Benefits of multiple AD sites

We are thinking of building two datacentres, both in the same city, but at opposite ends.

We're currently running Windows 2008 AD.

Could someone tell me the benefits of seperating the two datacentres into seperate AD sites, or just keeping them as one? I can't see what we gain by having two AD sites, one per centre?
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Mike KlineConnect With a Mentor Commented:
The two main benefits of sites are

1.  Control Authentication - Users use a local DC in their site for Auth
2.  Control Replication - intersite replication

More info here

There are also site aware apps (SMS/SCCM for example)

If both data centers are connected by high speed links then you may get away with having only one site.


Adam BrownConnect With a Mentor Sr Solutions ArchitectCommented:
Having a single site will cause computer to authenticate more or less randomly (with some constraints)  to any Domain Controller on your network. If you have two physical locations this can cause a lot of traffic to cross your WAN link between sites. This will result in a significant amount of bandwidth being used up by regular domain traffic. As Mike stated in fewer words, setting up a site for each location will allow you to decrease this burden by having all computers at one site use DCs in the site they are assigned to. When this is done properly, the amount of bandwidth required for Active Directory between sites is quite low, as only replication traffic needs to go between sites to make sure that all the Domain Controllers are up to date and have the same information. If you have a slow link, it makes a lot of sense to have separate sites for each location.

I agree with mkline71 and acbrown2010

Localizing access to resoures is very important because active directory is sharing the WAN connection with other traffice. The last thing you want to do is increase WAN utilization due to login and Global Catalog traffic.  It is also important from an application standpoint. Certain applications such as Microsoft Exchange rely on the global catalog for directory look ups so having a local GC server is highly reccommended for optimal performance.  
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AwinishConnect With a Mentor Commented:
Protection when something wrong goes to complete datacenter so other will be available to server you & help you to bring the business to normality due to power failure or anything like that.

Joe_BuddenAuthor Commented:
Thanks all for the great replies!

As regards MS Exchange -  you say that it needs GC's for it to function. Do you know how Exchange (or any application really) finds out the closest GC's to it? What's the actual process?
Adam BrownSr Solutions ArchitectCommented:
The Global Catalog servers are listed in DNS under the _msdcs zone (which controls SRV records for the Domain). If all the DNS servers that clients are configured with fail, or if all the global catalogs fail, there is no way for clients and servers to know where Domain information is served. The SRV records for Global Catalogs are separated by site in DNS as well, and sites are determined by the subnet range assigned to the site. So a Computer will look up DNS for the appropriate site information and attempt to contact a Global Catalog in its site. If none is found, it will go through other sites based on link cost to find a Global Catalog.
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