After performing the IP change of a Domain Controller, the Exchange Active Directory Topology Service could not start, generating the following events: Event ID 4027 The LDAP server is unavailable.
Process msexchangerepl.exe (PID=3840). WCF request (Get Servers for contoso.lab) to the Microsoft Exchange Active Directory Topology service on server (TopologyClientTcpEndpoint (localhost)) failed. Make sure that the service is running. In addition, make sure that the network ports that are used by Microsoft Exchange Active Directory Topology service are not blocked by a firewall. The WCF call was retried 3 time(s). Error Details
Active Directory server is not available. Error message: Active directory response: The LDAP server is unavailable. —-> The LDAP server is unavailable.
Process Microsoft.Exchange.Directory.TopologyService.exe (PID=1440) Forest contoso.lab. Topology discovery failed, error details
Active Directory server is not available. Error message: Active directory response: The LDAP server is unavailable..
Given how the log is very clear about where is the issue, I started to look at my only Domain Controller to check if something was wrong. In fact, performing the DCDIAG command I noticed a Directory Services error. Since the error occurred after the IP change, I did some troubleshooting to identify that the network was OK.
The second point was checking the DNS, and indeed the issue was there.
After some checks, I noticed that the DNS records (same as parent) were still with the old IP.
After modifying the records with the new Domain Controller IP, I cleaned the cache in DNS and cleaned the local cache of the Exchange Server with the ipconfig / flushdns command.
As a result, the issue has been solved, and the Exchange AD Topology service has started successfully.