Primary Domain controller is failed

1. It should take over the job of the failed DC. Btw. How long is "temporarily"?
2. Clients will try to authenticate to the unavailable DC. If they fail, they will authenticate to another DC. This is why you have two DCs.
3. Answered here: https://www.experts-exchange.com/questions/29180754/DNS-is-not-resolving-when-Primary-dns-server-is-down.html

The way DNS works, your clients machines will not attempt to reach the secondary DNS server immediately so your clients will get timeouts.  This will only work if the primary DNS server goes offline for an extended period of time which is usually tens of minutes.
If you reboot a server and a client can reach the server and the DNS Server (service) hasn't started yet, the client machines aren't smart enough to know to switch over to the secondary server.  As long as the server is up, it assumes that DNS must be up as well.

Also, you should configure a DC to point to the second DNS server and vice versa.
And configure both DNS servers in DHCP Scope Options.

Hi There

thank you for quick reply

how can I make sure DNS and domain service are working fine?
I tried with dcdiag .it is giving all successful

USe nslookup, and enter 'server servername' so that you can then test dns lookup on other dns servers and not just the first one.

To check where fsmo roles are installed, run netdom query fsmo or use powershell : get-adforest | fl, get-addomain | fl to check where fsmo roles are.

As well as DCdiag, check Services.msc and check the following are running ok:

Active directory domain services
Netlogon
DNS
Group Policy client