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IP Conflict

Posted on 2008-10-03
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Last Modified: 2013-11-09
I am going to try and explain this as best I can.  I have seen this issue before, I have applied the fix, but never really understood technicall why it occurs.  This time around, I need your help to understand why the following has occured.

I have a terminal server running Windows 2003 Server.  This server has a dual BroadCom NIC.  The primary card has been enabled since day one; the secondary card is disabled and all it's services.  Today, after running for 2 years, the primary NIC stop working.  From a workstation, if you ping the IP Address - it replies timed out and every so often it replies.  

* The server was rebooted, and nothing happened.  Same thing.  
* We unconfigured the static IP Address and disabled that card.
* Enabled the secondary card and reconfigured it with the static address from the primary.  Right away, the system responds with "Duplicate IP Address".  We rebooted the system again, and same message.  

* We checked the primary card to make sure it does not have that IP Address taken.  And it did not.  
* We reenabled the primary NIC and it got an IP address dynamically from the DHCP server... BUT, that IP would not respond from any workstations, nor could we connect to it.  

So, we decided, that NIC is dead.  On the secondary NIC, we again tried to put the orginal IP Address and we still got "IP Address Conflict".  We enabled DHCP, it received and IP Address, and we were able to connect to that server.  But we needed the original IP or we would have had to change the connection settings on all the PC's.

Once my level II tech explain the situation to me, I knew right away how to fix it...  Here's what I did, but I don't understand why it fixes it...

* Enabled the original NIC.
* Allowed it to get a DHCP address.
* Then disabled it, and renabled it and statically entered the ORINAL IP ADDRESS.
* Then renable DHCP - and at this point is where the error went away.
* I permanently disabled the primary card and configured the secondary NIC with the original IP Address.

So some how, the primary NIC was locking out the original IP Address?
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Question by:BygRob
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by:Mikealcl
ID: 22636809
This almost sounds like an issue with the switch.  Like its not updating the mac table until the network card reports using a different ip address.  Once that is changed the switch mac table assigns the original ip from nic 1's port to the port for nic 2?


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by:freefromspam
ID: 22636846
Was the server part of a cluster?
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by:BygRob
ID: 22636859
No cluster.
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by:BygRob
ID: 22636874
Good point Mikealcl.  I will look into that switch.
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Patrick49er earned 500 total points
ID: 22636984
This isn't a switch issue but rather an issue with you DHCP and DNS servers.  Switches care more about MACs, which can be noted by the fact you cannot tell what the IP address is for a device connected to a switch port but you can the MAC.

Make sure that you do not have a WINs entry that is hanging up with the name of the server to the IP.  DNS will also have this listed.  If you use entries in your DHCP server for reserved IP addresses, you will have an issue since it assigns the IP according to the MAC you supply.  However, with your solution that you did, it does not sound like you use reserved addresses.

With that in mind, I would bet your DNS and/or WINs was not updating.  If you are forced into this situation, you can always go into your DNS and/or WINs servers and manually remove references to the server for that IP.  By doing the procedure you did, there was a logical removal of the IP address and MAC address from the server name.
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by:ChiefIT
ID: 22640240
When you get an IP address dynamically, the DHCP server provides a DHCPoffer. Within the offer packets, it tells the client requesting the DHCP address to send out an ARP ping to see if the IP address is taken. If the address is either reserved in DHCP or a ARP ping reply is returned, the offer is no good and the DHCP client will request again. If no reply or reservation, the offer goes through and a lease is assigned.

ARP stands for Address resolution protocol and it is used to map IP addresses to MAC addresses.

As Patrick49r was saying, a DHCP reservation will reserve an IP for a MAC address. If reserved, no other NIC can get that IP because it is reserved by MAC address, not by servername. Your DHCP server will reply to a ARP ping in the event the IP address is reserved. If it compares the two MACs and finds them to be dissimilar, you will get the error you are seeing.

So, Patrick49r had a very good analasys of the situation.

DHCP basics: (for your viewing pleasure. LOL)
http://support.microsoft.com/kb/169289

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by:Patrick49er
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Is this one closed?
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