2 DHCP servers on the same subnet - Reservations?!

Hi,

We are looking at sorting out our DHCP finally, and am looking at having 2
DHCP servers on our 1 subnet (Using the 80/20 rule etc)...

We have 1 group of computers which we would like to perform DHCP
reservations for so it may be like this:

DHCP Server 1: 192.168.0.1-192.168.0.200
Exclusion Range: 192.168.0.201-192.168.0.254

DHCP Server 2: 192.168.0.201-192.168.0.254
Exclusion Range: 192.168.0.1-192.168.0.200

For example we would then put the reservations in for our special group of
192.168.0.1-192.168.0.50 in the scope of DHCP Server 1.

The problem is, what happens if a client who has not got a lease, requests
an address and DHCP Server 2 answers first, as it will not know about the
reservation??

"UselessUser" wrote:

> Hi,
>
> We are looking at sorting out our DHCP finally, and am looking at having 2
> DHCP servers on our 1 subnet (Using the 80/20 rule etc)...
>
> We have 1 group of computers which we would like to perform DHCP
> reservations for so it may be like this:
>
> DHCP Server 1: 192.168.0.1-192.168.0.200
> Exclusion Range: 192.168.0.201-192.168.0.254
>
> DHCP Server 2: 192.168.0.201-192.168.0.254
> Exclusion Range: 192.168.0.1-192.168.0.200
>
> For example we would then put the reservations in for our special group of
> 192.168.0.1-192.168.0.50 in the scope of DHCP Server 1.
>
> The problem is, what happens if a client who has not got a lease, requests
> an address and DHCP Server 2 answers first, as it will not know about the
> reservation??

Good news for you!

Reservations can (and should) be made from the addresses excluded from
distribution for your DHCP servers. So define them on both servers to achieve
redundancy.

So you subnet is mapped to three exclusive ranges:
Reservations
First-come-first-served DHP server1
First-come-first-served DHP server2

Now with enough reservations you can split the first-come-first-served
ranges 50/50 instead of 80/20. After all, you don't want to be called back
from holiday because a DHCP server has gone down.

--
Regards,
Newell White

"UselessUser" <(E-Mail Removed)> wrote in message
news:08D2522F-989A-4DFC-A2CC-(E-Mail Removed)...
> Hi,
>
> We are looking at sorting out our DHCP finally, and am looking at having 2
> DHCP servers on our 1 subnet (Using the 80/20 rule etc)...

Most DHCP servers will object if they detect another DHCP server. I think
(but I'm not 100% about this) that the Windows DHCP server will shut down if
it detects another DHCP server on the network. So I'm not sure that your
arrangement will work. Sorry.
--
Brian Cryer
www.cryer.co.uk/brian

"Brian Cryer" <not.here@localhost> wrote in message
news:(E-Mail Removed)...
> "UselessUser" <(E-Mail Removed)> wrote in message
> news:08D2522F-989A-4DFC-A2CC-(E-Mail Removed)...
>> Hi,
>>
>> We are looking at sorting out our DHCP finally, and am looking at having
>> 2
>> DHCP servers on our 1 subnet (Using the 80/20 rule etc)...
>
> Most DHCP servers will object if they detect another DHCP server. I think
> (but I'm not 100% about this) that the Windows DHCP server will shut down
> if it detects another DHCP server on the network. So I'm not sure that
> your arrangement will work. Sorry.

This may just be an issue with Windows SBS ...

"Brian Cryer" wrote:

> "UselessUser" <(E-Mail Removed)> wrote in message
> news:08D2522F-989A-4DFC-A2CC-(E-Mail Removed)...
> > Hi,
> >
> > We are looking at sorting out our DHCP finally, and am looking at having 2
> > DHCP servers on our 1 subnet (Using the 80/20 rule etc)...
>
> Most DHCP servers will object if they detect another DHCP server. I think
> (but I'm not 100% about this) that the Windows DHCP server will shut down if
> it detects another DHCP server on the network. So I'm not sure that your
> arrangement will work. Sorry.
> --
> Brian Cryer
> www.cryer.co.uk/brian
>
We have 2 x W2k3 standard DCs and run 2 DHCP servers .
So do many other posters.

