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DHCP Relay Agent and DHCP server on same network segment. RRS feed

  • Question

  • Hello,

    I am in the middle of a cross forest AD migration. The old domain / forest has DHCP running on a 2008R2 Server. I have copied the scopes and other options to a separate 2012R2 DC in the new domain / forest. The scopes are disabled on the new server until a maintenance window is scheduled. The servers are on the same network segment (subnet) and there is no router or firewall between them. At the time of cut over, if I add the DHCP relay agent to the older DHCP server, configure it to forward to the new server, and disable the scopes on the old box and activate them on the new server will everything work OK? I guess the goal is to not have to reconfigure the helper address on any of the switches or routers. 

    Thursday, April 26, 2018 1:23 AM

Answers

  • IMHO, I wouldn't bother with a relay agent.  You'll just be left with a piece of equipment you'll need to cleanup later. And honestly, DHCP is pretty basic and dumb. Just disable the NIC on the old DHCP server and give your new DHCP the same IP address if you're worried about helpers.

    I would also export then import the DHCP config from old to new, rather then copy them just to make sure you didn't miss anything in your config. The config from 2008r2 to 2012r2 will migrate fine.

    Like I said, DHCP is really simple, so yea, your way will absolutely work, but the end will be a bit messy.

    Either way, when your new server starts leasing, do a reconcile on your scopes!!! ESPECIALLY if you clone your config instead of exporting it. You really don't want your new server to start leasing addresses that are already in use. That's recipe for a perfect storm and really REALLY bad day.


    ZdPav

    • Marked as answer by wedqwdqwedwd Thursday, April 26, 2018 4:22 PM
    Thursday, April 26, 2018 3:37 AM

All replies

  • IMHO, I wouldn't bother with a relay agent.  You'll just be left with a piece of equipment you'll need to cleanup later. And honestly, DHCP is pretty basic and dumb. Just disable the NIC on the old DHCP server and give your new DHCP the same IP address if you're worried about helpers.

    I would also export then import the DHCP config from old to new, rather then copy them just to make sure you didn't miss anything in your config. The config from 2008r2 to 2012r2 will migrate fine.

    Like I said, DHCP is really simple, so yea, your way will absolutely work, but the end will be a bit messy.

    Either way, when your new server starts leasing, do a reconcile on your scopes!!! ESPECIALLY if you clone your config instead of exporting it. You really don't want your new server to start leasing addresses that are already in use. That's recipe for a perfect storm and really REALLY bad day.


    ZdPav

    • Marked as answer by wedqwdqwedwd Thursday, April 26, 2018 4:22 PM
    Thursday, April 26, 2018 3:37 AM
  • Thank you for the info ZdPav! I wish I got to make all the decisions, but as a consultant, I often find I have to do things the customer's way - which is not always the best way. 
    Thursday, April 26, 2018 4:22 PM
  • Hi,

    Thanks for your question.

    Your DHCP migration is OK, and don't worry about old DHCP on the same subnet even not need to disable its NIC. 

    If you would like to old DHCP also to serve for clients, you may configure DHCP failover on the old DHCP as the new DHCP partner with hot standby/load balance mode.

    You may refer to the following article,

    https://blogs.technet.microsoft.com/keithmayer/2012/10/28/step-by-step-scoping-out-the-new-dhcp-failover-in-windows-server-2012-31-days-of-favorite-features-part-28-of-31/

    Hope above information can help you.

    Highly appreciate your effort and time. If you have any questions and concerns, please feel free to let me know.

    Best regards,

    Michael


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    Friday, April 27, 2018 2:57 AM
  • Well, it is done. It did NOT go smoothly. When you have the DHCP Relay Agent and the DHCP server on the same sub net, you should lower the Boot Threshold to zero (see the screenshot below). The symptom was that all the requests were being discarded. I saw the message "dropping REQUEST with secs-since-boot" in the IPBOOTP.LOG tracing log, and researched based on that error. Also, this server was running NPS, which added additional complexity. Avoid this configuration if you can!

    


    Saturday, April 28, 2018 3:44 PM