Something new in ConfigMgr 2012 is that Microsoft added a new site type, which is the CAS (Central Administration Site)
Microsoft says the following about it” The central administration site coordinates inter-site data replication across the hierarchy by using Configuration Manager database replication. It also enables the administration of hierarchy-wide configurations for client agents, discovery, and other operations. Use this site for all administration and reporting for the hierarchy”

This role is needed for large scale deployment and sits at the top of the hierarchy in ConfigMgr 2012. The top reasons for using this role is:
       * If you have over 100.000 clients in your environment.
       * If you wish to split up your environment in general.
       * If you need more then 1 Primary site

Top reasons not to use this role:
        * One extra server to maintain with licensing, monitoring and such.
        * SQL enterprise is most likely needed since this supports up to 400.000 clients (SQL server standard)  
           only support up to 50.000 clients.
        * The CAS role needs some heavy machinery since it needs to process a lot of data. Microsoft
            recommends 16 cores, 64 GB of ram and 1.5 TB disk space for all the files.
            If your environment  consists of 100.000 clients the database files alone will take approx. 500 GB                    
The CAS can support up to 25 child primary sites.

A typical CAS setup would look like this (If we had 2 domains)


Something that you need to know about the CAS role

It does not process data from clients

It does not accept client assignments

It participates in database replication

So this is a role that just “sits” there and receives data from other sites in the hierarchy.  Since the CAS also participates in database replication all inventory data and information will be replicated to the CAS server, therefore it is very useful to use for reporting services.

Also something to consider, in a multiple domain forest and multiple sites. Who has access to the CAS, who has admin access on the CAS? The users that have access to the CAS has access to every sites and its db information.

But remember if you need to use a CAS site in your environment, remember that you have to install the CAS role BEFORE you install a primary site. If you installed the primary site first, the only way to join that primary site to the CAS is to reinstall it.

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