Universal groups (UG) are stored in the Global Catalog (GC). But what exactly is the Global Catalog, and how does it store objects? Does it store anything at all! And how do Universal Groups work anyway?
Active Directory Domain Controllers (DC) have exactly one database. It is stored in %windir%NTDS and is called NTDS.DIT. DIT means Directory Information Tree. It is an MDB database. This database is the only place a Domain Controller stores directory data. So that means that the Global Catalog is not a separate database.
Domain Controller databases store Naming Contexts (NC). These are also called directory partitions, or just partitions, and those names actually makes it easier to understand what they are. By default a DC has at least 3 NCs. The first is the Domain NC, also called the Default Naming Context, which stores all the objects belonging to a domain; users, groups, computers etc. The DC has a copy of this NC because it is a DC for the same domain. The second is the Schema Naming Context which stores all the object definitions for a particular forest. And finally the Configuration Naming Context, which stores the common configuration for a forest, e.g. sites, subnets, forest-wide services etc. In addition to these we can have application partitions, used by DNS for example, but they are not important here. The Configuration and Schema partitions are stored by every DC in a forest and contain the same data on all DCs. If you are a DC in a forest you always store the full Configuration and Schema NCs. The Domain Naming Context is stored in full by every DC in the given domain. A DC can only be a DC for one domain, meaning it only ever stores one complete Domain Naming Context. In addition to its own full Domain Naming Context, the DCs that are designated as Global Catalogs also store partial Naming Contexts for all other Domain Naming Contexts in the forest. So the Global Catalog is not a separate Naming Context.
Now, the point of a Global Catalog is to have one or more places where you can get information, but not all the information, from all the domains in the entire forest. To be a GC, a machine must also be a DC. You cannot just be a GC. A GC knows a little bit about every object in every Domain NC in the forest. It also knows everything in the Configuration and Schema partitions, because the GC is also a DC and every DC holds the Configuration and Schema partitions. A forest must have at least one Global Catalog, and every site in a forest must have a Global Catalog. A user must be able to talk to a Global Catalog to log on. You talk to a GC using LDAP or ADSI. A regular DC listens for LDAP traffic on port 389 TCP (and 689 TCP for LDAPS), and on these ports you can retrieve all the information about any object in any of its full NCs. The Global Catalog, which is not a separate database or naming context, remember, listens for LDAP traffic on port 3268 TCP.
What controls the information the GC has about each object from the Domain NCs in the forest is called the Partial Attribute Set (PAS), and it is defined on the attributes in the Schema. Some attributes are part of the PAS, but most are not. If you would like to add an attribute to the PAS you can do so with the Schema Management Console Snap In. So every attribute definition in the schema has a flag that says if that particular attribute should be replicated to the Global Catalog. Which is not a separate database or naming context, but has its own port, but not its own protocol.
So let’s try to sum this up. Objects in domains in Active Directory are stored in Naming Contexts. An object is made up of several attributes, some of which are flagged as being part of the Partial Attribute Set. A DC designated as a GC stores a copy of every domain NC in the forest, containing all the objects in that NC, but only the attributes that are flagged as being part of the PAS. That means that the Global Catalog is just a copy of a regular domain NCs,but with a little less data. A DC/GC also has only one directory service process, LSASS.EXE, which listens on both port 389 TCP and 3268 TCP. If traffic comes in on port 389 you can ask about the Configuration, Schema or full Domain NCs. But if traffic comes in on port 3268 you can only retrieve data about attributes that are part of the PAS, and only from domain NCs (any domain NC on the DC/GC).
What I am trying to say here is that there really is no difference between the regular DC function and the GC function. To say that something is stored in the Global Catalog really does not make sense, since it is in the same database and in the same Domain Naming Context. The only difference is what you can “see” while looking at the GC, as opposed to looking at the DC. Think of this as having DC glasses and GC glasses. When viewed with the GC glasses you can see a fuzzy outline of an object, but when viewed with the DC glasses, the same object is clear and crisp.
When we say that a universal group is stored in the Global Catalog what we are really saying is that a Universal group is a group, stored in the Domain Naming Context of the domain where it was created, which has its members attribute in the Partial Attribute Set. To demonstrate this lets look at a Global Group in a full domain NC (with our DC glasses). You will see the members attribute populated with its members. Now look at the same global group on a GC (with GC glasses). The members attribute is missing so the GC does not know who is a member of the Global Group. A Universal group’s members attribute will look identical when viewed in the full Domain NC and when viewed in the partial Domain NC.
Also, think about this. If Domain B is a child domain of Domain A. What happens to a Universal group created in Domain B when Domain B is decommissioned? It will disappear! Because, it was just a group who had its members attribute in the PAS. If a Universal group was really stored in a separate Global Catalog, it would not be visible in any domains in the forest, it would only be visible when you connected to the Global Catalog partition. Instead, in real life. you can see Universal groups in Active Directory Users and Computers in the domain where you created them, but not in any other domains in your forest. Another effect of this is that when User A in Domain B is a member of a Universal Group created in Domain C, you cannot see that User A is a member of the Universal Group when you look at him in ADUC in Domain B. But you can see that he is a member when you look at the Universal Group’s members in Domain C.
So the Global Catalog is an illusion and saying that you replicate something to it is nonsense. What you are doing is applying a filter to objects.
(Some illustrations would probably be a good idea for this post, but it’s just too late. Sorry!)