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Ip conflicts

Posted on 2014-02-27
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Last Modified: 2014-03-10
Im getting ip conflicts all across my network. I have plenty of ip's available in my dhcp server. Yesterday I lowered the lease time to 1 hour and then deleted all the leases to see if I could get machines to update DNS. Could this have caused this? If so what can I do to remedy the situation? I changed the lease time back to 5 days.
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Question by:Thomas N
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by:Dave Baldwin
ID: 39893501
If the other computers were still on during that, they would not have necessarily 'seen' the change in 'lease time' so yes, I think it could.  It won't clear up until all the computers check in for new DHCP settings.
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by:Thomas N
ID: 39893507
Is there anything I can do to force them to check in. Renew their lease? I have over 3k machines.
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by:Dave Baldwin
ID: 39893559
I don't know.  Click on "Request Attention" above to get more people to look at your question.
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by:KCTS
ID: 39893563
If you were running out of IP leases then I would expect to see failures to obtain an IP address, not IP conflicts.

If you are getting IP conflicts then it suggests either than you have machines with static IPs that are conflicting with your DHCP scope, or you have multiple DHCP servers giving out the same range of IPs.
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by:Thomas N
ID: 39893575
No I checked. We only have 1 valid DHCP server.
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by:Thomas N
ID: 39893577
I do see some "bad_address" ips in the dhcp server.
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by:Fred Marshall
ID: 39893748
No I checked. We only have 1 valid DHCP server.
But you didn't confirm that there are no static IP addresses assigned to devices. ??

And, you didn't confirm that there are no "invalid" DHCP servers.  ??
That's a likely bet.
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by:Thomas N
ID: 39893945
There are static ips assigned to our servers and routers but I doubt that is the issue. Our network has over 4k machines and we are getting the ip conflicts all over the network.

 I checked the DHCP server to check and it was the only one authorized.

This happened after we lowered the leased IP last night and deleted all the leases from the DHCP scopes hoping they would register with DNS.

We were having issues with machines registering to DNS and found out it was because the DNS update queue limit was exceeded.
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by:Fred Marshall
ID: 39896749
There should be no doubt that the static IPs are an issue.  They should be assigned strictly outside of the DHCP range.  

How does one check to see that a DHCP server is "authorized"?  It's easy to plug a commodity router from home into the network in order to have one's own wireless access point and suddenly have multiple DCHP servers on the network.

I'm not at all sure what DNS has to do with DHCP leases. DNS may be included but it's not a driving force.  Or have I missed something here?  I just don't know what "registering with DNS" means.

It's hard to determine what changing the lease period would be without the numbers before and after.  And, if all the leases were deleted it's not clear what might happen.  The computers think they have a valid lease and the server doesn't...... So maybe the server starts handing out new leases for what are still latent "used" addresses out there.

As far as I'm concerned, it makes little sense to have the leases longer than a day.
The leases will be renewed by the clients half way through - or 12 hours for 24 hour leases.  After that time you should be clear IF there's but one DHCP server that the addresses are all OK.  It probably would be better to shorten the lease time and not delete the current leases and simply wait for the old leases to run out.
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KCTS earned 500 total points
ID: 39896966
IP conflicts are not caused by failure to register IPs in DNS, they are caused by two or more machines on your network having the same IP. There are only two possible causes for this.

1. Static IPs are conflicting with your DHCP range
2. There are multiple DHCP servers (authorized or not - could be a router with DHCP enabled), issuing IP addresses.
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