This post helps you to undetstand about SCCM CAS site VS Primary Site vs Secondary Site
For SCCM 2007 Use this link http://configmgr2oo7.blogspot.com/2011/08/choose-sccm-dp-vs-secondary-site-vs-bdp.html
 
Ms Link http://technet.microsoft.com/en-us/library/bb693570.aspx
 
Below is FOR SCCM 2012:-
 
Use the following table to help you plan the type of sites that you
might require in your hierarchy.

Server Purpose More information
Central administration site The recommended location for all administration and reporting for the
hierarchy.
  • SQL Server is required.
  • Does not process client data.
  • Does not support client assignment.
  • Not all site system roles are available.
  • Participates in database
    replication.
Primary site A required site that manages clients in well connected networks. All clients
are assigned to a primary site.
  • SQL Server is required.
  • Additional primary sites provide support for a higher number
    of clients.
  • Cannot be tiered below other primary
    sites.
  • Participates in database
    replication.
Secondary site Manages clients in remote locations where network bandwidth control is
required.
  • SQL Server Express or a full instance of SQL Server is
    required. If neither is installed when the site is installed, SQL Server Express
    is automatically installed.
  • A management point and distribution point are automatically
    deployed when the site is installed.
  • Secondary sites must be direct child sites below a primary
    site, but can be configured to send content to other secondary
    sites.
  • Participates in database
    replication.
When you plan a Configuration Manager hierarchy, consider the
following:
  • You can schedule and throttle network traffic when you
    distribute deployment content to distribution points. Therefore, you can use a
    distribution point instead of a site for some remote network locations.
  • Discovery data records (DDRs) for unknown resources transfer
    by using file-based replication from a primary site to the central
    administration site for processing. Because discovery can create a large number
    of DDRs, plan where to place your central administration site and consider at
    which sites discovery operations will run to minimize the transfer of DDRs
    across low-bandwidth networks. DDRs for known resources are processed at the
    first primary site to receive them and do not transfer by using file-based
    replication to the central administration site. Instead, after being processed
    at the primary site, the discovery information replicates to other sites by
    using database replication.
  • Role-based administration provides a central administrative
    security model for the hierarchy, and you do not have to install sites to
    provide a security boundary. Instead, use security scopes, security roles, and
    collections to define what administrative users can see and manage in the
    hierarchy.
  • Alerts in the Configuration Manager console provide
    state-based information for operations throughout the
    hierarchy.
Use the following sections to help you determine whether to install
Configuration Manager sites and site systems.

Determine
Whether to Install a Central Administration Site

Install a central administration site if you require multiple
primary sites. However, unless you support more clients and devices than a
single primary site can support, you can install a stand-alone primary site and
reduce your administrative overhead and avoid unnecessary database replication
between a primary site and a central administration site. In a stand-alone
hierarchy design, a stand-alone primary site provides the same functionality as
a central administration site. Prior to Configuration Manager SP1, this was a
permanent decision. Beginning with Configuration Manager SP1, you can expand a
stand-alone primary site into a hierarchy with a central administration site,
and then add additional primary sites. However, System Center 2012
Configuration Manager does not supported the removal of a central administration
site from a hierarchy to convert a hierarchy to a stand-alone hierarchy
design.
Use a central administration site to configure hierarchy-wide
settings and to monitor all sites and objects in the hierarchy. This site type
does not manage clients directly but it does coordinate inter-site data
replication, which includes the configuration of sites and clients throughout
the hierarchy.
Use the following information to help you plan for a central
administration site:
  • The central administration site is the top-level site in a
    hierarchy.
  • When you configure a hierarchy that has more than one primary site, you must
    install a central administration site, and it must be the first site that you
    install.
  • The central administration site supports only primary sites as child
    sites.
  • The central administration site cannot have clients assigned to it.
  • The central administration site does not support all site
    system roles. For more information, see Planning
    Where to Install Sites System Roles in the Hierarchy.
  • You can manage all clients in the hierarchy and perform site management
    tasks for any primary site when you use a Configuration Manager console that is
    connected to the central administration site.
  • When you use a central administration site, the central administration site
    is the only place where you can see site data from all sites. This data includes
    information such as inventory data and status messages.
  • You can configure discovery operations throughout the hierarchy from the
    central administration site by assigning discovery methods to run at individual
    sites.
  • You can manage security throughout the hierarchy by assigning different
    security roles, security scopes, and collections to different administrative
    users. These configurations apply at each site in the hierarchy.
  • You can configure file replication and database replication to control
    communication between sites in the hierarchy. This includes scheduling database
    replication for site data, and managing the bandwidth for the transfer of
    file-based data between sites.

