Site and services - Failover to alt AD?

I have an AD with 4 sites, each site has it's own AD ctrl.

Setting up sites and services with each site having it's own network in AD

Questions

1. What happens when the AD ctrl at a site is down?  does it randomly pick a different ctrl?
2. Is there a way to set the 2nd best option?
erikwhitewayAsked:
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MaheshConnect With a Mentor ArchitectCommented:
that can be controlled over alternate DNS settings under tcp/ip setting on client machines

I assume that all DCs are DNS server as well...

if local DC goes down, client will try to pick alternate DC (DNS) server

and to find best DC if local DC is not available, is depends upon how your site links are configured, it will try to get authentication from another member site in site link

so basically I suggest, you can create 3 site links total and put two sites in each site link, one site should be common in all site links

also users still can be logged on if no site to site connectivity is available and also if local DC is also down because of cached credentials on workstations...

Mahesh.
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Dan McFaddenConnect With a Mentor Systems EngineerCommented:
Here is a simple description of the process:

1. When a client is booted, it queries DNS for a list of DCs.
2. It checks if the DC is in the local site by pinging the list of DCs.
3. If the DC is in the client's site, it uses that DC. Otherwise, it looks for the closest responding DC from the list in #1.  If no local DC responds, it uses the closest DC.

Here is an article about Site and Services config/design with some additional tech details.

Link:  http://blogs.msmvps.com/acefekay/2013/02/24/ad-site-design-and-auto-site-link-bridging-or-bridge-all-site-links-basl/

Another article about setting up site links.

Link:  https://windorks.wordpress.com/2014/03/14/single-ad-site-link-dont-do-it/

2. essentially you can control connection paths between sites by creating site links and setting the weight to control "routing" info.  Similar to weight in routing tables in network equipment (loosely said).  

Microsoft overview of AD Sites and Services.

Link:  https://technet.microsoft.com/en-us/library/cc731907(v=ws.11).aspx#BKMK_SiteLinks

Dan
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