VLAN clients not receiving IPv6 addresses from Windows DHCP servers

Question

I noticed a few months ago that clients connected to my VLANs stopped receiving IPv6 addresses from my Windows DHCP servers (both virtualized). DHCP requests on the network are forwarded to the Windows DHCP servers by the router, and they receive IPv4 addresses just fine. A packet capture of traffic on one of the VLANs shows that there are DHCPv6 solicitations that are being transmitted (destination of ff02::1:2), but the Windows DHCP servers don't respond. This problem has occurred with both Windows Server 2022 and 2025.

Answer 1

It occurs to me that you might check the firewall on your Domain Controller/DNS/DHCP server.

Basically, you have a DHCP relay that relaying DHCP discovery packets from the PC in vlan 100 to the default vlan where the DC is located. I would check to see if the Windows Firewall on the DC is blocking those packets. I know that Windows Server's default firewall settings often block a lot of broadcast packets to DCs, you might need to make some changes to firewall settings there. I would start out just turning Windows Firewall logging on and setting it to log dropped packets. Generate some DHCP requests with the PC in vlan 100 (just connect it to the network) then check the firewall log in Windows Server. Note that in Windows Server the firewall log can lag behind a little in updating, so give it a couple of minutes at least.

You probably also need a static route in the router too. The route will have it's detination set to vlan 100's subnet and the gateway

Your firewall will need to allow the DHCP response traffic into vlan 100 as well. I believe this response traffic will be addressed to the DHCP relay agent's IP address and will run on port 67. Your firewall will probably need a firewall rule to allow this traffic.