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Dual DHCP on one network

Posted on 2011-09-14
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Last Modified: 2012-05-12
Have an issue here where some machines are not getting the proper DNS servers.

I have a windows 2003 server setup on our network that handles the DHCP duties ... so everyone that logs on gets the iP and DNS servers (192.168.123.xxx).

We have a contractor here that sets up a subnet (10.135.6.xxx) for his PLC programming.

Everything works OK if he isn't connected to my network.  When he runs a line from his router to a switch on my network, we have some (just a few) users that will pick up the IP from my DHCP server, but the DNS servers from his router ...

Is there an expklanation for why this happens? How can we make sure that the DNS servers are picked up from my DHCP server?

Thanx ...


Ming ...
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Question by:gmpon
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Expert Comment

by:Matt V
ID: 36538302
First, why are you letting anyone plug a router into your production network?

Aside from that, the reason this happens is because DHCP is a braodcast request that takes the first answer it gets.  So if a client expires while the contractor router is on your LAN, and the DHCP response from his router reaches the client before the response from the 2003 server, the client will take the contractor's IP.

I can think of no good reason why he needs to be plugging a router running DHCP into your production LAN.
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Expert Comment

by:jrgcomputing
ID: 36538316
Do they just get the DNS servers or a different IP address as well?
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Author Comment

by:gmpon
ID: 36538514
They plug so that they can get internet and my data server access.

They are officially part of our company, albeit in a separate group. The problem is they need to duplicate the field operations within their office. Coding is then identical to what they'll send out to the field.

This is the strange thing ... The IP addresses come from my server, the DNS comes from their router.

So iI end up with an IP in the 192.168.1234.xxx range (my IP's, my dns' are 192.168.123.241 and .242) ...and the DNS is 10.135.6.254 (which is what they use, but goes nowhere anyways) ...

Ming ...
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Expert Comment

by:jrgcomputing
ID: 36538540
If you run ipconfig /all what is the ip address of the dhcp server?
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Author Comment

by:gmpon
ID: 36538576
DHCP server shows up as my server ...

Ming ...
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Expert Comment

by:jrgcomputing
ID: 36538597
Have you setup your DNS servers as part of the DHCP Scope Options or Server Options? I suspect they are part of the Server Options so I would add them as part of the Scope Options and see what happens.
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Accepted Solution

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nativevlan earned 250 total points
ID: 36538613
Divide the switch into vlans and go from there, there is no good reason to have two L3 networks on the same L2 network.
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Author Comment

by:gmpon
ID: 36538654
Both of my DNS servers are already listed as part of the DHCP scope options  ...

006 DNS Servers  192.168.123.241, 192.168.123.240

Ming ...
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Expert Comment

by:jrgcomputing
ID: 36538677
Have you checked the effected computers to make sure they don't have a static dns server
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Author Comment

by:gmpon
ID: 36538712
I'll have to check whether or not I can separate the 2 at the switch level ...

Affected machines don't have static DNS ... when we pull off the other router .. all works OK ... when we put it in, the DNS servers are wrong.


Ming ...

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Expert Comment

by:jrgcomputing
ID: 36538763
Vlans would be a good resolve for this, however you will need to make sure your switch can handle vlans and second can it do vlan routing. You will also have to update any routing tables you might have at remote sites.

An alternate to this may be to create a VPN Tunnel between the two networks.
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Assisted Solution

by:ArneLovius
ArneLovius earned 250 total points
ID: 36539412
as nativevlan, divide the L2 network

If you don;t have managed switches, make it a physical disconection, you'll need "something" for the second network to access the Internet, but I'm going to guess that it doesn't need extreme performance, and do a "domestic" cable (not ADSL) router (with an Ethernet WAN port) might suffice.
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Author Closing Comment

by:gmpon
ID: 36550320
Looks like the VLAN's were the way to go, but we fixed the problem by just physically disconnecting them from our switch. Then plugging our switch into a router on their end ... access for them to our network is then via a machine that has 2 network cards .. one connected to each side ... and that appears tobe working OK ...

Thanx ...

Ming ...
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