



So, Clayton best regards for you, and all others (Particularly Russ: I hop I
didn't offend you, it wasn't my intention).



And for more information about DHCP check:

http://technet2.microsoft.com/WindowsServer/en/Library/10112946-1204-4ff4-b52c-599c303135d11033.mspx



Best Regards
Systems Administrator
MCSA + Exchange

 

[Russ Kaufmann [MVP]]

The problem with that Russ is that 95% of our addresses are reservations.
So if one DHCP server goes down we will have half of our users down until
we can come in and move the reservations over to the 2nd DHCP server.
That's what we are trying to avoid.

Just because a DHCP server is down doesn't mean taht all of its IP addresses
disappear from its clients.

I think your problems are multiple:
1. The architecture of your IP deployment is an issue in that you are using
reservations to basically force static IP addresses which makes little
business sense. What is the problem that you are trying to fix with this
solution?
2. You seem to make the assumption that if a DHCP server is down that all of
its clients will suddenly need new IP addresses. This is not the case.
3. You seem to think that you _have to_ copy the reservations between the
two servers when you don't. You can, but you are not forced to do so.

The solution?
1. ID the business requirements around IP deployment for servers and for
client machines.
2. Perform risk analysis for DHCP failure
3. Build DHCP recovery plans in the event of DHCP failure
4. Consider HA deployments based on risk analysis and recovery plans

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

 

[Russ Kaufmann [MVP]]

I'm sorry Russ but it seems that you misunderstood me.

What I said was:

You can have equal scopes among different servers (There's no question
about this: NOTHING prohibits you to configure Identical scopes between
your DHCP servers).

Hopefully professionalism and knowledge of how DHCP works would stop you or
anyone else from performing work that is guaranteed to cause duplicate IP
addresses and network failures.

I also SAID that if you do this you'll have problems.

I am not going to get into a flame war with you. Your response might have
been misinterpretted, but it was also extremely misleading. It is never a
good idea to say something can be done knowing full well that it if is done,
it will cause major problems and will break the very problem that you are
trying to fix.

A good analogy would be to tell some body that they can drive the wrong way
down the highway in their car. While it is true that it is possible, it
certainly isn't smart, certainly isn't recommended, and it should never be
stated because somebody might actually do it thinking it is OK.

I think we can just leave it at that.

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

 

[Jmnts]

A good analogy would be to tell some body that they can drive the wrong

way down the highway in their car. While it is true that it is possible,
it certainly isn't smart, certainly isn't recommended, and it should never
be stated because somebody might actually do it thinking it is OK.

You're absolutely right, but the problem is that some times we say that
something can't be done (as you said yourself), and other persons that you
come for backwards and say that you could do.. But you shouldn't.

That's why I always say that although you can do (it's possible) . You
shouldn't. So that the pp can understand that although some things are
possible to be done, some times that type of configurations aren't the best
path to chose.


--
Best Regards
Systems Administrator
MCSA + Exchange

 

[Scott Lowe]

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

Clayton,

I'm going against the grain here with this recommendation, but after
reading the thread, it seems to me that the easiest solution (and the
solution that requires the least human intervention) is to simply
cluster the DHCP servers as you originally suggested. If the
university is willing to spend the money to create the cluster, then
actually creating the cluster is very simple, and it will (to the best
of my knowledge) address all the concerns you expressed.

HTH.

 

[Clayton]

Hey Scott,

Thanks for the reply. I think your are right (If the university is willing
to spend the money). We have all of the SAN in place, we just need to
upgrade from Windows 2003 std. to ent and buy an HBA card for each server
and we are done.


Clayton

 

[Russ Kaufmann [MVP]]

Hey Scott,

Thanks for the reply. I think your are right (If the university is
willing to spend the money). We have all of the SAN in place, we just
need to upgrade from Windows 2003 std. to ent and buy an HBA card for each
server and we are done.

I still hate the idea, but... if there is a justifiable business reason for
it, then go for it. In this case, I don't understand the need for all of the
reservations, but assuming there is a need, MSCS is the way to go.

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

 

[Scott Lowe]

I still hate the idea, but... if there is a justifiable business reason
for it, then go for it. In this case, I don't understand the need for
all of the reservations, but assuming there is a need, MSCS is the way
to go.

You're certainly right, Russ--I don't understand the need for the
reservations, and a cluster is overkill just for DHCP. But, as you
pointed out, there is a justifiable business reason, and the
organization is willing to make the investment. Besides, I'm sure he
(Clayton) will be able to find other uses for that cluster....