What is/are the benefits of having one external/backup DNS Server (Server B) and the internal DNS (Server A) server?

Hi Sara,

Do you mean what are the benefits of having a secondary DNS server?

The major benefits of having a secondary DNS server are as follows:

Firstly, when the primary DNS server is unavailable, the secondary DNS server can provide name resolution, so the client query wont be affected.

Secondly, a secondary server can be used to offload DNS query traffic where a DNS zone is heavily queried by client.

For more information about DNS server, you may refer to the following documents:

Adding a secondary DNS server:

 https://technet.microsoft.com/en-us/library/Cc776953(v=WS.10).aspx

Best practices for configuring and administering DNS:

https://technet.microsoft.com/en-us/library/Cc959288.aspx

Best regards,

Anne he