Windows DHCP does not shut down when it detects another DHCP server, as many
discover when they plug in a router that has built-in DHCP server configured
as default, and suffer chaos!

--
Regards,
Newell White

OK, I think I know what I need to do, but let me expand on what I have
already inherited from a previous IT guy...

2 DHCP servers, 1 LAN

Our network is on a 10.200.0.0/16

Each server has a scope (Not a superscope or anything) which is 10.200.0.0/16

However each server has different address pools..

Server 1 - Scope 10.200.0.0/16
Address Pool - 10.200.1.1-10.200.2.254

Server 2- Scope 10.200.0.0/16
Address Pool - 10.200.6.1-10.200.9.254

And thats it! No exclusion ranges (I believe this is why we are getting
NACKS as clients are attempting to get addresses from the other server rather
than the one they got their original address from).

What I am looking to do is sort this out properly so from what people have
said I need to do this:

Server 1 - Scope 10.200.0.0/16
Address Pool - 10.200.2.1-10.200.2.254
Exclusion Range - 10.200.6.1-10.200.9.254
Static Reservations - 10.200.1.1-10.200.1.254

Server 2- Scope 10.200.0.0/16
Address Pool - 10.200.6.1-10.200.9.254
Exclusion Range - 10.200.2.1-10.200.2.254
Static Reservations - 10.200.1.1-10.200.1.254

How does that look, will this avoid the NACK's and allow my static
reservations to work?

"Newell White" <(E-Mail Removed)> wrote in message
news:7B93377E-C418-45A1-9A06-(E-Mail Removed)...
>
> "Brian Cryer" wrote:
>
>> "UselessUser" <(E-Mail Removed)> wrote in message
>> news:08D2522F-989A-4DFC-A2CC-(E-Mail Removed)...
>> > Hi,
>> >
>> > We are looking at sorting out our DHCP finally, and am looking at
>> > having 2
>> > DHCP servers on our 1 subnet (Using the 80/20 rule etc)...
>>
>> Most DHCP servers will object if they detect another DHCP server. I think
>> (but I'm not 100% about this) that the Windows DHCP server will shut down
>> if
>> it detects another DHCP server on the network. So I'm not sure that your
>> arrangement will work. Sorry.
>> --
>> Brian Cryer
>> www.cryer.co.uk/brian
>>
> We have 2 x W2k3 standard DCs and run 2 DHCP servers .
> So do many other posters.
>
> Windows DHCP does not shut down when it detects another DHCP server, as
> many
> discover when they plug in a router that has built-in DHCP server
> configured
> as default, and suffer chaos!

Actually that was when I discovered it, but I think that may be an SBS
foyble since Windows SBS server likes to be in control. I agree that it
doesn't happen here with the two Windows standard servers that are running
on the same network.

Get rid of the /16 bit mask. You don't need 16,777,214 hosts in each subnet

Use a /24 bit mask,...limit your subnets to 254 host each. Ethernet
effieciency degrades after about 300 Hosts on a segment. The degredation is
less noticable on a Gigabit speed system, but the degredation is still
there.

The following example provides 4 subnets of 254 Hosts giving a total of 1016
Hosts
It requires a LAN Router to route between the subnets and requires that the
Router have the ability to forward DHCP Queries to the DHCP Servers. The
DHCP Servers have only one Nic and "live" on only one subnet (doesn't matter
which subnet). Also,....No Superscopes!

