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In my environment, i got 2 DCs Windows Server 2012. But when the primary is down i cant seams to login or find the path in the network to access another PC.
I have checked and all the DCs has GC and when i change the DNS in the PC from primary to secondary then it works, it just the PCs are not going to the second when the primary is down which i have set all the PCs with a secondary DNS and i am using static IPs for all PCs and servers.
If your clients are set to DHCP, then why don't you just add the secondary DC as secondary DNS server, assuming DNS is also running on the secondary DC?
Also, a trick to log a client in when the DC is down is to simply pull the network cable when entering the password.
EDIT i just saw that the IP's are static. Try an ipconfig -flushdns on a client and see if it makes a difference.
First, check your DCs IP configurations. Make sure the secondary DC can see itself as a DNS server for the domain. If it is depending on the offline server for DNS, you will have this problem.
Next, look into "DNS Client Side caching". It may be a timeout issue at the client that has previously seen the primary. http://blogs.msmvps.com/acefekay/2009/11/29/dns-wins-netbios-amp-the-client-side-resolver-browser-service-disabling-netbios-direct-hosted-smb-directsmb-if-one-dc-is-down-does-a-client-logon-to-another-dc-and-dns-forwarders-algorithm/#section7
https://blogs.technet.microsoft.com/stdqry/2011/12/14/dns-clients-and-timeouts-part-2/
Whenever I've had problems with authentication, removing the Ethernet connection will force the PC to use cached credentials. So long as that user has logged onto the PC before it should at least get you logged in to troubleshoot, then reconnect Ethernet.
In a deleted answer, you commented that "1st and 2nd they are configured fine and both DCs are pointing to themselves and clients can do nslookup on both server". This configuration is incorrect.
On a domain controller, the first and second DNS servers should be other domain controllers. The third DNS server should be 127.0.0.1. All of your clients should have at least 2 domain controllers as their DNS server.
