The DC +DNS server (Win2012R2) is installed on subnet 10.1.124.0/24, and the computers on this subnet are functioning correctly within the domain.

However, computers on subnet 10.1.125.0/24 are not working well in the domain, despite one of them previously being added to the domain. These computers are unable to resolve IP addresses from the DNS server, as shown by failed pings and failed attempts to locate the host.

The problem may be related to configuration issues with the DNS server, incorrect host settings, or routing difficulties between the subnets.

It appears that your issue is related to DNS rather than routing. The DNS appears to be malfunctioning, resulting in the domain not functioning.

Can you provide more information on what type of router is being used between subnets? Is it a hardware or server-based router? Additionally, are there any firewalls in place that could be blocking DNS traffic between clients and the DC?

It would also be helpful to know if there are any additional settings on the firewall of the controller itself. Which Windows firewall profile is enabled on the client and on the DC? Finally, please attach the hosts file from the problematic workstation for further analysis.

Your difficulty lies on a different level and is not related to subnets, so the domain controller cannot be of any help. Since the pings are successful, it's not a routing issue, although there might be some other problem.

It seems that 10.1.124.3 and 10.1.125.3 could be the same router, but are they? Has anyone acknowledged this? Is there a possibility of a random NAT connection between the networks? Is there any enabled firewall that might be a source of trouble?
Additionally, it would be helpful to know more about the DNS configuration.

1) It appears that there are some issues with the reverse zone.
2) Has the second subnet been registered on the domain controller?
3) What is the output of the "ping servert-01.test.local" command?
4) In case the network configuration is as depicted in the diagram, kindly remove DNS 8.8.8.8 and bring up the secondary domain controller while raising the second DNS.

It seems like there could be several potential causes for the issues you're experiencing with the computers on subnet 10.1.125.0/24 not working well in the domain. Here are a few possible explanations, as you mentioned:

1. DNS Server Configuration: Double-check the configuration of the DNS server on the DC. Ensure that it is configured to listen on all necessary interfaces, including those that serve the 10.1.125.0/24 subnet. Also, confirm that the DNS server has the correct entries for the computers on subnet 10.1.125.0/24.

2. Incorrect Host Settings: Verify that the computers on subnet 10.1.125.0/24 have the correct DNS server settings in their network configurations. Ensure that they are pointing to the correct DNS server, which should be the DC running the DNS server for the domain.

3. Routing Difficulties: Check if there are any routing issues between the subnets. Confirm that the necessary routing rules and firewall configurations are in place to allow traffic between subnet 10.1.125.0/24 and the subnet where the DC + DNS server is located (subnet 10.1.124.0/24).

4. Network Connectivity: Ensure that the computers on subnet 10.1.125.0/24 have proper network connectivity to the DC + DNS server. Check if there are any physical or logical network issues, such as faulty cables, switches, or routers. You can test this by trying to ping the DC + DNS server from a computer on subnet 10.1.125.0/24 and vice versa.

5. Firewall Settings: Verify that the necessary firewall rules are in place to allow DNS traffic between the subnets. Make sure that port 53 (UDP and TCP) is open on the DC + DNS server for both incoming and outgoing connections. Additionally, ensure that the Windows Firewall or any other software firewalls on the client computers are not blocking DNS traffic.

6. Subnet Configuration: Double-check the subnet configurations on the routers or switches that connect subnet 10.1.124.0/24 and subnet 10.1.125.0/24. Confirm that the correct subnet masks and gateways are configured on both sides.

7. DNS Forwarders: Consider configuring DNS forwarders on the DC + DNS server. This will allow it to resolve external DNS queries for the computers on subnet 10.1.125.0/24. Add the IP addresses of reliable external DNS servers (such as those provided by your ISP) as forwarders in the DNS server configuration.

8. DNS Zone Replication: If the DC + DNS server is hosting a DNS zone for the domain, ensure that zone replication is functioning correctly. Check the DNS event logs on the DC for any errors related to zone transfers or replication.