Constant DHCP Conflicts on Windows 2003 Server

Hi,

I seem to be having big problems on our windows 2003 active directory,
with dhcp addresses on our client machines. Despite the fact that the
ip scope far exceeds the number of ip addresses we need, we are getting
ip conflicts on many of our machines (we have around 700 machines in
total, 95% of which are running Windows XP SP2). When you manually
release and renew the ip address on the PC, it gives the following
error message:

Windows IP Configuration

An error occurred while renewing interface Local Area Connection : The
DHCP client has obtained an IP address that is already in use on the
network. The local interface will be disabled until the DHCP client can
obtain a new address.

Trying to do an ipconfig -release and then an ipconfig -renew comes up
with the same error. After doing several ipconfig -renew commands, it
finally gets an ip address, but usually the one it had before, which is
having the conflict.

I originally thought that this was because we had two dhcp servers, one
which was NT4 and not registered with AD. I disabled this server, but
still got the same problems. I have then created another dhcp server
within the AD, running on Server 2k3, but as before, the problems
reoccur.

This is causing major problems with packets dropping and the PCs being
very slow on the network, along with various error messages which are
annoying the users, but I cannot work out what the problem is.

Has anyone had this same problem or something similar, or more
importantly, anyone know how to resolve it?

Many thanks,

Laurence

Hi,

Are both (all DHCP) server serving same IP range? If so -- they should
_not_. DHCP servers on the LAN do not talk to each other (unless you set
them up in cluster) -- so they can all server same IP addresses if you
configured them with same scope.

What you can do is the following:

- on DHCP server A set the DHCP scope to give out IP addresses from e.g.
10.1.1.10 to 10.1.1.254/16
- on DHCP server B set the DHCP scope to give out IP addresses from e.g.
10.1.2.10 to 10.1.2.254/16

These ranges do not overlap between two servers.

What you can also do is configure Conflict detection. Right click on the
name of your DHCP server and select Properties. Now click on Advanced tab.
Here you have Conflict detection attempts which is by default 0. You can set
it up to any number e.g. I usually set it to 3.
What this does is it tries to ping IP address before DHCP issues it to the
client. If it gets ICMP reply it will not give IP address to the client.
Note: configure conflict detection on all your DHCP server...

--
Mike
Microsoft MVP - Windows Security

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ps.com...
> Hi,
>
> I seem to be having big problems on our windows 2003 active directory,
> with dhcp addresses on our client machines. Despite the fact that the
> ip scope far exceeds the number of ip addresses we need, we are getting
> ip conflicts on many of our machines (we have around 700 machines in
> total, 95% of which are running Windows XP SP2). When you manually
> release and renew the ip address on the PC, it gives the following
> error message:
>
> Windows IP Configuration
>
> An error occurred while renewing interface Local Area Connection : The
> DHCP client has obtained an IP address that is already in use on the
> network. The local interface will be disabled until the DHCP client can
> obtain a new address.
>
> Trying to do an ipconfig -release and then an ipconfig -renew comes up
> with the same error. After doing several ipconfig -renew commands, it
> finally gets an ip address, but usually the one it had before, which is
> having the conflict.
>
> I originally thought that this was because we had two dhcp servers, one
> which was NT4 and not registered with AD. I disabled this server, but
> still got the same problems. I have then created another dhcp server
> within the AD, running on Server 2k3, but as before, the problems
> reoccur.
>
> This is causing major problems with packets dropping and the PCs being
> very slow on the network, along with various error messages which are
> annoying the users, but I cannot work out what the problem is.
>
> Has anyone had this same problem or something similar, or more
> importantly, anyone know how to resolve it?
>
> Many thanks,
>
> Laurence
>

Miha Pihler [MVP] wrote:
> Are both (all DHCP) server serving same IP range? If so -- they should
> _not_. DHCP servers on the LAN do not talk to each other (unless you set
> them up in cluster) -- so they can all server same IP addresses if you
> configured them with same scope.
>
> What you can do is the following:
>
> - on DHCP server A set the DHCP scope to give out IP addresses from e.g.
> 10.1.1.10 to 10.1.1.254/16
> - on DHCP server B set the DHCP scope to give out IP addresses from e.g.
> 10.1.2.10 to 10.1.2.254/16
>
> These ranges do not overlap between two servers.

Yes, they are set up so that one dhcp server is offering 89.0.40.1 -
89.0.45.254. The other is 89.0.46.0.1 - 89.0.48.254. They're obviously
both on the same subnet (255.0.0.0)

> What you can also do is configure Conflict detection. Right click on the
> name of your DHCP server and select Properties. Now click on Advanced tab.
> Here you have Conflict detection attempts which is by default 0. You can set
> it up to any number e.g. I usually set it to 3.
> What this does is it tries to ping IP address before DHCP issues it to the
> client. If it gets ICMP reply it will not give IP address to the client.
> Note: configure conflict detection on all your DHCP server...

Conflict detection attempts was originally set to 3. I changed this to
6, which didn't make any difference, so I have now set it back to 4.
This has been set up the same on both servers. Also, the lease time is
30 days on both servers, should I try a shorter lease time?

Thanks,

Laurence

Yes, try shorter lease times (e.g. 8 days?).

It would also be very good if possible to check both computers that are in
IP conflict (e.g. run ipconfig /all on the computer that was given IP
address from DHCP server first and see which DHCP server gave it).

--
Mike
Microsoft MVP - Windows Security

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>
> Miha Pihler [MVP] wrote:
>> Are both (all DHCP) server serving same IP range? If so -- they should
>> _not_. DHCP servers on the LAN do not talk to each other (unless you set
>> them up in cluster) -- so they can all server same IP addresses if you
>> configured them with same scope.
>>
>> What you can do is the following:
>>
>> - on DHCP server A set the DHCP scope to give out IP addresses from e.g.
>> 10.1.1.10 to 10.1.1.254/16
>> - on DHCP server B set the DHCP scope to give out IP addresses from e.g.
>> 10.1.2.10 to 10.1.2.254/16
>>
>> These ranges do not overlap between two servers.
>
> Yes, they are set up so that one dhcp server is offering 89.0.40.1 -
> 89.0.45.254. The other is 89.0.46.0.1 - 89.0.48.254. They're obviously
> both on the same subnet (255.0.0.0)
>
>> What you can also do is configure Conflict detection. Right click on the
>> name of your DHCP server and select Properties. Now click on Advanced
>> tab.
>> Here you have Conflict detection attempts which is by default 0. You can
>> set
>> it up to any number e.g. I usually set it to 3.
>> What this does is it tries to ping IP address before DHCP issues it to
>> the
>> client. If it gets ICMP reply it will not give IP address to the client.
>> Note: configure conflict detection on all your DHCP server...
>
> Conflict detection attempts was originally set to 3. I changed this to
> 6, which didn't make any difference, so I have now set it back to 4.
> This has been set up the same on both servers. Also, the lease time is
> 30 days on both servers, should I try a shorter lease time?
>
> Thanks,
>
> Laurence
>