There are a multitude of things that can go wrong with an Active Directory Replication process, but like most preventative maintenance issues, issues can typically be resolved quickly if proper precautions are taken. Such problems include the DNS Lookup Failure coming up as an error in the logs. This is caused by a DNS zone having improper links to the child zones, the IP configuration of the DCs having misconfigured DNS servers, or the mapping of the current IP address to the domain controller isn't correct, along with many other things. In order to fix these types of errors, it's important to test the connectivity through the prompt of dcdiag and verify that the CNAME records. If things aren't verifying correctly, restarting netlogon should fix the issue. But if not, verifying the IP configuration and the DNS servers are correct should correct the error. If an Access Denied from Active Directory on a manual replication begins, there is most likely an issue with permissions with the replication synchronization. Only certain containers that have had that assignment are eligible for manual replication. To be able to fix this, run the repadmin or replmon and that should be able to make the replication work for that directory partition. A very likely issue that can arise between active directories is that the replication between all the sites can become slow. This can be caused by an increase in the latency in the lines, or even if the number of sites continues to go up, the time to replicate all of them will increase as well. When it comes to fixing this error, one has to keep an eye on the network speeds first off, to make sure that it has nothing to do with the active directory hardware. Keeping with the preventative maintenance, reviewing the logs for errors and making sure that the topology design that best fits the network is key to keeping the speeds as strong as they should be to keep the data flowing between them.