Determine
Whether to Install a Primary Site

Use primary sites to manage clients. You can install a primary site
as a child primary site below a central administration site in a larger
hierarchy, or as the first site of a new hierarchy. A primary site that installs
as the first site of a hierarchy creates a stand-alone primary site. Both child
primary sites and stand-alone primary sites support secondary sites as child
sites of the primary site.
Consider installing a primary site for any of the following
reasons:
  • To manage clients directly.
  • To increase the number of clients and devices you can manage
    with a single hierarchy. For information about the number of clients and devices
    each primary site supports, see the Clients
    per Site section in the Supported Configurations for
    Configuration Manager topic.
  • To provide a local point of connectivity for
    administration.
  • To meet organizational management requirements. For example,
    you might install a primary site at a remote location to manage the transfer of
    deployment content across a low-bandwidth network. However, with
    System Center 2012 Configuration Manager you can use options to throttle the
    network bandwidth use when transferring data to a distribution point and this
    capability can replace the need to install additional sites.
Use the following information to help you plan for primary
sites:
  • A primary site can be a stand-alone primary site or a child
    primary site in a larger hierarchy. When a primary site is a member of a
    hierarchy with a central administration site, the sites use database replication
    to replicate data between the sites. Unless you need to support more clients and
    devices than a single primary site can support, consider installing a
    stand-alone primary site. Beginning with Configuration Manager SP1, you can
    convert a stand-alone primary site into a larger hierarchy when your deployment
    exceeds the capacity of a single primary site.
  • A primary site supports only a central administration site
    as a parent site.
  • A primary site supports only secondary sites as child sites
    and can support one or more secondary child sites.
  • When you use Configuration Manager with no service pack, a
    primary site cannot change its parent site relationship after installation.
    However, beginning with Configuration Manager SP1, you can install a new central
    administration site as a parent site of an existing stand-alone primary
    site.
  • Primary sites are responsible for processing all client data
    from their assigned clients.
  • When a primary site installs, it automatically configures
    database replication with its designated central administration site.
  • Primary sites use database replication to communicate
    directly to their central administration site.
  • You can install typically used site system roles when you
    install a primary site. For a list of site system roles that are supported on
    primary sites, see Planning
    Where to Install Sites System Roles in the Hierarchy.

Determine
Whether to Install a Secondary Site


Use secondary sites to manage the transfer of deployment content and
client data across low-bandwidth networks.
You manage a secondary site from a central administration site or
the secondary site’s parent primary site. Secondary sites must be attached to a
primary site, and you cannot move them to a different parent site without
uninstalling them, and then re-installing them as a child site below the new
primary site. You can route content between peer secondary sites to help manage
the file-based replication of deployment content. To transfer client data to a
primary site, the secondary site uses file-based replication. However, a
secondary site also uses database replication to communicate with its parent
primary site.
Consider installing a secondary site if any of the following
conditions apply:
  • You do not require a local administrative user for the
    site.
  • You have to manage the transfer of deployment content to
    sites lower in the hierarchy.
  • You have to manage client information that is sent to sites
    higher in the hierarchy.
If you do not want to install a secondary site and you have clients
in remote locations, consider using Windows BranchCache or distribution points
that are enabled for bandwidth control and scheduling. You can use these content
management options with or without secondary sites, and they can help you to
reduce the number of sites and servers that you have to install. For information
about content management options in Configuration Manager, see Determine Whether to Install a Site or Use
Content Management Options.
Use the following details to help you plan for secondary sites:
  • Secondary sites automatically install SQL Server Express
    during site installation if a local instance of SQL Server is not
    available.
  • Secondary site installation is initiated from the
    Configuration Manager console when it is connected to the central administration
    site or a primary site.
  • When a secondary site is installed, it automatically
    configures database replication with its parent primary site.
  • Secondary sites use database replication to communicate
    directly to their parent primary site and to obtain a subset of the shared
    Configuration Manager database.
  • Secondary sites support the routing of file-based content to
    other secondary sites that have a common parent primary site.
  • Secondary site installations automatically deploy a
    management point and distribution point that are located on the secondary site
    server.

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