Server 1
-----------------
#1 Scope 10.200.0.0/24 (254 Hosts)
Address Pool - 10.200.0.1-10.200.0.254
Exclusion Range - 10.200.0.1-10.200.0.25 (normal expected Exclusion)
Exclusion Range - 10.200.0.115-10.200.0.254 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them
#2 Scope 10.200.1.0/24 (254 Hosts)
Address Pool - 10.200.1.1-10.200.1.254
Exclusion Range - 10.200.1.1-10.200.1.25 (normal expected Exclusion)
Exclusion Range - 10.200.1.115-10.200.1.254 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them
#3 Scope 10.200.2.0/24 (254 Hosts)
Address Pool - 10.200.2.1-10.200.2.254
Exclusion Range - 10.200.2.1-10.200.2.25 (normal expected Exclusion)
Exclusion Range - 10.200.2.115-10.200.2.254 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them
#4 Scope 10.200.3.0/24 (254 Hosts)
Address Pool - 10.200.3.1-10.200.3.254
Exclusion Range - 10.200.3.1-10.200.3.25 (normal expected Exclusion)
Exclusion Range - 10.200.3.115-10.200.3.254 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them

Server 2
-------------------
#1 Scope 10.200.0.0/24 (254 Hosts)
Address Pool - 10.200.0.1-10.200.0.254
Exclusion Range - 10.200.0.1-10.200.0.25 (normal expected Exclusion)
Exclusion Range - 10.200.0.26-10.200.0.114 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them
#2 Scope 10.200.1.0/24 (254 Hosts)
Address Pool - 10.200.1.1-10.200.1.254
Exclusion Range - 10.200.1.1-10.200.1.25 (normal expected Exclusion)
Exclusion Range - 10.200.1.26-10.200.1.114 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them
#3 Scope 10.200.2.0/24 (254 Hosts)
Address Pool - 10.200.2.1-10.200.2.254
Exclusion Range - 10.200.2.1-10.200.2.25 (normal expected Exclusion)
Exclusion Range - 10.200.2.26-10.200.2.114 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them
#4 Scope 10.200.3.0/24 (254 Hosts)
Address Pool - 10.200.3.1-10.200.3.254
Exclusion Range - 10.200.3.1-10.200.3.25 (normal expected Exclusion)
Exclusion Range - 10.200.3.26-10.200.3.114 (to create the 50/50)
Static Reservations - There is no range. The Exclusions are for covering
static ranges. Reservations are generally random and you should never have
more than a handful of them


--
Phillip Windell
www.wandtv.com

The views expressed, are my own and not those of my employer, or Microsoft,
or anyone else associated with me, including my cats.
-----------------------------------------------------


"UselessUser" <(E-Mail Removed)> wrote in message
newsDCECD3F-7C7C-4B8A-8B94-(E-Mail Removed)...
> OK, I think I know what I need to do, but let me expand on what I have
> already inherited from a previous IT guy...
>
> 2 DHCP servers, 1 LAN
>
> Our network is on a 10.200.0.0/16
>
> Each server has a scope (Not a superscope or anything) which is
> 10.200.0.0/16
>
> However each server has different address pools..
>
> Server 1 - Scope 10.200.0.0/16
> Address Pool - 10.200.1.1-10.200.2.254
>
> Server 2- Scope 10.200.0.0/16
> Address Pool - 10.200.6.1-10.200.9.254
>
> And thats it! No exclusion ranges (I believe this is why we are getting
> NACKS as clients are attempting to get addresses from the other server
> rather
> than the one they got their original address from).
>
> What I am looking to do is sort this out properly so from what people have
> said I need to do this:
>
> Server 1 - Scope 10.200.0.0/16
> Address Pool - 10.200.2.1-10.200.2.254
> Exclusion Range - 10.200.6.1-10.200.9.254
> Static Reservations - 10.200.1.1-10.200.1.254
>
> Server 2- Scope 10.200.0.0/16
> Address Pool - 10.200.6.1-10.200.9.254
> Exclusion Range - 10.200.2.1-10.200.2.254
> Static Reservations - 10.200.1.1-10.200.1.254
>
> How does that look, will this avoid the NACK's and allow my static
> reservations to work?

