We are setting up a stretched farm for a customer where they have 2 DC close to each other. Can anyone pls guide us on how many number of WFEs,App servers, SQL servers required at each DC? Also in case of Failover of one DC, how do we transfer all users to DC2 ?? Pls suggest if we going wrong anywhere!

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Microsoft recommends a ratio of 3 Web servers per DC (https://technet.microsoft.com/en-us/library/cc263061(v=office.12).aspx for 2007, maybe for 2010/2013 change this recomendation).

Some things to evaluate:

  • LDAP operations (such as authentication and user operations)
  • Number of users (RPS)
  • Type of authentication (basic, kerberos)

Active Directory uses DNS SRV records to locate domain controllers. Unless you've taken the time to set up a different AD Site where that other DC lives, chances are your clients are already using that other domain controller. When one DC is down, there will be some timeouts as clients attempt to talk to the dead DC but will automatically connect to any other DC in the domain.

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