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Windows 2016 DHCP server RRS feed

  • Question

  • Hi,

    In my lab I have had for many years a Windows 2008R2 acting as a DHCP server for my Cisco phones. The Cisco phones (around 10) are in a different VLAN. MY Cisco switches have the ip-helper address configured. The Windows servers and PC's are in vlan 1 with network 172.16.0.0/24 and the IP-phones are in vlan 20 with network 172.16.20.0/24

    My Windows 2008R2 and Windows 2016 have IP address 172.16.0.60 (they don't run at the same time) On Windows DHCP servers are two DHCP scopes configured: 172.16.0.0/24 and 172.16.20.0/24

    It seems that Windows 2016 doesn't give IP's for network 172.16.20.0/24, but some times it does. When I shutdown Windows 2016 DHCP and start Windows2008R2 DHCP, my phones get immediately an IP from the correct network, and register on Cisco Communication manager

    Is there a difference between Windows2008R2 DHCP and Windows2016 DHCP, especially with multiple scopes?

    Both Windows machines (actually all my servers and PC's) are running in vmware ESXi

    Thank you

    JH

    Monday, July 31, 2017 3:45 PM

Answers

  • Hello all

    OK, I found the problem, sometimes you do something stupid :(

    The network mask on my windows 2016 server was /16 instead of /24, so for a server with IP 172.16.0.60/16 the IP 172.16.20.1 is in the same network, so uses broadcast and not unicast. After changing the network mask on the Windows 2016 server to /24 all works perfectly

    Thank you for your help, and good to now there is an active forum here, might need some help with Exchange 2016 in the future

    Jan

    Friday, August 4, 2017 9:36 AM

All replies

  • Hi,

    >>Is there a difference between Windows2008R2 DHCP and Windows2016 DHCP, especially with multiple scopes?

    No difference between Windows2008R2 DHCP and Windows2016 DHCP with multiple scopes.

    what's new in WS2016 DHCP :

    https://docs.microsoft.com/en-us/windows-server/networking/technologies/dhcp/what-s-new-in-dhcp

    Have you configured DHCP Relay Agent in your WS2016

    The DHCP Relay Agent component relays DHCP messages between DHCP clients and DHCP servers on different IP networks. Because DHCP is a broadcast-based protocol, by default its packets do not pass through routers. A DHCP relay agent receives any DHCP broadcasts on the subnet and forwards them to the specified IP address on a different subnet. The DHCP Relay Agent is compliant with RFC 1542, "Clarifications and Extensions for the Bootstrap Protocol." For each IP network segment that contains DHCP clients, either a DHCP server or a computer acting as a DHCP Relay Agent is required.

    Configure the IPv4 DHCP Relay Agent

    https://technet.microsoft.com/en-us/library/dd469685(v=ws.11).aspx

    Please check scope options of two scopes in two servers if them are same.

    http://www.cisco.com/c/en/us/td/docs/security/asa/asa84/configuration/guide/asa_84_cli_config/basic_dhcp.html



    If all above suggestions don’t work, you can download Network Monitor to capture packet. To check DHCPDiscover ,DHCPOFFER,DHCPQUEST,DHCPACK packet.


    Microsoft Network Monitor

    https://www.microsoft.com/en-sg/download/details.aspx?id=4865

    Best Regards,

    Frank




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    Tuesday, August 1, 2017 6:49 AM
    Moderator
  • Hi,
    Just checking in to see if the information provided was helpful. Please let us know if you would like further assistance.

    Best Regards,

    Frank

    Please remember to mark the replies as answers if they help and unmark them if they provide no help.
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    Wednesday, August 2, 2017 8:07 AM
    Moderator
  • So you have two DHCP servers serving the same scope, but not at the same time exactly? That will cause issues as clients ask for new address but they are already allocated resulting in bad IP address and existing clients trying to renew.

    Are you just testing 2016 DHCP? Can you not create a new subnet and VLAN for testing or bring both servers online with the scope split and updated helpers?

    Wednesday, August 2, 2017 8:16 AM
  • Hi,

    My voice VLAN is VLAN 20, with a default gateway 172.16.20.1. The DHCP server is in VLAN 1 with IP 172.16.0.60.

    In VLAN 20 a DHCP broadcast is unicast to 172.16.0.60 (ip-helper address in Cisco). Then 172.16.0.60 should unicast this back to 172.16.20.1 and the switch then make sure the client gets an IP address. Because the source is 172.16.20.1, the windows DHCP server knows which scope to use

    With wireshark on 2008R2 I see this:

    172.16.0.60 unicast DHCP back to 172.16.20.1

    What I see on 2016 is that the source is still 172.16.20.1, but the DHCP server (172.16.0.60) never unicast back, but does a broadcast

    I don't think I need DHCP relay agent on 2016, because the relay is done by the switch, and the 2016 has only one network interface and never sees the the broadcast from VLAN 20

    Here is a screen shot from wireshark on 2016:

    The server (172.16.0.60) never unicast back to 172.16.20.1, only broadcast

    Very weird

    Jan


    • Edited by jhuizi123 Wednesday, August 2, 2017 10:37 AM
    Wednesday, August 2, 2017 10:29 AM
  • Apologies, your clearly a skilled network guy. I'm just reading up on the Cisco IOS page, I can't remember how the DHCP Server sends back to the client, surely it only knows the source MAC unless something is carried in the forwarded DHCP broadcast. If the DHCP server ARP's then surely it won't get a response.
    Thursday, August 3, 2017 8:48 AM
  • Hi,


    Thank you for your question. I am trying to involve someone familiar with this topic to further look at this issue. There might be some time delay. Appreciate your patience.

    Thank you for your understanding and support.

    Best Regards,

    Frank


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    Friday, August 4, 2017 5:30 AM
    Moderator
  • Hello all

    OK, I found the problem, sometimes you do something stupid :(

    The network mask on my windows 2016 server was /16 instead of /24, so for a server with IP 172.16.0.60/16 the IP 172.16.20.1 is in the same network, so uses broadcast and not unicast. After changing the network mask on the Windows 2016 server to /24 all works perfectly

    Thank you for your help, and good to now there is an active forum here, might need some help with Exchange 2016 in the future

    Jan

    Friday, August 4, 2017 9:36 AM
  • Hi,

    I am glad to hear that your issue was successfully resolved.If there is anything else we can do for you, please feel free to post in the forum.


    Best Regards,

    Frank


    Please remember to mark the replies as answers if they help and unmark them if they provide no help.
    If you have feedback for TechNet Subscriber Support, contact tnmff@microsoft.com.

    Friday, August 4, 2017 9:41 AM
    Moderator