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We used to have one AD box, AD1. We since added two new AD boxes: AD2 & AD3. The idea was to decommission the old box.

I went through DHCP Manager and set the scope and everything seems fine. Right click on DHCP and selecting "Manage authorized server" shows the new new boxes: AD2 and AD3.

I went to one workstation and did issued ipconfig /release followed by ipconfig /renew. The box releases right away and more importantly renews right away and shows AD2 as the DHCP server.

I then went to another workstation but any attempts to "renew" without AD1 turned on and authorized yields no connection. I even turned the box off and then back on. When connected, it shows AD1 as the DHCP server, obviously.

I did see this quesiton. I went to the old DHCP and verified that the DNS server option in DHCP record points to the new box. I also verified the first option in the forward lookup zone that the new servers show up in the Name Servers and the WINS tabs.

I checked and both workstations have obtain an IP address automatically and DNS servers automatically in the IPV4 properties for the adapter. Going to the Ethernet adapter on the second machine going to the old server and seeing if that has the old server listed rather than obtain an IP address automatically was my first thought.

Why can this second workstation not see the new DHCP server: AD2 and how do I fix the problem? Is the problem something that needs setting on our switch? (just a thought)

UPDATE:

I found a key piece to the puzzle. We have 3 subnets

  1. .70.*: Servers & Printers
  2. .71.*: Workstations
  3. .72.*: Phone

The workstation that works, when AD1 is off, gets an IP address on the 70 subnet, whereas the workstation that fails and requires AD1 has an IP address on the 71 subnet.

I also verified that when AD1 is off and I attempt to request an IP address, Microsoft Packet Sniffer shows that the failing workstation can indeed talk on the network, as I see packets flying by.

I would say the problem is with the switch, and still might be a configuration on our main switch, however both the AD1 and AD2 have IP addresses on the 70 subnet.

JoeQuerty said that a missing DHCP Relay setting in our Brocade ICX 6450-24P smart switch (router) is the problem. A Google search turned up this question, which says the same thing. As stated in my question, the only hesitation is that AD1 works and AD2 fails and both are on the 70 subnet and use the same switch with the current settings.

  • 1. The DHCP process is unrelated to DNS. 2. Was there a reservation for the PC in question in the DHCP scope on AD1? Run a packet capture on the PC and issue a release and renew. At the same time run a packet capture on AD1, AD2, and AD3. Then interrogate the captures, filter the captures for DHCP, and see what you see on the client and the servers. – joeqwerty May 6 at 19:13
  • Additionally, when migrating DHCP you can export and import the DHCP config using netsh or Powershell. This will ensure that you don't miss anything. – joeqwerty May 6 at 19:14
  • @joeqwerty no on the AD1 reservations. Someone else did the initial setup on AD2/3. I went back this morning and cleaned a few things up and eliminated all traces of AD1. I even picked a nice machine which was able to ipconfig /release /renew, just I found out that other workstations refuse to deal with any DHCP server that is not AD1. I will locate a packet sniffer. :-( I am not an expert in packet sniffing, so we shall see on that. I read up on DHCP config export/import this morning. That is not an issue or needed now. – Sarah Weinberger May 6 at 20:08
  • @joeqwerty The packet sniffer, see update to my question, was helpful as I though that there would be no network traffic, but in fact there is network traffic. The problem is (details above) that the failing computer is on our .71 subnet, whereas the computer that works gets an IP address on the .70 subnet. That points to either a switch issue or more than likely a difference in the configuration between AD1 and AD2. – Sarah Weinberger May 7 at 15:32
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    You're not dumb at all. You can't know something until you work with it. None of us are born knowing this stuff. It's experience and knowledge gained. Glad to help. – joeqwerty May 7 at 16:06
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The DHCP server is on one subnet. You have DHCP clients on other subnets. In order for the DHCP traffic from these other subnets to reach the DHCP server you need a DHCP relay agent running on your router that will forward the DHCP traffic from those other subnets to your DHCP server.

Your DHCP relay agent is currently configured to forward DHCP traffic to AD1. You need to configure the DHCP relay agent to forward DHCP traffic to AD2 and AD3. Find where your DHCP relay agent is and reconfigure it. This will fix the problem.

These have been immensely helpful to me in my IT career:

https://www.amazon.com/TCP-Illustrated-Protocols-Addison-Wesley-Professional/dp/0321336313

http://www.tcpipguide.com/

| improve this answer | |
  • he still knows his stuff. Hope things are well Joe. +1 – TheCleaner May 7 at 16:58
  • @TheCleaner Long time! We're good here. Hope you and yours are well! – joeqwerty May 7 at 17:20
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    @joeqwerty Our switch person made a change and my failed PC now talks to AD3, though I am having them change to AD2. Bingo on answer. – Sarah Weinberger May 7 at 19:26

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