2 DHCP Servers 2 Different Subnets Same LAN

March 17, 2010 at 13:37:59
Specs: Windows 2003
Hello,

I am rather new to networking, and just landed a job as a net admin a few months back.

The company I'm working for is in a transitionary period, and we currently have 2 DHCP servers with 2 different scopes on 2 different subnets on the same LAN. The company has been running this configuration for at least 6 months. I know this is not recommended, but I also know that it can work...sometimes :) As you would expect, we have been experiencing the oddball intermittent problem here and there, but nothing too catastrophic.

Recently, we have been seeing some very strange behavior.

1.) IP address conflicts (approximately 5 per day) when we had maybe 1 per week previously which we traced back to a machine with a static assignment that did not have a reservation/exclusion in DHCP.

2.) When checking one server today, everything from xxx.xxx.xxx.3 to xxx.xxx.xxx.200 was showing as available (no lease, no reservation). A minute or two later, I attempted to reserve one of the available addresses and got a message that the address was in use. So I refreshed DHCP and lo and behold, AT LEAST 50 more addresses showed with leases. I know for a fact that there was no hardware failure, and that 50 people didn't suddenly turn on their machines.

Any ideas what would cause this sort of behavior (besides having 2 DHCP servers on the same LAN with two different subnets)?

Notes:

My LAN has approximately 200 machines on the network at any given time spread across 3 buildings.

We rarely use static assignments - when we do, we make sure to add a reservation or exclusion for that address (excluding the 1 that was giving us problems previously).

Both servers and all main switches have had 100% uptime for 30 days.


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#1
March 17, 2010 at 17:49:06
When assigning a static IP address, I feel it's better to use an IP out of the DHCP range, instead of reserving one in the DHCP range, although I've done that too.

Are the 2 subnets as follows, 192.168.1.xxx & 192.168.2.xxx ?

With 5 IP conflicts per day, something is radically wrong but I have a feeling it's a simple fix.

I would look in the server room. There maybe a cable crossed to the wrong switch.

How do you know when a politician is lying? His mouth is moving.


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#2
March 18, 2010 at 08:38:40
The company I'm working for is in a transitionary period, and we currently have 2 DHCP servers with 2 different scopes on 2 different subnets on the same LAN. The company has been running this configuration for at least 6 months. I know this is not recommended,

This is an incorrect assumption.

You can, and I have done this, have two DHCP servers within the same subnet without issue as long as their scopes don't overlap.

I had a training lab I setup with two SOHO Router's (Linksys WRT54GL's using the Tomato firmware). Because I didn't want to overload either one with too many connections, I split the scope between the two routers so half of the wireless clients would connect to one, and the other half to the other router. It worked quite well.

You say you've reserved .3 through .200
Unless you require that many DHCP IP's I recommend you close your scope down to the number required plus 10 or 20 extra's. When statically assigning IP's do not use exclusions or reservations. Use an IP that's outside the DHCP Scope. Then you can never have a conflict.

When you do get IP conflicts, open Event Viewer on the client affected and find the error associated with the duplicate IP. Within that error it will show the MAC address of the other computer involved. Use that information to track down the other client and check. It's just possible you have some PC's with static assignments that you didn't know about.


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