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DHCP to new subnet

Anonymous
2025-02-07T15:04:40+00:00

Hi all,

Hoping to get some help here. We recently added a new physical location that is connected via fiber to our main datacenter. We are using intervlan routing with HP Aruba switches. I have worked with HP to ensure routing is working correctly, but the issue we are having is that DHCP will not service the new subnet.

If I set a static IP address, I can ping to and from the DHCP server to the new subnet, traceroute, and do everything that devices at the main datacenter can do.

I come from a VMware world so I'm not super familiar with Hyper-V. Is there somewhere in Hyper-V or the virtual switch that I need to add this new subnet?

Currently it services the 10.17.0.0/16 network (at the main datacenter) and the 10.16.0.0/16 network (another remote site connected via fiber). The new network is 10.18.0.0/16 and it's set up the same in terms of routing as the 10.16 network.

HP was unable to find anything that would stop us from receiving IP addresses from the DHCP server through the switches and routings and suggested maybe an issue with Hyper-V or the DHCP server.

The scopes are set up and activated. The DHCP server has an ip address of 10.17.100.7 and as stated, I can reach to and from it with pings from the new network.

Please let me know what other info I can provide that would be helpful.

Windows Server Networking

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  1. Anonymous
    2025-02-11T05:58:32+00:00

    Hello,

    Thank you for reaching out

    Problem Analysis and Solution for DHCP Not Servicing the New Subnet

    Based on your description, the core issue is that the DHCP server is not providing IP addresses to the new subnet (10.18.0.0/16), even though static IP addresses work fine, and routing is correctly configured. Here is a step-by-step analysis and possible solutions for your issue:

    Problem Analysis

    1. DHCP Relay Configuration
      DHCP relies on broadcast communication, and in the case where the DHCP server and client are in different subnets, the router needs to support DHCP relay. Since routing seems correct, the following should be checked:

    DHCP Relay Configuration: Make sure the DHCP relay feature is enabled on the network devices (e.g., routers or switches) between the new subnet (10.18.0.0/16) and the DHCP server. The relay agent forwards DHCP requests from clients in remote subnets to the DHCP server.

    Router/Switch Configuration: Verify that the router or switch on the new subnet (10.18.0.0/16) correctly forwards the DHCP requests to the DHCP server.

    1. Hyper-V and Virtual Switch Configuration
      If you are running virtualized environments on Hyper-V, the virtual switch might need to be configured to allow the new subnet's traffic. Improper virtual switch settings can result in virtual machines not getting DHCP addresses. You should check:

    Virtual Switch: Ensure the virtual switch in Hyper-V is properly configured to allow traffic from the new subnet (10.18.0.0/16). If it's an external virtual switch, make sure it's bound to the correct physical network interface that has access to all subnets.

    VM Network Adapter Settings: Ensure that the virtual machines' network adapters are properly configured and connected to the correct virtual switch.

    1. DHCP Server Configuration

    Scope Configuration: Check if the DHCP server has a scope set up for the new subnet (10.18.0.0/16). The server needs a defined range of IP addresses for this subnet.

    DHCP Relay Agent: If the DHCP server is located in a different subnet and requests are being forwarded, ensure that DHCP relay agents are correctly configured in the network to forward the requests to the DHCP server.

    1. Routing and Firewall Check
      Even though you can ping and traceroute, make sure that no firewall rules or ACLs (Access Control Lists) are blocking the DHCP traffic. DHCP uses UDP ports 67 and 68, which must be allowed through the network.

    Firewall Settings: Temporarily disable any firewalls or ACLs to check if they are preventing the DHCP request and response messages.

    Check Routing: Verify that the routing between the new subnet and the DHCP server is complete and correct.

    Solutions

    1. Enable DHCP Relay Functionality
      On your HP Aruba switch, ensure that DHCP relay is enabled, so that DHCP requests from the new subnet can be forwarded to the DHCP server. Here’s an example configuration:
    2. ip helper-address 10.17.100.7  # IP address of the DHCP server

    This setting tells the switch to forward DHCP requests to the DHCP server at 10.17.100.7.

    1. Check Hyper-V Virtual Switch Configuration
      Ensure that the virtual switch in Hyper-V is correctly configured to allow traffic from the new subnet (10.18.0.0/16). If you are using an External Virtual Switch, make sure it is connected to the physical network interface that has access to the entire network.

    In Hyper-V Manager, check the network adapter configuration of your virtual machines, ensuring they are connected to the correct virtual switch.

    1. Update DHCP Server Configuration

    On the DHCP server, make sure you have configured a scope for the 10.18.0.0/16 subnet. This scope should specify the range of IP addresses available for the new subnet.

    Example steps:

    1. Open the DHCP Management Console.
    2. Right-click on "Scopes" and select "New Scope."
    3. Set the IP range to 10.18.0.0/16, and activate the scope.

    Ensure that the scope is activated and there is no conflict with existing ranges.

    1. Check Firewall and ACL Settings
      Verify that firewalls and ACLs are not blocking DHCP traffic. DHCP uses UDP ports 67 and 68, so ensure that these ports are allowed through any firewalls between the new subnet and the DHCP server.

    If possible, temporarily disable firewalls to test if they are causing the issue.

    1. Check Network Interface Binding
      If you are using a virtual switch that is bound to a physical network interface, make sure the interface is configured to route traffic to the 10.18.0.0/16 network. Confirm that the routing table allows traffic to flow properly between the new subnet and the DHCP server.

    By following these steps, you should be able to diagnose and resolve the issue of DHCP not servicing the new subnet. If none of these steps resolves the issue, further inspection of network logs, routing configurations, and Hyper-V logs will be needed to identify the root cause.

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