Check domain health

dcdiag /v /c /d /e /s: > c:\dcdiag.txt
ipconfig /all (from all DCs and DNS servers)
repadmin /showrepl (from each DC)
repadmin /replsum
dcdiag /test:dns /s: /dnsbasic
repadmin /syncall /aped
Ping each DC by name and verify that the name resolves to the correct IP address.
Use nslookup to test DNS across different DCs.
Use tracert to test the routes between servers.
repadmin /bind servername – Can the DCs bind to each other?