Some of the questions that should be asked when you are determining the FSMO roles and the Global Catalogue (GC) placement are:
1. How many Domain Controllers are in your environment? If there is only one DC in your environment (Not recommended) then all the roles will be on the same server and I would recommend that there should be no more than 20 users. If you have more than 20 users you should have at least two DCs. Two domain controllers are recommended for environments of up to 200 users. It is important to not have the Domain Naming Master on the same server as the RID Master or the PDC Master, because if it stopped working it would be difficult to create a new DC to replace the failed DC as the Domain Naming Master must be live to use promote/create a new DC. The PDC (Primary Domain Controller) Master and RID (Relative Identifier) Master are the roles that have the biggest initial impact on the environment if lost to both users and Systems Administrators.
2. What servers/server are global catalog servers? All domain controllers can be a global catalog. When every domain controllers is a global catalog it does increase the replication work load, but this has minimal impact and speeds up the performance of AD. The global catalog provides the ability to locate objects from any domain without having to know the domain name. A global catalog server is a domain controller that, in addition to its full, writable domain directory partition replica, and it stores a partial, read-only replica of all other domain directory partitions in the forest. The additional domain directory partitions are partial because only a limited set of attributes is included for each object. By including only the attributes that are most used for searching, every object in every domain in even the largest forest can be represented in the database of a
References: Al-Beruni, R. (2014). MicrosoftGURU. Retrieved from MicrosoftGURU web site: http://microsoftguru.com.au/2012/07/29/microsoft-active-directory-best-practice-part-ii/ kiransawant. (2012, July 16). Best Practices for Assigning FSMO Roles. Retrieved from Fellowme: http://kiransawant.wordpress.com/2012/07/16/best-practices-for-assigning-fsmo-roles/ Microsoft. (2012, April 26, 2012 26). Planning Operations Master Role Placement. Retrieved from Technet.microsoft: http://technet.microsoft.com/en-us/library/cc754889(v=WS.10).aspx Petri, D. (2009). Whale Web Inc. Retrieved from Whale Web Inc web site: http://www.petri.co.il/planning_fsmo_roles_in_ad.htm Tulloch, M. (2004, June 15). Best Practices for Assigning FSMO Roles. Retrieved from Windows Devcenter: http://www.windowsdevcenter.com/pub/a/windows/2004/06/15/fsmo.html Wiley, J. (2012). Windows Server 2008 Active Directory Configuration. Hoboken: John Wiley and Sons, Inc.