Clients no longer able to receive IP addresses from the primary DHCP server

What would cause a DHCP server to stop handing out IP addresses to a subnet?  We have a single DHCP server that handles two different subnets - 1 & 2, that are on different VLANS on our network.  After working for several months, clients that originally have an IP leased from "subnet 1", began having trouble getting an IP assigned automatically from "subnet 2" - they just sit dormant, retaining the bad IP from the other "subnet 1".   To our knowledge, nothing has changed on the network - although something MUST have changed, somehow.  Again - this had been working fine for some time...and then stopped.  Any suggestions is much appreciated.

Update: I checked the DHCP server log, and I can see it is handing out IP's for subnet 1 to client requests coming from subnet 2.  This is the problem, but I do not know why its doing this.
Damian GardnerIT AdminAsked:
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IvanSystem EngineerCommented:
Hi,

maybe silly question, but have you tried restart dhcp service? Do you have dhcp helper (relay) on some router, maybe it's bugging atm.
Check if scope is not fully used. Any info in event viewer?

Regards,
Ivan.
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Damian GardnerIT AdminAuthor Commented:
thanks or your help.;  I checked event viewer, but nothing in there.  the DHCP log however shows requests are being answered incorrectly - subnet 1 IP's are being handed out to clients who move from the "subnet 1" area  over to "subnet 2" area of the network (another building).  since requests are making it through, it confirms its not an IP helper issue.  also have rebooted the DHCP server a couple times.
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Damian GardnerIT AdminAuthor Commented:
Could it somehow be related to the existing lease for this test machine I'm using - there is a lease on "subnet 1" that exists.  this is the IP that the server keeps giving the machine, even when its on subnet 2.  Could the lease somehow be "locking the machine in"??
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MaheshArchitectCommented:
some issue at router / switch relay side, probably VLAN / subnet 2 switch / router reboot would fix the issue, check if IP helper config has correct DHCP server IP configured for subnet 2
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Damian GardnerIT AdminAuthor Commented:
I'm engaging Microsoft on this.  will let you know what the problem was.
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DrDave242Commented:
Could it somehow be related to the existing lease for this test machine I'm using - there is a lease on "subnet 1" that exists.  this is the IP that the server keeps giving the machine, even when its on subnet 2.  Could the lease somehow be "locking the machine in"??

Yes, this is likely what's happening. At 50% of the lease duration, the client begins attempting to renew the existing lease, initially via unicast DHCPREQUEST packets to the DHCP server that gave out the address. If there's no response to any of these attempts, then at 87.5% of the lease duration, the client begins broadcasting DHCPREQUEST packets in the hope that any available DHCP server will respond. This is still an attempt to renew the existing lease, though. The client won't attempt to obtain a completely new address until either the lease duration completely expires or a DHCP server responds to the request with a DHCPNAK packet (essentially, "That address is no longer available - start the lease process over from the beginning").

Wireshark captures on the DHCP server and an affected client should give a good idea of exactly what's happening.
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Damian GardnerIT AdminAuthor Commented:
Dr Dave - thanks for your replay.  Assuming that this IS what's happening, how do I counteract that?  Also - its interesting that this process of clients walking back and forth between buildings here (different VLANs), has been working ok for atleast a few months (since November) when we installed the new leg of the network.  It just suddenly stopped working.
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DrDave242Commented:
Assuming that this IS what's happening, how do I counteract that?
That is a good question. I'm afraid I don't have an answer off the top of my head, but I'll see if I can come up with something. No changes were made to the network infrastructure around the time the issue started?
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Damian GardnerIT AdminAuthor Commented:
thanks for your help guys.  problem turned out to be due to the scopes being included in a superscope.  we don't know how that happened.  thanks again
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