"UselessUser" wrote:

> OK, I think I know what I need to do, but let me expand on what I have
> already inherited from a previous IT guy...
>
> 2 DHCP servers, 1 LAN
>
> Our network is on a 10.200.0.0/16
>
> Each server has a scope (Not a superscope or anything) which is 10.200.0.0/16
>
> However each server has different address pools..
>
> Server 1 - Scope 10.200.0.0/16
> Address Pool - 10.200.1.1-10.200.2.254
>
> Server 2- Scope 10.200.0.0/16
> Address Pool - 10.200.6.1-10.200.9.254
>
> And thats it! No exclusion ranges (I believe this is why we are getting
> NACKS as clients are attempting to get addresses from the other server rather
> than the one they got their original address from).
>
> What I am looking to do is sort this out properly so from what people have
> said I need to do this:
>
> Server 1 - Scope 10.200.0.0/16
> Address Pool - 10.200.2.1-10.200.2.254
> Exclusion Range - 10.200.6.1-10.200.9.254
> Static Reservations - 10.200.1.1-10.200.1.254
>
> Server 2- Scope 10.200.0.0/16
> Address Pool - 10.200.6.1-10.200.9.254
> Exclusion Range - 10.200.2.1-10.200.2.254
> Static Reservations - 10.200.1.1-10.200.1.254
>
> How does that look, will this avoid the NACK's and allow my static
> reservations to work?

Philip is of course correct about not overloading your subnet with Ethernet
broadcasts. I have benefited from his advice in the past, he is tops.

But you are living with the existing traffic, so it is by definition
tolerable.

Your proposed structure is a little messy.
My rules to follow are:

1) All DHCP servers in a scope have the same address pool.

2) Each DHCP server has one exclusion range (within that pool) in common, in
which reservations are defined (same MAC<->IP mapping on all servers).

3) Each DHCP server has additional exclusion ranges to stop it treading on
the toes of other DHCP servers.

4) Now it awards addresses to DHCP clients from the remaining portion of the
pool, which is excluded from all other servers.

So I would recommend re-organising as:

Server 1 - Scope 10.200.0.0/16
Address Pool - 10.200.1.1-10.200.9.254
Exclusion Range - 10.200.1.1-10.200.1.254 (reservations)
Exclusion Range - 10.200.6.1-10.200.9.254 (server 2)

Server 2- Scope 10.200.0.0/16
Address Pool - 10.200.1.1-10.200.9.254
Exclusion Range - 10.200.1.1-10.200.1.254 (reservations)
Exclusion Range - 10.200.2.1-10.200.5.254 (server 1)

--
Regards,
Newell White

"Newell White" <(E-Mail Removed)> wrote in message
news:F18B7D98-77B2-4E02-BBC1-(E-Mail Removed):
> Your proposed structure is a little messy.
> My rules to follow are:
>
> 1) All DHCP servers in a scope have the same address pool.

Why? What's the advantage?

> 2) Each DHCP server has one exclusion range (within that pool) in common, in
> which reservations are defined (same MAC<->IP mapping on all servers).

Typically we locate our servers at the beginning of the range:
192.168.1.2-192.168.1.19 for example. Then we just start the DHCP range
at .20 and go from there. We don't have to worry about reservations or
exclusions because the server addresses aren't even in the pool.

> 3) Each DHCP server has additional exclusion ranges to stop it treading on
> the toes of other DHCP servers.

Which seems unnecessary if you just split the pool on each server.

Server 1 - Address Pool - 10.200.2.1-10.200.5.254
Server 2 - Address Pool - 10.200.6.1-10.200.9.254

No need for exclusions or reservations. I can understand doing
reservations if you want to hand out addresses and other network
information to servers or other devices using DHCP instead of
statically. But I'm not as clear why we really need to do exclusions if
we just leave those addresses out of the pool to begin with.

I may be missing something and I love to learn, so please let me know if
I've overlooked a good reason not to do it that way.

Best wishes and aloha,


Ben M. Schorr, MVP
Roland Schorr & Tower
http://www.rolandschorr.com
http://www.officeforlawyers.com/outlook.htm