I just installed a second Active Directory server into my test domain. Dead easy – just install the role on Windows Server 2008 R2 SP1 and follow the prompts. I chose to install DNS at the same time (the AD setup wizard defaults to ticking this so don’t need to change anything).
At the end it synched my AD domain from the original server. It brought over all my DNS zones automatically, including: both the AD forward lookup zone and a manual forward lookup zone; my reverse lookup zone and DNS settings such as my forwarder IP addresses (which the server uses to resolve external DNS names on behalf of clients).
Now I have two DNS servers I can point my clients (including the DNS servers themselves) at both of them and should have basic networking (AD logons & DNS) even if one server goes down.
I also added the new DNS server to the DHCP Server Options so the new DNS server should get offered to DHCP clients automatically. My Windows 7 client picked this up without needing a DHCP release/renew which makes me suspect it got the new DNS server IP from somewhere else. I removed the new DNS server IP from the DHCP Server Options again and will check to see if clients are finding it on their own somehow.